The researchers obtained chromosome preparations using lymphocyte cultures and examined the samples with conventional staining, C-banding, and silver staining for nucleolus organizer regions (NORs). C-banding showed heterochromatin confined to the centromeric portions of the standard chromosomes, with no differences among populations. Every individual had one NOR block on the long arm of a pair of submetacentric chromosomes, while the B chromosomes lacked NOR signals and were completely heterochromatic.

Despite the conserved basic karyotype, the sampled populations differed in their B-chromosome profiles. The Apa River population was the only one in which all analyzed individuals lacked B chromosomes. Mogi Guaçu, Sapucaí, and Grande contained the three previously described morphologies: small acrocentric, medium metacentric, and large submetacentric B chromosomes. Metacentric variants predominated in the Paraná population, which also included one individual with an acrocentric B chromosome.

Only metacentric B chromosomes were observed at the two sampling points in the Batalha River basin, Reginópolis and Bauru. A Reginópolis individual carried a previously unreported metacentric macrochromosome B, which was also completely heterochromatic. One-way analysis of variance found significant differences among localities for the frequencies of metacentric, submetacentric, and acrocentric variants. The respective statistics were F = 18.1307 for metacentric, F = 6.5932 for submetacentric, and F = 8.1826 for acrocentric B chromosomes, with P < 0.0001 and six degrees of freedom in each comparison.

Earlier studies cited by the authors provide a possible framework for these geographic differences. The metacentric B chromosome is the only variant in P. lineatus reported to have a transmission rate above Mendelian expectations, with kb = 0.587, whereas the acrocentric and submetacentric variants have lower values of 0.333 and 0.385, respectively, consistent with elimination during transmission. The new observations are compatible with accumulation of the metacentric variant and the apparent disappearance of the other two from the Paraná, Reginópolis, Bauru, and Apa populations. This is a spatial pattern in the present samples, not direct evidence of a decline through time.

All three morphologies share repetitive sequence content, and previous fluorescence in situ hybridization work has located satellite DNA sequence PliSat05-178 on the B chromosomes and in the telomeric region of autosomal pair 4. Repetitive sequences are also shared by B chromosomes in P. lineatus, P. costatus, and P. argenteus. The authors therefore interpret the common Prochilodus B chromosomes as having a single origin before the diversification of at least these three carrier species. Within P. lineatus, the proposed sequence is an ancestral acrocentric form giving rise to an isochromosome and the metacentric variant, with the submetacentric form derived subsequently.

The absence of B chromosomes in the Apa population remains open to two explanations in the authors’ assessment. Either the elements were eliminated from that historically isolated population, or they were never present there; they reject the second explanation because the shared repetitive content indicates that the B chromosomes arose long before diversification within Prochilodus. The Apa population had been isolated above Sete Quedas waterfall, which was destroyed during construction of the Itaipu hydroelectric plant. The authors also note that B chromosomes could appear there through contact with fish from other populations, although the present survey does not establish that this has occurred or identify a genetic-introduction mechanism.

Geographic variation in B-chromosome prevalence is not unique to P. lineatus. In the characid fish Psalidodon paranae, reported B-chromosome carrier frequencies in three stretches of the Cascatinha stream ranged from 65.7% near the source to 10% in each of the two downstream stretches. The authors also cite evidence from grasshoppers in which nonfunctional microspermatids arise during spermiogenesis and from P. paranae in which polar-body elimination has been associated with lower transmission from B-carrying females.

This was a geographically bounded cytogenetic survey rather than a direct test of the mechanisms driving those differences. It included 148 fish from seven sampling points, with sample sizes ranging from eight at Bauru to 32 at Mogi Guaçu. The study did not directly measure transmission rates, meiotic drive, historical change, or fitness effects in the newly sampled populations, and the proposed evolutionary sequence for the variants remains an interpretation based on chromosome structure, repetitive DNA, and earlier inheritance studies.

Subject of Research: Cytogenetic variation in B chromosomes of Prochilodus lineatus across the Paraná River basin

Article Title: Evolutionary dynamics of the B chromosomes in the fish species Prochilodus lineatus Valenciennes, 1837 of the Paraná River Basin

Article References: Penitente, M., Goes, C. A. G., dos Santos, R. Z., Utsunomia, R., Foresti, F., & Porto-Foresti, F. (2025). Evolutionary dynamics of the B chromosomes in the fish species Prochilodus lineatus Valenciennes, 1837 of the Paraná River Basin. Comparative Cytogenetics, 19, 1-12. https://doi.org/10.3897/compcytogen.19.135127

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.135127

Keywords: Prochilodus lineatus, B chromosomes, cytogenetics, Paraná River basin, fish karyotype, chromosome variants, heterochromatin, population variation, meiotic drive, Evolutionary, dynamics, chromosomes

The investigation was a cytogenetic study conducted during routine screening of Kangayam bulls before their use for semen collection. The sample comprised 14 bulls from the Buffalo Frozen Semen Station, 16 from the Kangayam Cattle Research Station and 16 field-progeny animals tested in Tamil Nadu. Chromosomes were obtained through a short-term lymphocyte culture technique in the Cytogenetics Laboratory of the Department of Animal Genetics and Breeding at Madras Veterinary College in Chennai.

More than 200 metaphase spreads were screened, and chromosome preparations from the exceptional bull were subjected to further analysis with CBG and GTG banding. Forty-five of the screened bulls had a male chromosome complement consisting of 29 pairs of acrocentric autosomes, one large submetacentric X chromosome and the smallest acrocentric Y chromosome. The exceptional bull had a broadly similar complement, but its Y chromosome was classified as subacrocentric because a small p arm was visible.

This contrasted with the typical acrocentric Y chromosome found in the other animals, in which no prominent short arm could be distinguished. The subacrocentric chromosome was unpaired, and it appeared in every spread screened from this bull, indicating a consistent feature of the sampled metaphases rather than an isolated observation in individual cells. The unusual morphology alone did not establish whether the extra arm represented a structural translocation.

A similar chromosome complement had previously been reported in Chainina cattle and interpreted as a reciprocal translocation between the Y chromosome and chromosome 9. That possibility prompted the investigators to examine the Kangayam bull with CBG banding, which stains constitutive heterochromatin, and GTG banding, which distinguishes chromosome regions through a characteristic pattern of light and dark bands. The purpose was to determine whether the visible p arm belonged to the Y chromosome itself or contained a segment derived from an autosome.

CBG analysis showed characteristically dark centromeric bands on all the acrocentric autosomes, while the X chromosome stained lightly across its length. The exceptional Y chromosome behaved differently: it stained darkly throughout most or all of its length and lacked a clearly delineated centromeric band like that seen on the autosomes. Its small extra p arm was also darkly stained, rather than showing the light appearance characteristic of the autosomes.

These observations identified the additional arm as heterochromatic material associated with the Y chromosome, not a translocated autosomal segment. The GTG-banding results provided a second basis for characterizing the chromosome. The Y chromosome retained a recognizable band pattern, but its distribution differed between the subacrocentric variant in the exceptional bull and the typical acrocentric form.

In the variant, the pattern was divided between two visible arms, designated p and q; in the acrocentric form, the bands were situated together on the q arm. All autosomes retained the typical G-banding pattern expected in cattle. The investigators therefore found no indication of a translocation between the Y chromosome and an autosome, while confirming that the visible short arm was part of the Y-chromosome structure.

The result adds a rare observation to evidence that the Y chromosome of zebu cattle can vary in morphology. Earlier work on the Sahiwal, Sindhi, Brahman, Santa Gertrudis and Belmont Red breeds found differences in the position of the centromere relative to the end of the small p arm. In some bulls, no chromatin was visible beyond the centromere; in others, the p arm was distinct.

Sahiwal bulls were also described as having a visible subtelocentric Y chromosome, although that terminology can be problematic because the definitions of acrocentric and subtelocentric belong to different chromosome-classification systems. For the small Kangayam Y chromosome, the researchers used “subacrocentric” to emphasize the visible short arm relative to the typical acrocentric form seen in other breeding sires. Chromosomal rearrangements, including centric fusions, inversions and changes in the amount of heterochromatin, have been associated with the differentiation of chromosome complements across bovid species.

