Meiotic recombination is the process by which chromosomes swap segments during the formation of pollen and egg cells, creating the novel allele combinations that fuel evolution and agriculture alike. The number and position of these crossovers, or exchanges of genetic material between homologous chromosomes, are tightly controlled by chromosome architecture, a phenomenon called crossover interference, and by the choice of DNA repair pathway. Yet recombination is also famously plastic in response to the environment, and temperature is among the strongest modulators known in plants. Previous work had shown that in barley, temperature redistributes crossovers between proximal and distal chromosome regions, with similar though more moderate remodelling reported in wheat. In Arabidopsis itself, both warming and cooling can elevate crossover frequency. What remained unclear was how much of this response varies within a species, which genes are responsible, and how temperature connects mechanistically to the choice between the two biochemical routes that generate crossovers.

Those two routes are central to the new findings. In most eukaryotes, including plants, crossovers are produced by two pathways: the interference-sensitive ZMM, or class I, route, and the interference-insensitive MUS81-EME1, or class II, route. Class I crossovers are spaced apart along chromosomes by interference, which prevents exchanges from occurring too close together, while class II crossovers are largely free of such constraints. The researchers began by surveying nine genetically diverse Arabidopsis accessions crossed to lines carrying fluorescent seed reporters that allow crossover frequencies to be measured in two defined intervals on chromosome 3: a subtelomeric interval called 420, near the chromosome end, and a pericentromeric interval called 3.9, near the centromere. When the hybrids were grown at 12 degrees Celsius instead of the standard 21 degrees, most accessions showed a consistent signature: crossover frequency rose in the subtelomeric interval and fell in the pericentromeric one, indicating a redistribution of exchanges toward the chromosome arms.

The magnitude of the shift varied considerably from cross to cross, ranging from a 7 to 32 percent increase in the 420 interval and an 11 to 25 percent decrease in the 3.9 interval. Notably, the response did not correlate with the spring temperatures of the accessions’ native habitats or with their flowering time, consistent with the idea that most modern Arabidopsis accessions belong to a cosmopolitan lineage shaped by post-glacial expansion and human dispersal. Two accessions, however, stood out. Ler-0 and Se-0 showed no significant crossover changes at either interval when crossed to the reporter background, revealing that cold-induced recombination remodelling depends on genetic background and is not universal across the species. Intriguingly, when the researchers backcrossed the reporters into a mostly Ler genetic background, the subtelomeric cold response came back even stronger than in the reference Col-0 accession, roughly 2.5-fold greater, suggesting that Col and Ler achieve their contrasting behaviours through distinct underlying mechanisms.

To see the whole picture, the team turned to genome-wide crossover mapping. They generated large F2 populations from two hybrids, Col-420 × Per-1, which responds strongly to cold, and Col-420 × Ler, which does not, and genotyped hundreds of individual plants by whole-genome sequencing to locate crossovers as switches between parental SNP genotypes. In the responsive cross, cold produced a clear genome-wide redistribution: crossovers per individual rose on chromosome arms from 4.25 to 5.03 on average, while pericentromeric crossovers trended downward, and the landscape shifted visibly toward the distal ends of chromosomes. In the Col × Ler cross, by contrast, nothing changed significantly at any scale. Crucially, analysis of the coefficient of coincidence, a measure of interference, showed that interference patterns were essentially identical across temperatures and genotypes, extending over distances of up to roughly 8 megabases. Cold, in other words, was not weakening interference; it was changing which pathway produced the crossovers.

