Gourami clade shows high chromosome-number variability

A cytogenetic study of seven species in the spiral egg clade of the gourami family found unusually low and variable diploid chromosome numbers, ranging from 2n = 16 in the chocolate gourami, Sphaerichthys osphromenoides, to values of 34 or fewer across the genera examined. The researchers reported first karyotype descriptions for five species and confirmed the previously reported karyotypes of two others.

The study focused on the spiral egg clade, a monophyletic group within Osphronemidae that includes Sphaerichthys, Luciocephalus, Parasphaerichthys and Ctenops. The clade was initially proposed because of the projections arranged in a spiral pattern on the eggs of its members, a feature later supported and refined by molecular evidence. Its members also have an angular jaw shape, most pronounced in the pike-like morphology of the piscivorous Luciocephalus.

Most osphronemids have a comparatively stable diploid chromosome number of 2n = 46–48. By contrast, the species sampled in this study had low counts and substantial variation within and among genera. Teleost fishes collectively show unusually wide karyotype diversity, but the researchers noted that most species in Osphronemidae remain unstudied cytogenetically, leaving the broader distribution of chromosomal variation in the family uncertain.

All specimens were obtained from the aquarium trade and maintained in species-specific tanks at Reed College under institutional animal-care approval. The researchers prepared chromosomes from gill arches using a modified Kligerman and Bloom method. Specimens were incubated in 0.005% colchicine for 6–7 hours, and gill arches were treated with 0.4% KCl for 20–30 minutes, fixed in ethanol-acetic acid fixative and stained with 10% Giemsa in pH 6.8 phosphate buffer. At least 35 complete metaphase spreads were photographed from each specimen.

Chromosomes were classified as metacentric, submetacentric, subtelocentric or acrocentric according to their arm ratios. The fundamental number, or FN, counted metacentric and submetacentric chromosomes as biarmed and subtelocentric and acrocentric chromosomes as uniarmed. This distinction allowed the researchers to compare not only the number of chromosomes but also the number of chromosome arms in each karyotype.

All four sampled Sphaerichthys species had different diploid numbers, from 2n = 16 to 28, while their FN values varied less, from 28 to 38. The sister species S. osphromenoides and S. selatanensis had nearly identical FN values despite differing by 12 in diploid chromosome number: S. osphromenoides had a primarily biarmed karyotype, whereas S. selatanensis had a primarily uniarmed karyotype. The other sister pair, S. acrostoma and S. vaillanti, had mixed biarmed and uniarmed chromosomes and differed in both 2n and FN.

The two sampled S. selatanensis individuals also differed from one another. One had 28 uniarmed chromosomes, described as 2n = 28 and 2st+26a, while the other had 2n = 27 with a single unpaired metacentric chromosome, 1m+2st+24a. That metacentric chromosome was approximately twice the size of the largest acrocentric chromosome and may have arisen from a fused acrocentric pair. Gross examination of external features and gonads suggested that both individuals were male, but the authors cautioned that histological analysis would be needed to assess a possible sex-biased chromosomal arrangement.

In Luciocephalus, L. pulcher had an entirely uniarmed karyotype consisting of 20 acrocentric chromosomes. Its congener, L. aura, had a primarily biarmed karyotype with a lower 2n but a higher FN. Parasphaerichthys ocellatus had higher 2n and FN values than any of the sampled Sphaerichthys or Luciocephalus species, although its counts remained below the value reported for Ctenops nobilis in the literature. Overall, Sphaerichthys, Luciocephalus and Parasphaerichthys showed low diploid numbers, 2n ≤ 34, and high within-genus variation in both 2n and FN.

The authors associated the large reduction in chromosome number with predominantly fusion rearrangements, but the current data could not conclusively establish the mechanisms or driving forces behind the clade’s karyotype differentiation. Molecular dating places the divergence of the spiral egg clade at about 25 million years ago, and the authors suggest that the highly variable chromosome evolution began after that divergence. They recommend histological analysis of gonads for more accurate sex determination and further karyotyping of Osphronemidae species to determine whether other highly differentiated clades occur within the family.

Subject of Research: Chromosome-number variation in the spiral egg clade of gourami fishes

Article Title: High variability in chromosome number from a clade within the gourami family ( Teleostei , Osphronemidae )

Article References: Mobley, B., & Anderson, A. P. (2025). High variability in chromosome number from a clade within the gourami family (Teleostei, Osphronemidae). Comparative Cytogenetics, 19, 19-28. https://doi.org/10.3897/compcytogen.19.132971

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.132971

Keywords: cytogenetics, gourami fishes, Osphronemidae, chromosome number, karyotype evolution, Sphaerichthys, Luciocephalus, Parasphaerichthys, teleost fishes, High, variability, chromosome