Extraordinary Diversity in the Origins of Sex Chromosomes in Anurans Inferred from Comparative Gene Mapping

Abstract

Sex determination in frogs (anurans) is genetic and includes both male and female heterogamety. However, the origins of the sex chromosomes and their differentiation processes are poorly known. To investigate diversity in the origins of anuran sex chromosomes, we compared the chromosomal locations of sex-linked genes in 4 species: the African clawed frog (Xenopus laevis), the Western clawed frog (Silurana/X. tropicalis), the Japanese bell-ring frog (Buergeria buergeri), and the Japanese wrinkled frog (Rana rugosa). Comparative mapping data revealed that the sex chromosomes of X. laevis, X. tropicalis and R. rugosa are different chromosome pairs; however, the sex chromosomes of X. tropicalis and B. buergeri are homologous, although this may represent distinct evolutionary origins. We also examined the status of sex chromosomal differentiation in B. buergeri, which possesses heteromorphic ZW sex chromosomes, using comparative genomic hybridization and chromosome painting with DNA probes from the microdissected W chromosome. At least 3 rearrangement events have occurred in the proto-W chromosome: deletion of the nucleolus organizer region and a paracentric inversion followed by amplification of non-W-specific repetitive sequences.

Sequential Turnovers of Sex Chromosomes in African Clawed Frogs (Xenopus) Suggest Some Genomic Regions Are Good at Sex Determination

“…In both species, SOX3 and AR are located on the sex chromosomes (Fujii et al 2014; Uno et al 2015). Interestingly, this inference extends even farther: orthologs of AR , SOX3 , and FMR1 are also present on the X chromosome of therian mammals, including humans (Uno et al 2013), and SOX3 is a new trigger for sex determination in a fish ( O. dancena ; Takehana et al 2014).…”

Sex Determination and Sex Chromosomes in Amphibia

“…data]. Further, Cyp17 is a candidate female-determining gene on sex chromosome 9 of B. buergeri as it is located in the inverted region of the W chromosome, and the divergence of nucleotide and amino acid sequences is evident between Cyp17 of Z and W chromosomes [Uno et al, 2015]. In the latter 2 species, the 2 candidate female-determining genes are primarily involved in testis differentiation and thus allow us to speculate that the genes on the W chromosomes would inhibit male-determining functions of homologous genes on the Z chromosomes to determine the female sex ( Table 1 ).…”

Did sex chromosome turnover promote divergence of the major mammal groups?

“…In sticklebacks, too, novel sex genes have rapidly arisen 18, 19, and in cichlid fish of Lake Victoria, rampant speciation is often accompanied by sex chromosome turnover 20. In frogs, too, different sex determining genes and sex chromosome systems have arisen, and one species is polymorphic for XY and ZW systems 21, 22, 23, 24.…”