The chromosome complements of Bos indicus and Bos taurus are otherwise highly similar, but the Y chromosome differs in shape: it is generally acrocentric in Bos indicus and submetacentric or metacentric in Bos taurus. Comparative cytogenetic studies have related this morphological difference to a pericentric inversion, while variation in Y-chromosome size among subspecies has been associated with differences in heterochromatin content. The banding evidence in the Kangayam bull is consistent with heterochromatin variation as the explanation for its unusual p arm.

Because the affected animal was phenotypically healthy and the investigators did not classify the chromosome complement as an abnormality, the study reports a possible chromosomal polymorphism rather than a demonstrated reproductive defect. Its scope remains limited: the variant was detected in one of 46 bulls, and the study did not establish that it affects fertility, productivity or the transmission of the chromosome to offspring. The authors state that the productive and reproductive performance of the bull and its descendants should be studied if the bull is used for breeding.

They also note the possibility of culling an animal after screening, making evaluation relevant when a bull is retained. The central finding is the identification, through CBG and GTG banding, of a stable subacrocentric Y-chromosome variant in Kangayam cattle.

Subject of Research: Cytogenetic identification of a subacrocentric Y-chromosome variant in a Kangayam bull

Article Title: A rare chromosomal polymorphism in a Kangayam bull ( Bos indicus ) of south India

Article References: Harshini, V., Kumarasamy, P., & Karthickeyan, S. M. K. (2021). A rare chromosomal polymorphism in a Kangayam bull (Bos indicus) of south India. Comparative Cytogenetics, 15(4), 459-465. https://doi.org/10.3897/compcytogen.v15.i4.71295

Image Credits: AI Generated

DOI: 10.3897/CompCytogen.v15.i4.71295

Keywords: Kangayam cattle, Bos indicus, Y chromosome, chromosome polymorphism, cytogenetics, heterochromatin, CBG banding, GTG banding, Tamil Nadu India, cattle breeding, rare, chromosomal

B chromosomes, often abbreviated to Bs, are additional, dispensable components of the karyotype. Unlike the standard A chromosomes, they are not required for survival, do not follow Mendelian rules of inheritance, and typically do not recombine with the regular chromosome set, instead pursuing what researchers describe as their own evolutionary destiny. They have now been detected in roughly three percent of all analysed species across animals, plants, and fungi, and in 85 mammalian species. Yet their distribution is strikingly uneven: some lineages carry them routinely, while others — including most bats — almost never do. Usually Bs originate from A chromosomes of the same species, though hybridization between closely related species can also generate them, and recent studies suggest many are transcriptionally active and may even shape the traits of their carriers.

The new study focused on four adult male Leisler’s bats captured with mist-nets over water bodies at Bebića Luka in Western Serbia and Zlot in Eastern Serbia during 2020. Tissue samples from the plagiopatagium, the membrane stretching between body and wing, were taken with sterile biopsy punches and transported to the laboratory within 24 hours, where primary fibroblast cell cultures were established. Once sufficient cells had grown, researchers arrested cell division with colchicine, treated the cultures with ethidium bromide to lengthen the chromosomes, and prepared chromosome spreads for staining. Every animal was safely released immediately after sampling under permits from Serbia’s Ministry of Environmental Protection.

The cytogenetic results were unambiguous. The standard complement of Nyctalus leisleri counts 44 chromosomes — 42 autosomes plus a pair of sex chromosomes — and all four males exceeded this baseline. Three bats from Bebića Luka carried karyotypes ranging from 44 plus one to two Bs, and 44 plus three to five Bs, while the single male from Zlot carried between two and four. All the extra chromosomes were heterochromatic microchromosomes, tiny elements composed largely of constitutive heterochromatin concentrated at their centromeres. Crucially, the number of Bs varied not only between individuals but also between cells within the same individual, a hallmark of B chromosome biology that reflects their unstable, non-Mendelian behaviour during cell division.

Detailed banding analyses added texture to the picture. G-banding, which reveals the characteristic light and dark stripes of condensed and less condensed chromatin, confirmed the morphology of the standard set: three pairs of large metacentrics, one small submetacentric pair, and 17 pairs of acrocentrics, with a medium-sized metacentric X and a small acrocentric Y. C-banding showed heterochromatin in the centromeric regions of all autosomes, sex chromosomes, and Bs alike. Silver staining identified active nucleolus organizer regions — the sites of ribosomal RNA genes — on two chromosome pairs, numbered 8 and 11 in the karyotype. Notably, earlier work on specimens from Greece and Turkey had placed these regions on pairs 8 and 15, a discrepancy the authors attribute either to genuine spatial diversity among populations or to differences in chromosome condensation during preparation, a known pitfall when karyotypes contain many similarly sized acrocentrics.

With this study, Leisler’s bat becomes the holder of the highest B chromosome count recorded in the species, surpassing the maximum of three Bs documented by Volleth in 1992 in males from Turkey, Germany, and Greece. Only four bat species are known to carry Bs at all: three vespertilionids — Pipistrellus tenuis, Myotis macrodactylus, and Nyctalus leisleri — and the megadermatid Megaderma spasma, whose Bs, though among the smallest chromosomes in its karyotype, are not microchromosomes and do not appear fully heterochromatic. In the least pipistrelle, Bs take metacentric form, while the big-footed Myotis carries a single micro B that can be acrocentric or metacentric. Intriguingly, every analysed Leisler’s bat karyotype reported to date — all of them males — has contained Bs, and the only female karyotype on record, from Poland, probably carried two micro Bs that the original authors, analysing a single animal, did not report as such.

The rarity of Bs in bats compared with rodents is the study’s central puzzle. Rodents, the largest mammalian order, boast 61 species with B chromosomes; bats, the second-largest order with more than 1440 species, have just four. The authors propose that genome size may hold part of the answer. Bats average a remarkably small genome of about 2.35 picograms of DNA, ranging from 1.3 to 3.2 pg — a range comparable to birds, whose genomes span a narrow 2 to 4 pg and whose repeated sequence content, at 15 to 20 percent, is the lowest among vertebrates. Both birds and bats are the only living vertebrates capable of active flight, and it has long been hypothesized that the extreme metabolic demands of flight impose a constraint on genome size. Smaller genomes mean smaller cells, and smaller cells have a higher surface-area-to-volume ratio, improving gas exchange to satisfy metabolic needs. A streamlined genome, the reasoning goes, may simply have no room to tolerate the genomic baggage of Bs — a pattern echoed in flowering plants, where B chromosome presence correlates positively with total genome size and small-genomed species rarely feature them.

Life history may reinforce the genomic argument. Bats are among the longest-lived mammals for their size, with slow reproductive rates and small litters, typically one or two young per year. Rodents, by contrast, live fast and die young, producing large litters that give B chromosomes far more opportunities to establish, spread, and persist within populations. A dispensable chromosome that confers no immediate benefit needs many generations and many reproductive events to gain a foothold; a bat’s leisurely life cycle offers few such chances. The authors also note that the apparent rarity of Bs in bats may partly reflect how few bat karyotype studies have been conducted compared with the extensive cytogenetic literature on rodents.

The Serbian setting adds ecological intrigue of its own. Leisler’s bat is a medium-sized woodland species distributed across Europe up to 57 degrees north, considered rare almost everywhere except Ireland, where it thrives. It roosts in tree holes, forages over canopy, forest trails, and water, hibernates in tree cavities, and undertakes long seasonal migrations — movements exceeding 1500 kilometres have been recorded in females breeding in Germany and wintering on the Iberian Peninsula. In Serbia, the species has been recorded at seven localities, almost always as single males captured at foraging grounds between July and September, with no known roosts and no evidence of reproduction. This male-dominated pattern likely reflects the species’ migration dynamics, in which males occur sporadically in northern regions but dominate southern populations, and it explains why the only published female karyotype comes from Poland.