The genetic dissection that followed pinpointed the culprit. Quantitative trait locus mapping in the Col × Ler F2 populations, using subtelomeric crossover frequency as the trait, revealed a major locus on chromosome 4, dubbed rQTL4, whose effect intensified dramatically in the cold. At 21 degrees Celsius the locus explained 20.1 percent of the variance in crossover frequency; at 12 degrees it explained 48.6 percent, with the LOD score jumping from 8.48 to 23.14. A second locus on chromosome 1, likely corresponding to the known recombination modifier HEI10, showed only modest temperature modulation. The critical experiment came from F6 lines that differed exclusively at a 15.6-kilobase interval on chromosome 4 encompassing SNI1 and four neighbouring genes, fixed either for the Col or the Ler haplotype while the rest of the genome was identical. Cold increased subtelomeric recombination more than twofold more strongly in lines carrying the Ler version of the interval, and a significant genotype-by-temperature interaction confirmed that this small stretch of DNA governs the differential cold response.

SNI1, it turns out, is the Arabidopsis orthologue of NSE5, a component of the SMC5/6 complex, a conserved structural maintenance of chromosomes machine increasingly implicated in meiotic recombination and in chromosome architecture. The team tested the gene’s role directly using sni1-1, a hypomorphic allele that reduces but does not abolish SNI1 function. In Col inbreds, wild-type plants showed the canonical cold response, but sni1-1 mutants eliminated it entirely: heterozygotes measured 21.3 centimorgans in the 420 interval at 21 degrees and 22.2 at 12 degrees, while homozygotes measured 31.1 at both temperatures. Reducing SNI1 dosage, in other words, fixed the recombination landscape in a constitutively cold-like, subtelomere-biased state that no longer responded to temperature. Conversely, in the buffered Col × Ler hybrid, lowering SNI1 dosage unmasked and amplified cold responsiveness, with subtelomeric crossovers rising by up to nearly 6 centimorgans at 12 degrees in sni1-1 homozygotes. Genome-wide sequencing of sni1-1 populations confirmed the pattern, showing significant cold-driven increases in total, arm and subtelomeric crossover counts that were absent in wild-type hybrids.

The mechanistic link to pathway choice came from cytology in zip4 mutants, which are defective in the class I pathway and therefore reveal class II activity directly. In the Col background, zip4 chiasmata, the cytological manifestations of crossovers, nearly doubled at 12 degrees, rising from 1.15 to 2.04 per cell, demonstrating that cold promotes class II crossover formation. In zip4 sni1-1 double mutants, chiasmata were already elevated at 21 degrees, reaching levels comparable to cold-treated zip4, and did not increase further in the cold. The interpretation is elegant: SNI1 normally inhibits class II crossover formation at warm temperatures, and cold increases class II output primarily by relieving this SNI1-dependent constraint. In Ler, the cold-driven rise in zip4 chiasmata from 0.86 to 2.73 was likewise abolished by the sni1-1 mutation, while in hybrids, adequate SNI1 activity buffered the response and reduced dosage unmasked it. Inter-homologue polymorphism, the sequence differences between chromosomes from different parents, appears to dampen class II crossovers, which may explain why hybrid responses were consistently weaker than those of inbreds.

The study also probed chromosome architecture using super-resolution microscopy of the synaptonemal complex, the protein scaffold that zips homologous chromosomes together during meiosis. In Col, cold shortened total synaptonemal complex length from 217.2 to 172.1 micrometres, and sni1-1 mutants displayed a constitutively short complex even at 21 degrees, measuring 163.2 micrometres. Yet the relationship proved subtle rather than deterministic. Ler increased class II crossovers without detectable complex shortening, and Col × Ler hybrids shortened their complexes in the cold without any corresponding crossover response in the assayed intervals. The authors therefore interpret total complex length as a broad readout of SNI1/SMC5/6-dependent chromosome organization rather than a causal variable that directly dictates crossover outcome. Structural modelling added a final layer: the Ler and Se-0 alleles of SNI1 carry a unique I235V amino acid substitution at the interface with the partner protein ASAP1, plus four promoter variants that raise transcript levels, a compound haplotype that may explain why these accessions buffer the cold response despite expressing more SNI1 messenger RNA.

