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sex chromosome; pseudoautosomal region; recombination; sex determination; African clawed frogs; Xenopus tropicalis
Abstract:
Sex chromosome divergence has been documented across phylogenetically diverse species, with amphibians typically having cytologically
nondiverged (“homomorphic”) sex chromosomes. With an aim of further characterizing sex chromosome divergence of an
amphibian, we used “RAD-tags” and Sanger sequencing to examine sex specificity and heterozygosity in theWestern clawed frog
Silurana tropicalis (also known as Xenopus tropicalis). Our findings based on approximately 20 million genotype calls and approximately
200 polymerase chain reaction-amplified regions across multiple male and female genomes failed to identify a substantially
sized genomic region with genotypic hallmarks of sex chromosome divergence, including in regions known to be tightly linked to the
sex-determining region.Wealso foundthat expression andmolecular evolutionof genes linked to the sex-determining region did not
differ substantially from genes in other parts of the genome. This suggests that the pseudoautosomal region, where recombination
occurs, comprises a large portion of the sex chromosomes of S. tropicalis. These resultsmay in part explainwhy African clawed frogs
have such a high incidence of polyploidization, shed light onwhy amphibians have a high rate of sex chromosome turnover, and raise
questions about why homomorphic sex chromosomes are so prevalent in amphibians.