For the researchers, the combination of a record-breaking B chromosome count, the absence of female karyotype data, and scarce ecological information makes Nyctalus leisleri a compelling model for future B chromosome research. The modern view of Bs frames them as an extra genomic compartment with huge potential and still unknown biological significance — elements once dismissed as inert heterochromatic leftovers now suspected of contributing actively to the phenotypes of their carriers. Whether the five Bs of a Serbian Leisler’s bat carry genes, influence traits, or simply ride the waves of non-Mendelian inheritance remains to be discovered, but this small woodland bat has made clear that even the most streamlined genomes can still surprise us.

Subject of Research: Cytogenetics

Article Title: The new highest number of B chromosomes ( Bs ) in Leisler’s bat Nyctalus leisleri (Kuhl, 1817)

Article References: Rajičić, M., Budinski, I., Miljević, M., Bajić, B., Paunović, M., Vujošević, M., & Blagojević, J. (2022). The new highest number of B chromosomes (Bs) in Leisler’s bat Nyctalus leisleri (Kuhl, 1817). Comparative Cytogenetics, 16(3), 173-184. https://doi.org/10.3897/compcytogen.v16i3.89911

Image Credits: AI Generated

DOI: 10.3897/CompCytogen.v16i3.89911

Keywords: B chromosomes, Leisler's bat, Nyctalus leisleri, cytogenetics, karyotype, Chiroptera, heterochromatin, supernumerary chromosomes, genome size, Serbia, highest, number

The research team, led by cytogeneticists working with the enzymatic maceration and flame-drying method, faced a formidable technical challenge. Cucurbit chromosomes are notoriously small and look frustratingly similar under a conventional microscope, which has hampered chromosome identification since cytogenetic studies of the family began in the 1920s. To overcome this, the researchers germinated seeds from commercial suppliers in China, treated actively growing root tips with a chemical that arrests cell division at metaphase, and digested the cell walls with an enzyme cocktail of cellulase and pectolyase before flame-drying the preparations onto glass slides. The result was spreads of well-separated, clearly outlined mitotic chromosomes suitable for precise measurement.

The real magic came from two sequential labeling steps. First, the team applied CPD staining, a mixture of the fluorochromes propidium iodide and DAPI that simultaneously highlights GC-rich regions in red and AT-rich regions in blue. These bands mark heterochromatin, the densely packed, repetitive DNA that tends to cluster around centromeres and chromosome ends. Second, the researchers performed fluorescence in situ hybridization, or FISH, using a biotin-labeled probe for the 45S ribosomal RNA gene cluster, the tandemly repeated DNA that encodes the structural RNA components of ribosomes. The green fluorescent signals revealed exactly where these gene clusters sit on each chromosome, providing landmarks that distinguish otherwise identical-looking chromosomes.

The results revealed striking diversity hidden beneath superficial similarity. The diploid chromosome numbers ranged from 14 in cucumber to 40 in pumpkin, with watermelon, bitter gourd and bottle gourd carrying 22, melon and wax gourd carrying 24, and sponge gourd carrying 26. Despite these differences, all eight species possessed symmetrical karyotypes built from metacentric and submetacentric chromosomes, meaning their centromeres sit near the middle or slightly off-center. Metacentric chromosomes dominated the complements, accounting for nearly 87 percent of all chromosomes counted. According to Stebbins’ classic classification system, every karyotype fell into the most symmetric categories, 1A, 1B or 2A.

The heterochromatin patterns told a story of deep evolutionary divergence. Five species, including cucumber, melon, watermelon, bitter gourd and sponge gourd, showed GC-rich bands at their centromeres, while wax gourd, bottle gourd and pumpkin lacked them entirely. Cucumber stood out with additional GC-rich heterochromatin at its chromosome tips, a feature absent even in its close relative melon. Pumpkin displayed yet another pattern: after the FISH procedure, DAPI counterstaining revealed pericentromeric AT-rich bands on four chromosome pairs, a distinct type of heterochromatin that differs from both the GC-rich and canonical AT-rich classes. The researchers suggest that the centromeric GC-rich heterochromatin seen in most species is likely an ancestral feature of the subfamily, and that the three species lacking it may have lost the signal through a reduction in GC content after speciation.

The ribosomal DNA sites proved equally informative. FISH detected two 45S rDNA loci in five species and five loci in cucumber, sponge gourd and pumpkin, with most sites positioned at the terminal ends of chromosome arms. Pooling their data with previous surveys covering 58 cucurbit species, the authors found that roughly three quarters of all 45S rDNA loci in the family occupy terminal positions, consistent with the general pattern across flowering plants. Four of the eight crops studied, watermelon, wax gourd, bottle gourd and bitter gourd, shared the same configuration of two terminal rDNA loci, hinting at a close kinship that matches molecular phylogenies. The researchers speculate that the ancestor of the tribe Benincaseae, which includes many of these crops, carried exactly two terminal 45S rDNA sites.

Cucumber emerged as the star of the study. By combining the CPD banding pattern with the 45S rDNA signals, the team could unambiguously identify every one of its seven chromosome pairs, each with a unique fingerprint of red bands and green signals. This provides plant breeders and geneticists with a simple, reliable method for chromosome identification in a crop whose genome has already been fully sequenced. In pumpkin, by contrast, the size of the rDNA signals on one chromosome pair differed markedly between the two homologous copies, a sign that the accession analyzed was heterozygous at that locus, a reminder that even within a cultivar, genomes can vary.

To place the eight species in a quantitative framework, the researchers calculated four karyotype asymmetry indices, including the coefficient of variation of chromosome length, which measures differences in size between chromosomes, and the mean centromeric asymmetry, which captures variation in centromere position within chromosomes. A scatter plot of these two indices cleanly separated all eight species, with sponge gourd showing the most symmetric karyotype and melon and bottle gourd the most asymmetric by the intra- and interchromosomal measures respectively. A principal coordinate analysis based on six karyological parameters then grouped the species into two clusters that largely matched the relationships inferred from DNA sequence data, with watermelon, bottle gourd and bitter gourd clustering tightly together and pumpkin occupying the most isolated position.

The study also shed light on genome size evolution. Total chromosome length correlated strongly with nuclear DNA content across the eight species, with wax gourd carrying the largest genome and bitter gourd the smallest. Genome sequencing studies attribute these differences mainly to the accumulation of transposable elements rather than whole-genome duplication, with wax gourd’s large genome driven by a substantial load of LTR retrotransposons. Pumpkin, meanwhile, is the product of an ancient allotetraploidization event, and cucumber and melon evolved from a shared twelve-chromosome ancestor through inversions, centromere repositioning and chromosome fusions. Yet despite all this upheaval, karyotype symmetry remained remarkably stable across the family.

The authors conclude that multivariate karyotype asymmetry analysis deserves a place alongside molecular phylogenetics as a complementary tool for tracing chromosome evolution in Cucurbitaceae. For a family that supplies humanity with some of its most important vegetables, the ability to read chromosomes with fluorescent precision opens new avenues for breeders seeking to transfer disease resistance and fruit quality traits between related species, and for evolutionary biologists piecing together how a single ancestral genome diversified into the gourds, melons and squashes that now fill markets around the world.

Subject of Research: Comparative karyotype analysis of eight Cucurbitaceae crop species using fluorochrome banding and 45S rDNA FISH

Article Title: Comparative karyotype analysis of eight Cucurbitaceae crops using fluorochrome banding and 45S rDNA-FISH

Article References: She, C.-W., Jiang, X.-H., & He, C.-P. (2023). Comparative karyotype analysis of eight Cucurbitaceae crops using fluorochrome banding and 45S rDNA-FISH. Comparative Cytogenetics, 17(1), 31-58. https://doi.org/10.3897/compcytogen.v17.i1.99236

Image Credits: AI Generated

DOI: 10.3897/compcytogen.v17.i1.99236

Keywords: Cucurbitaceae, karyotype, fluorochrome banding, FISH, 45S rDNA, heterochromatin, cucumber, melon, watermelon, pumpkin, cytogenetics, genome evolution

Sabanejewia loaches are diminutive fish, rarely exceeding 15 centimeters in total length, and they lead a secretive benthic existence in river habitats across a vast swath of Eurasia. Their range stretches from northern Italy to the Aral Sea basin, touching tributaries of the Black, Caspian, Baltic and Mediterranean Seas. What sets them apart from all other members of the loach family Cobitidae is a peculiar sexual dimorphism: males develop distinctive lateral body swellings. Despite this clear signature, the taxonomy of the group has been anything but settled, with the highest diversity long assumed to reside in the Danube basin itself.