The implications stretch well beyond Arabidopsis. Because cold shifts crossovers toward chromosome arms without disrupting interference, modulating SNI1 dosage or allele state could allow breeders to bias genetic shuffling toward gene-rich distal regions under mild cold, avoiding the recombination-poor pericentromeric drag that limits the recovery of novel combinations. The allele-by-temperature interaction uncovered here argues for haplotype-aware breeding strategies in which the choice of SNI1 variant and growth regime are deployed together predictably across germplasm. More broadly, the work provides a concrete molecular mechanism for a long-standing evolutionary idea: that environmentally modulated recombination can accelerate adaptation by rebalancing the number and placement of crossovers, thereby altering the supply of selectable variation. As climate change shifts temperature regimes across growing seasons, a cold-sensitive brake on the class II pathway may prove to be one of the levers by which plants, and potentially the crops we depend on, tune their own genetic futures.

Subject of Research: Temperature-dependent control of meiotic crossover pathway choice by the SMC5/6 component SNI1 in Arabidopsis

Article Title: SNI1/NSE5 is a cold-sensitive brake on class II crossovers in Arabidopsis

Article References: SNI1/NSE5 is a cold-sensitive brake on class II crossovers in Arabidopsis. (n.d.). https://doi.org/10.1038/s41477-026-02400-x

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02400-x

Keywords: meiotic recombination, crossovers, SNI1, SMC5/6 complex, Arabidopsis thaliana, temperature response, class II crossovers, synaptonemal complex, QTL mapping, subtelomeric recombination, plant genetics, climate adaptation

The study, published in Advanced Biotechnology, rested on an ambitious sequencing effort. The researchers generated more than fifty-fold short-read whole-genome sequencing coverage for each of 106 individuals from a two-generation family: two parents and 104 F1 offspring. Rather than aligning these reads to an existing reference genome, which would have failed because of the low sequence similarity between the laboratory animals and the published assembly, the team used Platanus-allee, a haplotype-aware assembler designed for highly heterozygous regions, to assemble each parent’s genome de novo. In these assemblies, heterozygous regions appear as paired ‘bubble’ contigs, with a longer primary sequence and a shorter secondary sequence representing the two alleles. Homozygous regions, by contrast, collapse into single non-bubble contigs. Merging the two parental assemblies produced a custom reference to which offspring reads could be mapped accurately, because offspring inherit their DNA directly from these parents.

The logic of the recombination detection is elegant. Each bubble contig pair acts as a biallelic marker: an offspring can inherit either the primary or the secondary contig from a given parent, but not both. Using the R package hapi, the researchers phased these markers and reconstructed chromosome-level haplotypes for each parent. Any switch in the parental haplotype along an offspring’s chromosome signals a crossover (CO) event that occurred during that parent’s meiosis. Because bubble contigs can themselves contain internal crossovers, the team also searched for breakpoints within contigs, where reads from an offspring align to complementary segments of both the primary and secondary contigs, generating four distinct haplotypes in the offspring population. Benchmarking with simulated meioses across a gradient of heterozygosity levels and family sizes showed that when parental heterozygosity exceeded 0.01 and more than 20 offspring were sequenced, the strategy recovered roughly 95 percent of simulated crossovers, with most misses confined to chromosome termini lacking marker coverage.

The results paint a vivid picture of amphioxus meiosis. Across all offspring, the researchers identified 2,329 paternal and 2,288 maternal crossovers, averaging 22.4 and 22.0 events per meiosis respectively. After adjusting for approximately 9 megabases of uncallable chromosome-end regions, this translates into recombination rates of 4.66 centimorgans per megabase in the father and 4.57 cM/Mb in the mother, substantially higher than the roughly 0.5 cM/Mb typical of mammals and the approximately 2 cM/Mb seen in birds. The authors note, however, that this elevated rate per unit of DNA partly reflects the compact size of the amphioxus genome: averaged per haploid chromosome, the count of about 1.2 crossovers per meiosis is close to that of vertebrates. Strikingly, unlike in humans and several other vertebrates where one sex shows markedly higher recombination, no significant sex bias emerged in either the quantity or the chromosomal distribution of crossovers, with paternal and maternal distributions correlating strongly across the genome.

