Sex chromosome conservation, DMRT1 phylogeny and gonad morphology in diploid Palearctic green toads (Bufo viridis...

Due to the prevailing sex chromosome homomorphy and large genome size, the knowledge on sex determination systems, sex chromosomes and sex-determining genes in amphibians remains scarce. Using 3 cross-amplifying sex-linked microsatellite markers, we uncover sex determination systems and sex chromosomes in purebred, diploid Palearctic green toads (Bufo viridis subgroup), which had so far only been characterized in laboratory-bred hybrids. Our data support an XY system in B. balearicus, B. viridis and B. variabilis. While females show recombination, it is strongly suppressed (or not detectable) in males. Markers corroborate the largest chromosome pair 1 (homologous to linkage group 1 of Xenopus tropicalis) to represent the sex chromosomes in diploid species of the B. viridis subgroup (B. siculus, B. shaartusiensis, B. balearicus, B. turanensis, B. variabilis, B. viridis, and probably B. boulengeri). This chromosome harbors DMRT1, a key gene of the sexual pathway in deeply divergent animal taxa. However, our phylogenetic analysis of a 600-bp fragment of that gene in diploid green toad taxa reveals that X and Y alleles cluster by species and not by gametolog. This suggests that XY-sequence similarity stems from occasional XY recombination within DMRT1, and we preliminarily reject its role as the master sex determination gene, pending future extension of this evidence to the entire DMRT1 gene. We further create a chain of evidence, which supports the hypothesis that linkage group 1 of X. tropicalis appears to be maintained as the largest chromosome (1), and thus is homologous in anuran karyotype evolution from pipid to hylid, bufonid and ranid anurans.