That assumption has been shaken in recent decades. Phylogenetic studies have revealed that several phenotypes within the Danube region are so closely related that they belong to what researchers call the Sabanejewia balcanica species complex, meaning the exact species composition in the area is still unknown. Sabanejewia bulgarica itself was originally described in 1928 by Drensky as a species of the genus Cobitis, occurring in the lowest Danube basin including the Danube River proper. For many years it was treated as merely a subspecies within the polytypic Balkan golden loach complex, but later analyses of geographical variation showed that bulgarica is quite distinct from all other populations and deserves full species rank.

The distinguishing features are subtle but telling. Sabanejewia bulgarica differs from its relatives in overall body coloration, the character of Gambetta’s zone, pigmentation patterns, and the character of spots at the end of the caudal peduncle and the base of the caudal fin. Perhaps most strikingly, it is the only deep-water riverine form within the genus. More recently, researchers examining individuals from the type localities of both S. balcanica and S. bulgarica have argued that Danubian golden loaches are genetically closer to S. bulgarica, and that S. balcanica proper should be restricted to the drainage of the Vardar River.

Against this backdrop of taxonomic uncertainty, chromosome studies offer an independent line of evidence. Yet the chromosomes of Sabanejewia loaches remain poorly studied. Before this new work, only a handful of species had been karyotyped: S. larvata, S. caspia and S. kubanica in earlier summaries, followed by reports on S. caspia and S. aurata from other locations, and S. baltica from the Bug River in Poland. All of these studies documented an invariable diploid chromosome number of 2n = 50 and remarkably small karyotype variability across the genus, making each new species description a valuable test of that pattern.

The new study examined five specimens of S. bulgarica, all females, collected in July 1997 from the Saint George Branch of the Danube Delta in Romania. The fish arrived at the laboratory of the Danube Delta Institute in Tulcea in poor condition and subsequently died, but not before yielding the material needed for chromosome preparation. One female displayed the color pattern and high body described in Drensky’s original 1928 account, while the other four showed different color patterns, a reminder of how variable these fish can appear even within a single sample.

Using conventional Giemsa staining and the C-banding technique, with chromosomes classified according to the widely used system of Levan and colleagues, the researchers determined that the diploid chromosome number was 2n = 50 in all five individuals. The detailed karyotype analysis was carried out on a single specimen whose morphological characters matched the original description. The karyotype comprised two pairs of metacentric chromosomes, with the first pair much larger than the second, six pairs of submetacentric chromosomes, and seventeen pairs of subtelocentric to acrocentric chromosomes.

The C-banding procedure, which stains constitutive heterochromatin, revealed conspicuous pericentromeric blocks of heterochromatin in most of the chromosome pairs, with particularly prominent blocks in the largest metacentric chromosome and the first submetacentric element. These heterochromatic regions, clustered around the centromeres, provide a fingerprint that can be compared across species and populations, offering clues about how genomes are organized even when chromosome counts and shapes appear similar.

When the researchers compared the new karyotype with those of all karyologically studied Sabanejewia species, the similarities were striking. Every species examined so far shares the same diploid number of 2n = 50, a low number of metacentric chromosomes, and comparable proportions of submetacentric and subtelocentric-to-acrocentric elements. The karyotype phenotype of S. bulgarica proved to be essentially the same as that found for S. balcanica from the Northern Carpathian Mountains in Slovakia. Some apparent differences in karyotype structure among published accounts, the authors note, may stem from methodological choices rather than biology: some researchers distinguished subtelocentric from acrocentric chromosomes while others combined them, and the gradual range of centromere positions in small acrocentric-like chromosomes makes precise assignment difficult. The contraction of chromosome arms during mitosis, influenced by the timing and dose of colchicine treatment, further complicates comparisons between studies.

The conclusion is clear: the karyotype of S. bulgarica from the Danube Delta is very similar to those of other Sabanejewia species in chromosome number and morphology, and nearly identical to that of S. balcanica from the Northern Carpathian range. Whether this chromosomal conservatism reflects genuine evolutionary stasis or simply masks differences too subtle for conventional staining to detect remains an open question. The authors suggest that further detailed cytotaxonomic and genetic surveys of Sabanejewia populations throughout the genus’s distribution will be needed to resolve the systematics of the Danubian golden loaches, a group whose true diversity may still be hiding in the murky depths of Europe’s rivers.

Subject of Research: Karyotype analysis of the Bulgarian golden loach Sabanejewia bulgarica from the Danube Delta, Romania

Article Title: Karyotype of Sabanejewia bulgarica (Drensky, 1928) ( Teleostei , Cobitidae ) from the Danube Delta, Romania

Article References: Hnátková, E., Majtánová, Z., Bohlen Šlechtová, V., Bohlen, J., & Ráb, P. (2023). Karyotype of Sabanejewia bulgarica (Drensky, 1928) (Teleostei, Cobitidae) from the Danube Delta, Romania. Comparative Cytogenetics, 17, 157-162. https://doi.org/10.3897/compcytogen.17.103152

Image Credits: AI Generated

DOI: 10.3897/compcytogen.17.103152

Keywords: Sabanejewia bulgarica, Cobitidae, karyotype, C-banding, Danube Delta, cytogenetics, chromosomes, heterochromatin, freshwater fish, cytotaxonomy, golden loach, Sabanejewia

The research team, led by scientists from Brazilian and Argentine institutions, applied a combination of classical cytogenetic techniques and modern molecular mapping methods to five species within the P. gracilis group. Their goal was ambitious: to test a long-standing hypothesis about chromosome homology across these closely related frogs, and to determine whether the nucleolus organizer regions (NORs)—the chromosomal sites where ribosomal RNA genes cluster—occupy the same evolutionary position in different species, or whether they have shifted over time through structural rearrangements. The answer, as it turns out, is more nuanced than anyone expected.

Physalaemus evangelistai proved to have a diploid number of 22 chromosomes and a fundamental number of 44, which is entirely consistent with the karyotypic configuration seen across the entire genus. Chromosome pairs 1, 5, 6, and 10 were metacentric, while pairs 2 through 4 and 7 through 11 were submetacentric. C-banding revealed small blocks of constitutive heterochromatin in the centromeric regions of all chromosomes, a pattern that is remarkably conserved across the group. But the real surprise came when the researchers examined chromosome 8 in detail.

In every specimen analyzed, chromosome 8 carried an interstitial NOR on its long arm, coinciding with a secondary constriction visible under standard Giemsa staining. Yet the NOR was not uniform in size across individuals. The team identified two distinct morphotypes: chromosome 8a, bearing a small NOR, and chromosome 8b, carrying a much larger one. Three of the four males examined were heterozygous for this trait, while both females analyzed were homozygous for the smaller variant. More intriguingly, both females carried an additional NOR on one homologue of chromosome pair 9—a feature entirely absent from all four males and from every other karyotyped species in the group.

This sex-specific pattern immediately raised the possibility that P. evangelistai possesses a ZZ/ZW sex determination system, with the NOR serving as a cytogenetic marker distinguishing the sex chromosomes. Such a mechanism has been documented in other anuran species, including Physalaemus ephippifer, Gastrotheca riobambae, Buergeria buergeri, and Hyla femoralis, where NORs have been identified as the sole cytogenetic difference between sex chromosomes. However, the researchers are careful to note that NOR polymorphisms are relatively common among frogs in general, and the limited sample size—particularly the small number of females—prevents any definitive conclusion. Whether this heteromorphism is truly sex-linked or represents an autosomal polymorphism remains an open question that will require larger sampling efforts to resolve.