One of the most intriguing findings concerns recombination coldspots. The paternal genome contained roughly 209 megabases of regions spanning at least five megabases with no crossovers in any offspring, while the maternal genome harbored about 142 megabases. These coldspots could not be explained by marker sparsity, since many showed dense marker coverage while crossovers were detected in marker-poor regions elsewhere. Instead, the evidence points to large-scale heterozygous inversions. Amphioxus genomes are known to carry structural variation rates approximately thirty times higher than those observed between humans and chimpanzees, and the team identified four large heterozygous inversions in the parental genomes, all situated within coldspots. In one maternal example on chromosome 3, one haplotype carried an inversion spanning roughly 1.4 to 13.5 megabases, likely preventing crossovers by disrupting pairing and recombination repair in that region. Because inversion polymorphisms appear common in wild amphioxus populations, individual genomes may differ substantially in where recombination is suppressed, which may explain why the pedigree-based crossover map correlated only weakly with historical recombination rates inferred from population genomic variation.

The analysis also probed crossover interference, the phenomenon by which one crossover reduces the likelihood of another nearby. About a quarter of chromosomes in the offspring carried two or more crossovers in both parental genomes. After excluding coldspots and comparing observed inter-crossover distances with a shuffled null distribution, paternal crossovers showed no evidence of interference, while maternal crossovers exhibited significant negative interference, meaning one crossover actually increased the chance of another nearby (Wilcoxon signed-rank test, p = 7.07 x 10^-8). Negative interference remains rare among studied species and its molecular basis is largely unknown, making this maternal effect in amphioxus a curious exception that may shed light on how interference mechanisms evolve across chordates. The researchers also detected crossovers near the sex-determining region on the fused chromosome 16-20 in the paternal genome but not the maternal one, consistent with recombination suppression in the heterogametic ZW sex of this species.

Examining genomic features, the team found that crossover density correlated positively with transposable element density in both parents, with Pearson correlations of 0.43 in the father and 0.40 in the mother, and all major TE classes, including SINEs, LINEs, LTRs, MITEs and DNA transposons, individually showed significant associations. This positive relationship contrasts with the negative correlation often reported in other organisms, underscoring the complex and lineage-dependent interplay between transposable elements and recombination. Gene density also correlated with crossover rates, though more weakly, plausibly reflecting the more open chromatin configuration of gene-rich regions that facilitates binding of recombination proteins. GC content showed no significant correlation with paternal crossovers and only a weak negative correlation with maternal ones, suggesting that amphioxus recombination is not strongly shaped by base composition, unlike in many mammals where PRDM9-directed hotspots often coincide with GC-rich sequence.

Beyond crossovers, the high heterozygosity of amphioxus enabled an unusually sensitive search for non-crossover (NCO) gene conversion events, in which a short segment of one homolog is copied onto the other without a reciprocal exchange. Within analyzable bubble regions, the researchers identified 9,992 paternal and 5,850 maternal NCO events, averaging 96.1 and 56.3 per offspring. Most tracts were short, with 90.9 percent under 200 base pairs and an average length of 99 base pairs, and 83.6 percent covered only a single converted marker. Extrapolated genome-wide, the NCO-to-CO ratios of 28.8 in males and 20.5 in females far exceed those reported for humans and zebra finch, although the authors caution that the amphioxus figures may be inflated by the exceptional marker density and are likely underestimates overall since NCOs in marker-free regions cannot be detected. The conversion spectrum revealed GC-biased gene conversion, favoring C/G over A/T alleles, in most event classes, particularly in the paternal genome, indicating that this force, well documented in mammals, also operates in the basal chordate lineage.

