More sex chromosomes than autosomes in the Amazonian frog Leptodactylus pentadactylus

Abstract

Heteromorphic sex chromosomes are common in eukaryotes and largely ubiquitous in birds and mammals. The largest number of multiple sex chromosomes in vertebrates known today is found in the monotreme platypus (Ornithorhynchus anatinus, 2n = 52) which exhibits precisely 10 sex chromosomes. Interestingly, fish, amphibians, and reptiles have sex determination mechanisms that do or do not involve morphologically differentiated sex chromosomes. Relatively few amphibian species carry heteromorphic sex chromosomes, and when present, they are frequently represented by only one pair, either XX:XY or ZZ:ZW types. Here, in contrast, with several evidences, from classical and molecular cytogenetic analyses, we found 12 sex chromosomes in a Brazilian population of the smoky jungle frog, designated as Leptodactylus pentadactylus Laurenti, 1768 (Leptodactylinae), which has a karyotype with 2n = 22 chromosomes. Males exhibited an astonishing stable ring-shaped meiotic chain composed of six X and six Y chromosomes. The number of sex chromosomes is larger than the number of autosomes found, and these data represent the largest number of multiple sex chromosomes ever found among vertebrate species. Additionally, sequence and karyotype variation data suggest that this species may represent a complex of species, in which the chromosomal rearrangements may possibly have played an important role in the evolution process.

Sex Chromosome Evolution and Genomic Divergence in the Fish Hoplias malabaricus (Characiformes, Erythrinidae)

“…In support of this view, in several another fish species, the sex determining regions might be very tiny (reviewed in Schartl et al, 2016 ), with the extreme case of fugu genome, Takifugu rubripes , where the Y-specific sex-determining gene differs from the homologous region on the X chromosome by a single non-synonymous substitution (Kamiya et al, 2012 ). Moreover, even in the platyfish, Xiphophorus maculatus , with genetically defined sex chromosomes, no visible differences between X and Y were evidenced after CGH (Traut and Winking, 2001 ), similarly to what had been occasionally observed in some other animals (Koubová et al, 2014 ; Altmanová et al, 2016 ; Green et al, 2016 ; Gazoni et al, 2018 ). In yet another case, however, CGH proved to be resolute even in sex chromosomes of a very young age (Montiel et al, 2017 ).…”

Meiotic analyses show adaptations to maintenance of fertility in X1Y1X2Y2X3Y3X4Y4X5Y5 system of amazon frog Leptodactylus pentadactylus (Laurenti, 1768)

“…The present results confirm that 2n = 22 is conserved in the Amazon frog L. pentadactylus. However, the number of meiotic ring components was found to vary intraspecifically, with ten chromosomes observed in the present study and 12 in a previous study of this species 34 . These cytotypes differ in relation to a large bivalent observed in our sample.…”

Patterns of Sex Chromosome Differentiation in Spiders: Insights from Comparative Genomic Hybridisation

“…In some organisms, differentiation of neo-Y or neo-W chromosomes starts early after their origin [82,[107][108][109][110][111]. Despite this, our data did not reveal any differentiation of Y chromosome of this species by the male-specific probe, which resembles the pattern found in neo-sex chromosomes in some other CGH-based studies [50,112,113]. Instead, the male probe differentiated a distal end of an acrocentric chromosome, which is involved in the formation of standard acrocentric bivalent during meiosis and the most likely explanation for this observation is the intraspecific variability in repetitive DNA content as discussed above in Section 4.1.…”