Beyond the novel karyotype description, the study made a significant contribution to the ongoing debate about chromosome homology within the P. gracilis group. Four of the six species had been karyotyped previously, and their NORs occupy seemingly different positions: interstitial on chromosome 8 in P. carrizorum and P. lisei, terminal on chromosome 8 in P. gracilis, and terminal on chromosome 10 in P. barrioi. Because chromosomes 8 through 11 in these karyotypes are very similar in size and morphology, determining whether the NOR-bearing chromosome of P. barrioi is truly homologous to chromosome 8 of the other species has been extremely challenging. The new study found a subtle but consistent clue: an interstitial DAPI-positive C-band on the short arm of the NOR-bearing chromosomes in P. evangelistai, P. carrizorum from Palmas, and P. barrioi. This shared heterochromatic feature, though faint and easily missed in some metaphase preparations, provides the first independent cytogenetic evidence supporting the hypothesis that these chromosomes are indeed homologous across the group.

The researchers also mapped two repetitive DNA sequences—PcP190 satellite DNA and the U2 small nuclear RNA gene—across the karyotypes of the analyzed species. PcP190 is a satellite DNA family widely distributed among hyloid anurans, having been reported in seven families to date. In the P. gracilis group, it produces a small number of chromosomal clusters arranged in apparently species-specific patterns. The new mapping revealed that P. carrizorum from Palmas, Paraná, carries PcP190 clusters on chromosomes 1 and 3, while P. evangelistai has a cluster only on chromosome 1, and P. barrioi shares the chromosome 1 and 3 pattern. This distribution is phylogenetically informative: the species carrying the chromosome 1 cluster (P. evangelistai, P. carrizorum, and P. barrioi) form one clade, while P. lisei and P. gracilis, which lack it, form another. The most parsimonious explanation is that the chromosome 1 cluster was present in the ancestor of the group and subsequently lost in the lineage leading to P. lisei and P. gracilis.

Perhaps the most striking finding of the entire study concerns P. carrizorum itself. Specimens from Palmas in Paraná, Brazil, differ markedly from those previously described from Misiones, Argentina—localities separated by approximately 242 kilometers. The Palmas population shows a terminal NOR on chromosome 8 and PcP190 clusters on chromosomes 1 and 3, whereas the Misiones population has an interstitial/pericentromeric NOR on chromosome 8 and only a single PcP190 cluster on chromosome 3. Additional differences in heterochromatin distribution further distinguish the two populations. These cytogenetic divergences could reflect intraspecific variation, but they could equally signal the presence of cryptic species hidden within what is currently recognized as a single taxon. The authors emphasize that further taxonomic analyses, integrating morphological, molecular, and bioacoustic data, will be essential to determine which explanation is correct.

The U2 snRNA gene mapping provided a contrasting picture of evolutionary stasis. In all four analyzed species of the P. gracilis group, U2 snRNA gene clusters are restricted to the terminal region of the short arm of chromosome 6. This same condition has been observed in other species of Physalaemus and in the related genus Leptodactylus, supporting the hypothesis that chromosome 6 is homologous across these lineages. While the authors caution that mapping this sequence in additional leptodactylid genera is needed before drawing definitive conclusions, the consistency of this marker across such a broad taxonomic range makes it one of the most reliable chromosomal landmarks available for comparative cytogenetics in the family.

Taken together, these findings paint a picture of a frog group whose genomes are simultaneously conserved and dynamic. The fundamental karyotype—22 chromosomes with a fundamental number of 44—has remained stable across the genus, yet the positions of NORs, the distribution of satellite DNA clusters, and even the number of heterochromatic bands vary considerably between species and even between populations of the same species. This combination of stability and lability makes the P. gracilis group an excellent natural laboratory for studying how chromosomal rearrangements contribute to speciation, particularly in a lineage already known for its high frequency of cryptic diversity. As the authors note, much work remains to be done, but the characterization of P. evangelistai‘s karyotype and the identification of shared heterochromatic markers on NOR-bearing chromosomes represent important steps toward resolving the evolutionary history of this enigmatic group of Neotropical frogs.

Subject of Research: Comparative cytogenetics and karyotype characterization of frog species in the Physalaemus gracilis group

Article Title: Comparative cytogenetics of the Physalaemus gracilis group ( Anura , Leptodactylidae ) with characterization of the karyotype of Physalaemus evangelistai Bokermann, 1967

Article References: Mosquini, P. H. P., Souza, L. H. B., Ferro, J. M., & Lourenço, L. B. (2025). Comparative cytogenetics of the Physalaemus gracilis group (Anura, Leptodactylidae) with characterization of the karyotype of Physalaemus evangelistai Bokermann, 1967. Comparative Cytogenetics, 19, 171-188. https://doi.org/10.3897/compcytogen.19.171637

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.171637

Keywords: Physalaemus, cytogenetics, karyotype, NOR, satellite DNA, FISH, anurans, chromosome evolution, Leptodactylidae, heterochromatin, cryptic species, U2 snRNA

Scorpions occupy a special place in arachnid cytogenetics. They are the third most extensively studied order of arachnids from a chromosomal perspective, and their male meiosis is unusual: it is achiasmatic, meaning that the physical crossovers familiar from many other animals are absent. Correct chromosome segregation is instead guaranteed by the persistence of the synaptonemal complex, the protein scaffold that holds paired chromosomes together, until late stages of the first meiotic division. With the exception of the family Buthidae, whose members carry holocentric chromosomes with diffuse centromere activity, all other cytogenetically characterized scorpion families, including Bothriuridae, possess monocentric chromosomes with a single, clearly defined centromere.

Despite this foundation of knowledge, the data remain strikingly sparse. The family Bothriuridae comprises 168 described species, yet fewer than 6 percent have had their chromosomes examined, and only three genera have been studied at all. Within Brazil, the genus Bothriurus is the most diverse of the family, with 17 described species, but only five had been cytogenetically characterized before this work. The new study, led by researchers working with specimens deposited at the Federal University of Piauí, focused on two species from northeastern Brazil: Bothriurus asper and Bothriurus rochai, each the namesake of a species group still beset by taxonomic uncertainty.

The sampling effort was unusually broad for scorpion cytogenetics. Two populations of B. asper were analyzed, one from Teresina in Piauí and one from Igarassu in Pernambuco, the species’ type locality. Nine populations of B. rochai were sampled across Ceará, Paraíba, Pernambuco, Piauí, and Rio Grande do Norte, spanning both the Caatinga dry forest and, in the case of Igarassu, the Atlantic Forest. Gonadal tissue from adult specimens was prepared on slides and stained with Giemsa to reveal chromosome counts, meiotic configurations, and chromosome morphology, following established protocols for scorpion cytogenetic preparations.

The results for B. asper were remarkably consistent. Both geographically distant populations, separated by hundreds of kilometers and different biomes, showed a diploid number of 2n = 30, a strong indication that this is the fixed karyotype for the species. But the meiotic cells told a subtler story. In the Igarassu specimen, most post-pachytene nuclei displayed thirteen bivalents plus a chromosomal chain of four elements arranged in a cross-shaped configuration, while a minority showed fifteen simple side-by-side bivalents. In the Teresina specimens, every cell showed the four-element chain, but in an open configuration rather than a closed ring. Such multivalent associations are the classic signature of reciprocal chromosomal translocations, rearrangements in which segments are exchanged between non-homologous chromosomes, and the open configuration in the Teresina population suggests a second, non-reciprocal translocation that moved a fragment from the end of one chromosome into an interstitial position on another.

B. rochai proved far more volatile. Its populations split cleanly into two karyotypic forms: individuals from Brasileira, Floriano, São Raimundo Nonato, Icó, Quixadá, and Maturéia carried 2n = 18, while those from Apodi, Cajazeiras, and João Câmara carried 2n = 16. These are the lowest diploid numbers ever reported for the family Bothriuridae, a group whose previously characterized species range from 2n = 28 up to 2n = 50. The meiotic behavior varied accordingly. Some populations showed only simple bivalents, while others, notably João Câmara and Cajazeiras, consistently displayed five large bivalents plus a chain of six chromosomes, again pointing to reciprocal translocations layered on top of whatever fission or fusion events changed the chromosome count.