The broader significance of this work is methodological as much as biological. Previous direct detections of recombination without gamete sequencing typically required pedigrees spanning three or more generations, and long-read sequencing approaches can cost more than ten times as much per unit of data. By exploiting bubble contigs as natural markers, the new pipeline achieves contig-level parental haplotyping from just two generations and affordable short-read data, offering a template for studying recombination in the many non-model organisms burdened with high heterozygosity. The findings illuminate both conserved features, such as short NCO tracts and GC-biased gene conversion, and divergent ones, including the absence of sex-biased crossover rates and the prevalence of inversion-driven coldspots, in a species that anchors our understanding of vertebrate origins. As the authors acknowledge, results from a single family cannot capture population-level diversity, and regions of lower heterozygosity remain under-sampled. Nevertheless, by transforming amphioxus’s genomic eccentricity into a high-resolution lens on meiosis, the study opens the door to recombination mapping across a wide swath of the living world that has until now remained largely invisible to geneticists.

Subject of Research: Direct genome-wide detection of meiotic crossover and non-crossover recombination events in the highly heterozygous amphioxus Branchiostoma floridae

Article Title: Direct detection of meiotic recombination events in the highly heterozygous amphioxus genome

Article References: Tao, L., Xue, J., Cao, J., Li, G., & Li, C. (2025). Direct detection of meiotic recombination events in the highly heterozygous amphioxus genome. Advanced Biotechnology, 3(4), Article 30. https://doi.org/10.1007/s44307-025-00083-7

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00083-7

Keywords: amphioxus, meiotic recombination, crossover, non-crossover gene conversion, heterozygosity, Branchiostoma floridae, haplotype phasing, genome assembly, heterozygous inversions, GC-biased gene conversion, transposable elements, chordate evolution

The challenge that the team set out to overcome was technical rather than conceptual. For decades, scientists have known that G4-forming sequences exist throughout the genome, but the most repetitive parts of human DNA, such as centromeres and the long arrays of ribosomal DNA, resisted assembly with standard short-read sequencing technologies. These regions are composed of long, near-identical repeats that short reads cannot be uniquely placed, leaving gaps in even the most celebrated reference assemblies. The recent arrival of long-read sequencing and the resulting gap-less, telomere-to-telomere human genome assemblies changed the picture entirely. For the first time, researchers could scan the entirety of a complete human genome, from end to end, for sequences with the potential to form G-quadruplexes, and then compare those findings across many individuals.

In the new study, the researchers systematically identified potential G4-forming sequences in the gap-less reference human genome assembly and extended their analysis to 88 haplotypes, or individual genome copies, drawn from people of diverse ancestries. This breadth mattered. By looking across many genomes rather than a single reference, the team could ask not only where G4s sit in the genome but also how variable they are between people, a direct window into their evolutionary and mutational behavior. The scale of the analysis allowed them to move beyond anecdotal observations and build a population-level picture of G4 landscape and instability.

One of the most striking findings concerns where G4s concentrate. The team reports that G4-forming sequences are highly enriched in specific repetitive regions of the genome, including certain centromeric and pericentromeric repeat types, the specialized DNA that surrounds and structures the centromere, the point where chromosomes attach to the segregation machinery during cell division. They also found marked enrichment in ribosomal DNA arrays, the tandem repeats that encode the RNA components of ribosomes, the molecular machines that build proteins in every cell. These are precisely the regions that had been invisible to earlier G4 surveys, which means that a substantial fraction of the human G4 landscape had effectively been missing from the scientific record until now.

Importantly, the researchers did not rely on sequence patterns alone. Computational predictions of G4 formation are useful, but they can be misleading, because not every guanine-rich motif actually folds into a quadruplex under physiological conditions. To address this, the team experimentally validated the most prevalent G4 structures they detected, confirming that the sequences identified in these repetitive regions can indeed form the characteristic four-stranded architecture. This biophysical validation anchors the computational survey in laboratory reality and strengthens the case that the newly mapped G4s are genuine structural features of human DNA rather than artifacts of pattern matching.