To probe the physical consequences of these rearrangements, the researchers measured the diploid set length, the combined length of all chromosomes in a cell, using image analysis software and compared the values statistically with a generalized linear model. The differences were significant. Cells from individuals with 2n = 16 and from 2n = 18 individuals carrying the six-chromosome chain had systematically smaller chromosome sets than cells from 2n = 18 individuals with nine bivalents or a four-element chain. Because rearrangements such as fusions and fissions do not by themselves change total chromosome length, this pattern suggests that deletions or duplications of genetic material have accompanied the karyotypic divergence, adding a layer of complexity beyond simple one-step rearrangements. Determining which diploid number is ancestral, the authors note, will require phylogeographic or phylogenetic analyses.

Two molecular markers added further texture to the picture. Silver ion impregnation, which stains active nucleolus organizer regions, the chromosomal sites of the ribosomal RNA genes, revealed a single terminal pair of NORs in every B. rochai population examined, regardless of whether the population carried 16 or 18 chromosomes. This stability is notable: while NOR number and position are conserved in some scorpion lineages, other genera show variation, and a single terminal NOR pair is considered the ancestral condition for arachnids. In contrast, C-banding, which reveals constitutive heterochromatin, painted a more variable landscape. B. asper carried larger heterochromatic blocks than B. rochai, and within B. rochai the amount of heterochromatin differed among populations, with some populations showing no detectable C-banding signals at all, likely because their heterochromatin blocks are too small to resolve.

Perhaps the most intriguing correlation emerged from comparing heterochromatin quantity with meiotic instability. In scorpions, the amount of heterochromatin appears to be inversely related to the frequency of chromosomal rearrangements. Consistent with this trend, the B. rochai population with the least heterochromatin, Brasileira in Piauí, also showed the greatest diversity of post-pachytene configurations, including cells with nine bivalents, cells with a six-chromosome chain, and cells with a four-element chain. At the species level, B. asper, with its larger heterochromatic blocks, showed only a single distinct meiotic configuration. Fluorochrome staining added a final detail: prominent AT-rich regions detected with DAPI in some populations, while GC-rich regions stained uniformly with chromomycin A3, without specific bright signals.

Taken together, the study marks the first time multiple populations of any Bothriurus species have been cytogenetically characterized, and it substantially expands the karyotypic record for scorpions with monocentric chromosomes. The contrast between the stable karyotype of B. asper and the variable one of B. rochai suggests distinct evolutionary trajectories within the genus, with translocations and fission-fusion events playing a central role in reshaping genomes even as some features, like the single NOR pair, remain conserved. For a group whose species boundaries are still being untangled, such chromosomal characters may prove valuable diagnostic tools, and the authors hope the data will feed into broader biodiversity research in one of the world’s most understudied arachnid faunas.

Subject of Research: Comparative cytogenetics of chromosome number, meiotic behavior, heterochromatin, and nucleolus organizer regions in populations of two Bothriurus scorpion species from northeastern Brazil

Article Title: Comparative cytogenetics among populations of two Bothriurus species ( Scorpiones , Bothriuridae )

Article References: Lima, J. F. D., Schneider, M. C., Carvalho, L. S., & Pinto-da-Rocha, R. (2025). Comparative cytogenetics among populations of two Bothriurus species (Scorpiones, Bothriuridae). Comparative Cytogenetics, 19, 189-208. https://doi.org/10.3897/compcytogen.19.165160

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.165160

Keywords: scorpions, Bothriurus, cytogenetics, karyotype, chromosomal rearrangements, heterochromatin, nucleolus organizer regions, meiosis, Bothriuridae, Brazil, Caatinga, chromosome evolution

The central premise of the review is deceptively simple. Neurons are postmitotic cells, meaning they have permanently exited the cell cycle and cannot divide to replace themselves. This makes them extraordinarily long-lived but also vulnerable to the slow accumulation of unrepaired damage. Because the nucleus houses the genome and controls gene expression, RNA processing, DNA repair, and the traffic of molecules between nucleus and cytoplasm, any compromise to nuclear structure can cascade into dysfunction. In postmitotic neurons, LaminB is the dominant component of the nucleoskeleton, the filamentous scaffold that anchors chromatin to the inner nuclear membrane and connects, through the LINC complex, to the cytoskeleton and ultimately the extracellular matrix. This architecture allows the nucleus to act as a mechanosensor, translating physical forces from the environment into changes in genome organization and gene activity.

Napoletano proposes a framework of what he calls hallmarks of neuronal health, adapted from the broader hallmarks of health concept articulated by López-Otín and Kroemer. These include the integrity and repair of the cell body, axons, dendrites, and organelle membranes; long-term maintenance of molecular recycling and turnover; preservation of cell identity; alignment of neuronal output with environmental demands; sustained bioenergetic competence; and adaptive structural plasticity. Within this framework, the nuclear envelope and its LaminB scaffold emerge as a central hub, interfacing with essentially every other hallmark. When compartmentalization fails, for instance, nuclear DNA can leak into the cytoplasm, activating innate immune sensors such as cGAS-STING and triggering inflammatory pathways that accelerate degeneration.

The structural role of LaminB is well established. The protein is anchored to the inner nuclear membrane through a lipid modification called farnesylation, forming a meshwork that interacts with chromatin, RNA-binding proteins, transmembrane proteins of the LEM domain family such as Emerin and LAP2, and components of the nuclear pore complex. Mammals express two B-type Lamins, LMNB1 and LMNB2, with partially redundant functions, whereas Drosophila carries a single ortholog known as LaminDm0. This genetic simplicity has made the fly an exceptionally powerful system for dissecting LaminB function. When LaminB is knocked down in postmitotic fly neurons, the animals show neurodegeneration, reduced lifespan, and impaired locomotor ability, directly linking nuclear scaffold integrity to neuronal survival in vivo.

One of the most striking examples of LaminB dysfunction comes from Drosophila models of tauopathy, the family of neurodegenerative conditions that includes Alzheimer’s disease. In these flies, expression of mutated human Tau protein drives hyper-polymerization of actin fibers, which push against the nucleus through the LINC complex. The resulting mechanical load deforms the nuclear envelope, causing LaminB to buckle and RNA to accumulate abnormally within nuclear invaginations. This toxic buildup of messenger RNA inside deformed nuclei provides a tangible mechanistic link between cytoskeletal pathology and nuclear failure, illustrating how mechanical stress, long considered a peripheral concern in neurodegeneration research, can directly compromise the genome’s physical container.

Beyond its structural role, LaminB regulation emerges as a dynamic stress-integration system governed in part by post-translational modifications, particularly phosphorylation. The review highlights the enzyme Pin1, a phosphorylation-dependent peptidyl-prolyl isomerase conserved from yeast to humans. Pin1 recognizes serine or threonine residues that precede proline in target proteins, and by catalyzing cis-trans isomerization at these sites it acts as a conformational checkpoint that fine-tunes signaling. Pin1 is overexpressed in many cancers, where it promotes tumor progression, but it is reduced or inactivated during aging and in Alzheimer’s disease. Loss of Pin1 function in Drosophila mutants and in knockout mice causes premature aging phenotypes, including degeneration of brain neurons, retina, and gonads, positioning the enzyme as a molecular switch between proliferative stress responses and neuronal vulnerability.

Drosophila studies pioneered the discovery of a pathway linking Pin1 to genome stability under mechanical stress. When Pin1 levels or activity decline, as occurs with aging and in Alzheimer’s disease, the LaminB meshwork deforms under mechanical load. This deformation triggers degradation of Heterochromatin Protein 1 alpha, the major promoter of heterochromatin condensation, via the ubiquitin ligase Hecw, whose mammalian counterparts are NEDL and HECW-1/2. The loss of HP1a unleashes heterochromatin derepression and mobilization of transposable elements, mobile genetic sequences that insert themselves into DNA and cause cumulative damage. This cascade ultimately drives cell death and activates pro-inflammatory signaling, providing a mechanistic explanation for how mechanical stress in aging neurons translates into genome instability and degeneration.

The review also describes a distinct non-apoptotic cell death pathway termed karyoptosis, uncovered through Drosophila models of proteotoxic stress. When autophagy, the cellular recycling system, is chronically impaired, as occurs in polyglutamine diseases such as Dentatorubral-pallidoluysian atrophy, in Alzheimer’s disease, in C9ORF72-associated amyotrophic lateral sclerosis and frontotemporal dementia, and in autolysosomal disorders such as Vici syndrome, neurons activate a regulated death program involving p38-mediated site-specific phosphorylation of LaminB. This phosphorylation causes the nuclear scaffold to break down, leading to cellular expulsion of nuclear material. Unlike apoptosis, which relies on caspase-driven dismantling, karyoptosis represents a fundamentally different mode of neuronal demise centered on nuclear envelope failure, and its causal chain from LaminB phosphorylation to execution has not yet been fully dissected in mammalian neurons in vivo.

Therapeutically, these findings suggest multiple points of intervention. Preserving the Pin1-dependent LaminB-heterochromatin axis could protect neurons from mechanical stress-induced genome instability. Promoting nuclear envelope repair through LEM-BAF-ESCRT-III complexes might seal ruptures before they trigger inflammatory cascades. Inhibiting retrotransposon activity, for example with reverse transcriptase inhibitors, could limit the DNA damage caused by mobile elements. Blocking pathological LaminB phosphorylation by p38 MAPK might prevent karyoptosis, while enhancing autophagy-lysosomal function could reduce the proteotoxic stress that drives nuclear breakdown in the first place. The review notes that similar LaminB-dependent mechanisms operate in non-neuronal cells as well; in cancer cells, for instance, tethering of the transcription factor CREB3 to the Lamin nucleoskeleton maintains nuclear envelope integrity, and its abnormal cleavage under endoplasmic reticulum stress triggers karyoptosis.

Important caveats remain. B-type Lamins are dispensable for the survival and growth of mouse embryonic stem cells, and mice can develop without them, albeit with significant defects in neuronal shape, nuclear integrity, migration, and cortical layering. Germline duplication of LaminB1 causes autosomal dominant leukodystrophy, an adult-onset demyelinating disease driven by oligodendrocyte dysfunction rather than neuronal death, underscoring that LaminB biology is context-dependent. Nevertheless, the convergence of evidence from flies, mice, and human genetics positions LaminB regulation as a central determinant of neuronal homeostasis. As the population ages and neurodegenerative diseases impose an ever-growing burden, understanding how the nuclear scaffold integrates stress signals and decides between adaptation and death may prove one of the most consequential frontiers in modern cell biology.

Subject of Research: LaminB-dependent nuclear homeostasis in neuronal aging and cell death

Article Title: At the nucleus of neuronal health: Drosophila insights into LaminB-dependent homeostasis in aging and cell death

Article References: Napoletano, F. (2026). At the nucleus of neuronal health: Drosophila insights into LaminB-dependent homeostasis in aging and cell death. Frontiers in Cell Death, 5, Article 1954383. https://doi.org/10.3389/fceld.2026.1954383

Image Credits: AI Generated

DOI: 10.3389/fceld.2026.1954383

Keywords: LaminB, Drosophila, nuclear envelope, neurodegeneration, karyoptosis, Pin1, heterochromatin, transposable elements, mechanical stress, autophagy, p38 MAPK, neuronal aging

The research team examined Hirundichthys affinis, the fourwing flyingfish, from two Atlantic populations, one at the São Pedro and São Paulo Archipelago, a remote set of islets in the central Atlantic, and another at Caiçara do Norte on the northeastern coast of Brazil. They also analyzed two species of the genus Cheilopogon, C. exsiliens, the bandwing flyingfish, and C. furcatus, the spotfin flyingfish, both sampled at the archipelago. Together, these species span a family of roughly 80 species distributed across the Pacific, Indian, and Atlantic Oceans, animals that serve simultaneously as an economically valuable fishery resource, as the basis of prized roe, and as a critical trophic link converting energy from phytoplankton into biomass consumed by marine mammals, seabirds, and commercially important predatory fish.

The cytogenetic toolkit deployed in the study combined classic and molecular techniques. The researchers prepared mitotic chromosomes from kidney cells, stained them with Giemsa, and applied C-banding to reveal heterochromatin, the tightly packed, largely repetitive DNA concentrated around centromeres. Silver nitrate staining, known as Ag-NOR, highlighted active ribosomal gene clusters, while fluorescent in situ hybridization, or FISH, allowed the team to physically map three classes of repetitive sequences onto the chromosomes: the 18S and 5S ribosomal DNA genes and a (CA)15 dinucleotide microsatellite probe, each tagged with differently colored fluorophores so that their locations could be visualized under an epifluorescence microscope.

The headline result is a study in contrast. All three species share the same diploid number of 48 chromosomes, a count considered ancestral across the Percomorpha, the huge lineage of spiny-rayed fishes to which flying fish belong. However, the architecture of those chromosomes diverges in telling ways. Hirundichthys affinis carries an exclusively acrocentric karyotype, meaning every chromosome has its centromere at one end, yielding a fundamental number of 48 chromosome arms. Cheilopogon exsiliens and C. furcatus each possess one pair of bi-armed chromosomes, subtelocentric in the former and metacentric in the latter, raising their fundamental numbers to 50. That single structural difference between the two Cheilopogon species likely arose through pericentric inversions, a recognized engine of karyotypic diversification in marine percomorph fishes, or conceivably through centromere repositioning, a mechanism still poorly understood in fish evolution.

The most striking findings concern the ribosomal DNA markers. In all species, the 18S rDNA and Ag-NOR signals occupied a single locus, but that locus sat on entirely different chromosomes in each species and, remarkably, in each population of H. affinis. Fish from the Brazilian coast carried their 18S rDNA on the long arm of the large acrocentric pair 2, while individuals from the mid-Atlantic archipelago carried it on the short arm of the much smaller pair 21. The 5S rDNA showed parallel divergence: one site on pair 22 in the Brazilian population versus two sites, on pairs 18 and 22, in the archipelago population. Such a magnitude of rDNA variation between populations is unusual in marine fishes and can only be explained by a complex sequence of chromosomal events, including transposition and translocation.

Because H. affinis already shows documented population structure across the central western Atlantic, with earlier studies distinguishing at least three unit stocks among the eastern Caribbean, the southern Netherlands Antilles, and northeastern Brazil, the cytogenetic differences between the two sampled populations point to a genuinely complex scenario of transatlantic chromosomal diversification. The authors argue that the pronounced divergence in ribosomal gene organization raises the possibility of cryptic speciation, the emergence of reproductively isolated lineages that remain morphologically indistinguishable. Confirming that possibility will require molecular taxonomic approaches, but the chromosomal data now provide a clear set of candidate markers for detecting such hidden diversity in a species with vast interoceanic ranges and enormous population sizes.

The study also mapped the distribution of heterochromatin and microsatellite repeats, adding further resolution to the species comparisons. Heterochromatin in all three species was concentrated in centromeric and pericentromeric regions, but each species and population carried diagnostic additional blocks: extra telomeric segments on specific chromosome pairs in the Brazilian H. affinis, an interstitial block on pair 3 in the archipelago population, combined centromeric and telomeric staining on the first pair of C. exsiliens, and a terminal block on the first pair of C. furcatus that coincides with the Ag-NOR site. The (CA)15 microsatellite repeats, by contrast, showed a predominantly diffuse distribution along the chromosome arms, occasionally clustering in pericentromeric regions, a pattern consistent with reports from many other fish species and apparently a common genomic feature of the group.

The broader evolutionary picture that emerges challenges expectations for a pelagic fish family. Marine biologists have often noted an inverse relationship between geographic range and karyotypic variation in oceanic species: animals capable of widespread dispersal, and therefore extensive gene flow, tend to resist the fixation of structural chromosomal rearrangements, whereas reef fishes with fragmented populations accumulate them readily. Flying fish, despite their transoceanic distributions, large population sizes, and conserved diploid number, display conspicuous divergences in rDNA organization and chromosome structure at the population, species, and genus levels. The genus Cheilopogon is polyphyletic, with C. exsiliens and C. furcatus belonging to separate evolutionary clades, which may account for the independent chromosomal rearrangements each lineage has undergone, including possible homeologous divergence of the Ag-NOR-bearing chromosome pair through loss or gain of a major ribosomal site.