The study also examined the chemical modification landscape around G4s, with intriguing results. DNA methylation, the addition of a methyl group to cytosine bases, is a key epigenetic mark that influences whether genes are active or silent. The researchers found that G4-forming regions tend to show lower methylation than would be expected by chance. This observation fits with a growing body of evidence that alternative DNA structures can influence the epigenetic state of the sequences that carry them, potentially affecting how nearby genes are regulated. While the study establishes the correlation across the complete genome, it also raises new questions about cause and effect: whether G4 formation actively discourages methylation, or whether unmethylated regions are simply more permissive to quadruplex folding, remains an open avenue for future work.

Perhaps the most consequential finding relates to genome stability. By comparing G4 loci across the 88 haplotypes, the researchers could measure how often these sequences change from one generation to the next. The answer was unambiguous: G4-forming regions are genomically unstable, showing an excess of insertions and deletions relative to the surrounding genome. This makes mechanistic sense. A four-stranded structure folded out of one DNA strand creates an awkward substrate for the enzymes that copy and repair DNA, and stalled replication forks at stable secondary structures are a classic source of small insertions and deletions. The new data suggest that this mutagenic tendency is a genome-wide property of G4s, measurable at the population scale, and that these structures act as ongoing engines of genetic variation in repetitive DNA.

The team then connected G4s to one of the most dramatic events in the life of a genome: meiosis, the specialized cell division that produces sperm and eggs. During meiosis, programmed double-strand breaks are deliberately introduced at thousands of hotspots throughout the genome to initiate recombination, the shuffling of genetic material between chromosomes. Which sites are cut is largely determined by PRDM9, a protein whose DNA-binding domain varies between individuals and whose genotype therefore reshapes the recombination landscape from person to person. Remarkably, the researchers found that G4-forming motifs are consistently enriched at meiotic double-strand break hotspots across individuals with different PRDM9 genotypes. This consistency suggests that the physical propensity of DNA to fold into quadruplexes may contribute to the targeting or processing of meiotic breaks in a way that transcends the specific PRDM9 variant an individual carries, hinting at a structural layer of regulation overlaid on the well-known genetic one.

Taken together, the findings reframe G-quadruplexes not as curiosities of guanine-rich sequence but as dynamic, widespread, and functionally meaningful elements of the human genome. Their enrichment in centromeric and ribosomal DNA places them at the heart of chromosome architecture and protein synthesis; their reduced methylation ties them to epigenetic regulation; their excess of insertions and deletions marks them as drivers of mutation; and their presence at recombination hotspots links them to the machinery of heredity itself. Each of these threads also points toward disease. Genome instability at repetitive regions is a recurring theme in cancer and in a range of neurological and developmental disorders, and a complete map of where G4s form, and where they mutate, gives researchers a new tool for interpreting disease-associated variation in parts of the genome that were previously unreadable.

The study also illustrates how technological progress reshapes entire fields. Just as the telomere-to-telomere assemblies revealed previously hidden genes and structural variants, they have now revealed a hidden structural genome, one written not in the order of bases alone but in the capacity of those bases to fold. With 88 diverse haplotypes already surveyed, the stage is set for broader population studies that could trace how G4 landscapes differ across ancestries, how they evolve under selection, and how their instability contributes to both human evolution and human disease. For a structure first noticed as an oddity of guanine chemistry, the G-quadruplex has come a long way, and the complete human genome, it turns out, is full of it.

Subject of Research: Genomic landscape and mutational dynamics of G-quadruplex DNA structures in the complete human genome

Article Title: Landscape and mutational dynamics of G-quadruplexes across the complete human genome

Article References: Landscape and mutational dynamics of G-quadruplexes across the complete human genome. (n.d.). https://doi.org/10.1186/s13059-026-04255-z

Image Credits: AI Generated

DOI: 10.1186/s13059-026-04255-z

Keywords: G-quadruplexes, human genome, telomere-to-telomere assembly, centromeres, ribosomal DNA, genomic instability, DNA methylation, meiotic recombination, PRDM9, long-read sequencing, insertions and deletions, genome biology