The authors conclude that karyotypic evolution in flying fish is more dynamic than expected for a transoceanic pelagic group, and that cytogenetics offers a promising model for investigating speciation, reproductive isolation, and environmental adaptation in the epipelagic realm. Given the ecological centrality of flying fish to pelagic food webs and the commercial importance of their fisheries, the ability to detect cryptic lineages and stock boundaries through chromosomal markers carries practical implications for management and conservation. The study fills a long-standing gap in the cytogenetic record of Exocoetidae and positions these charismatic gliders as a valuable evolutionary model for one of the planet’s least understood habitats, where the interplay of gene flow, population structure, and chromosomal change continues to unfold across thousands of kilometers of open water.

Subject of Research: Cytogenomic and cytotaxonomic analysis of Atlantic flying fish species to investigate chromosomal evolution, population structure, and speciation in the epipelagic zone.

Article Title: Flying fish as useful model for evolutionary investigations in epipelagic zones. A cytogenomic and cytotaxonomic approach

Article References: Costa, G. W. W. F. D., Shibatta, O. A., Cioffi, M. D. B., Vidal, J. A. D., Bertollo, L. A. C., & Molina, W. F. (2026). Flying fish as useful model for evolutionary investigations in epipelagic zones. A cytogenomic and cytotaxonomic approach. Comparative Cytogenetics, 20, 141-155. https://doi.org/10.3897/compcytogen.20.190559

Image Credits: AI Generated

DOI: 10.3897/compcytogen.20.190559

Keywords: flying fish, Exocoetidae, cytogenetics, karyotype, rDNA, FISH, heterochromatin, microsatellites, speciation, epipelagic zone, population structure, Atlantic Ocean

The study, led by researchers at the Federal University of Rio Grande do Norte in northeastern Brazil, analyzed Coryphopterus glaucofraenum (Gill, 1863), a reef-dwelling species, and Evorthodus lyricus (Girard, 1858), a marine and estuarine species. Both range broadly along the western Atlantic, from the United States to southern Brazil, making them ideal candidates for a comparative analysis. Using classical cytogenetic techniques combined with base-specific fluorochrome staining, the team uncovered a level of karyotypic divergence between the two species that reinforces just how chromosomally dynamic the family Gobiidae really is.

Evorthodus lyricus exhibited a diploid number of 2n = 46, with a fundamental number (FN) of 92, corresponding to a karyotype composed of 22 metacentric and 24 submetacentric chromosomes. Coryphopterus glaucofraenum, by contrast, presented 2n = 40 with an FN of only 42, its karyotype comprising just one pair of submetacentric chromosomes and 38 acrocentric chromosomes. These are not subtle differences. The two species differ substantially in both chromosome number and chromosomal morphology, a divergence that places them at opposite ends of the karyotypic spectrum documented for Atlantic gobies.

The diploid number of 2n = 46 is considered the basal condition for the family Gobiidae, inherited from an ancestral karyotype of 46 acrocentric chromosomes. However, E. lyricus does not retain this ancestral morphology. Its extensive complement of two-armed chromosomes, reflected in the high fundamental number of 92, indicates that intense chromosomal rearrangements, likely pericentric inversions or centromere repositioning events, have reshaped the ancestral acrocentric karyotype into one dominated by metacentric and submetacentric elements. In other words, even a species retaining the basal diploid number has undergone substantial structural chromosomal evolution.

Coryphopterus glaucofraenum tells a different evolutionary story. Its diploid number of 2n = 40 falls below the basal condition, and the presence of acrocentric pairs that are disproportionately larger than the remaining chromosomes in the karyotype suggests that tandem fusion acted as the primary rearrangement mechanism. This pattern, in which chromosomes fuse end-to-end rather than at centromeres, has been documented in other gobiids, including Gobius paganellus, and appears to be a recurring theme in the karyotypic diversification of the family. The researchers note that conventional cytogenetic protocols lack the resolution needed to map precise structural breakpoints, leaving open questions about whether pericentric inversions or centromere repositioning, or tandem versus centric fusions, drove the observed patterns. High-resolution molecular cytogenetics will be needed to test these hypotheses directly.

Beyond chromosome number and morphology, the study examined heterochromatic regions using C-banding and identified the major ribosomal DNA sites using silver staining (Ag-NOR) followed by sequential staining with the fluorochromes CMA3 and DAPI. In both species, the Ag-NOR sites occupy terminal positions on the short arms of two-armed chromosome pairs, pair 2 in E. lyricus and pair 1 in C. glaucofraenum. These sites are heterochromatic and represent the only chromosomal regions showing a CMA3-positive, DAPI-negative staining pattern, indicating they are GC-rich. This condition, in which ribosomal sites are heterochromatic and GC-rich, is found in the nucleolar organizer pairs of various species within the family and constitutes a consistent cytotaxonomic marker for many fish groups.

The distribution of heterochromatin differed conspicuously between the two species. In E. lyricus, heterochromatic regions were prominent and terminally distributed on most chromosomes, while in C. glaucofraenum they were reduced and located primarily in centromeric regions. This heterogeneity matters because heterochromatic regions, rich in various types of repetitive sequences including 5S rDNA sites, have been suggested to facilitate chromosomal fusion events in marine Percomorpha species. Among Gobiidae, evidence of heterochromatic heterogeneity has been identified in several Mediterranean species, including Gobius paganellus, G. niger, G. cobitis and Zosterisessor ophiocephalus, as well as the Atlantic species Bathygobius soporator. This heterogeneity has been implicated in the occurrence and fixation of chromosomal modifications, including centric fusions and fissions, and has also been linked to the emergence of heteromorphic sex chromosomes in some gobiids.

Notably, the cytogenetic analyses revealed no sex-linked chromosome heteromorphisms between male and female specimens in either species. The researchers analyzed nine individuals of E. lyricus (four males, five females) and seven individuals of C. glaucofraenum (four males, three females), examining approximately thirty metaphase spreads per individual. While heterochromatin dynamics have been associated with sex chromosome emergence in other gobiid species, neither C. glaucofraenum nor E. lyricus shows any cytogenetic evidence of sex-specific chromosomal differences, suggesting that if sex chromosomes exist in these species, they are not yet morphologically differentiated.

The broader significance of these findings lies in what they reveal about the relationship between chromosomal change and speciation in Gobiidae. A karyotype of 2n = 46 acrocentric chromosomes is considered basal for the family, yet 84% of cytogenetically described gobiid species diverge from this ancestral pattern, with diploid numbers ranging from 2n = 30 in Neogobius euricephalus to 2n = 56 in Gobionellus microdon. This extraordinary numerical and structural variation suggests a close relationship between diversification processes and chromosomal modifications. Karyotype alterations have the potential to reduce recombination frequencies and produce chromosomally incompatible subpopulations, a mechanism that can drive speciation by creating reproductive barriers between groups with divergent karyotypes.

The majority of available cytogenetic data for gobies has historically come from studies on Mediterranean and Pacific species, with Atlantic representatives remaining understudied. By providing the first chromosomal characterization of two Atlantic goby species with broad geographic distributions, this study helps fill a significant gap and underscores the value of cytogenetic data for discovering new species, uncovering population structures, identifying sex chromosomes and pinpointing major shifts in repetitive DNA. As the field of goby cytogenetics continues to mature, each newly characterized species adds critical evidence to the emerging picture of a family whose evolutionary success has been accompanied, and perhaps enabled, by remarkable chromosomal flexibility.

Subject of Research: Karyotypic diversity in the Gobiidae : A comparative analysis of two Atlantic species

Article Title: Karyotypic diversity in the Gobiidae : A comparative analysis of two Atlantic species

Article References: de Lima-Filho, P. A., da Costa, G. W. W. F., da Silva, S. A. S., & Molina, W. F. (2026). Karyotypic diversity in the Gobiidae: A comparative analysis of two Atlantic species. Comparative Cytogenetics, 20, 187-194. https://doi.org/10.3897/compcytogen.20.206266

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DOI: 10.3897/compcytogen.20.206266

Keywords: Gobiidae, karyotype, cytogenetics, Atlantic gobies, chromosomal evolution, tandem fusion, Ag-NOR, heterochromatin, speciation, Comparative Cytogenetics, Karyotypic, diversity