Hyla_genotypesGenotypes of parents and offspring for crosses used to infer sex linkage. File contains 6 descriptive columns plus 2 per marker, and one row per individual. Missing data value: empty cell. species: Species cross: Collective identifier for 2 parents and their offspring indiv: Unique identifier for an individual stage: Adult, juvenile, or tadpole sex: Sex, observed or inferred from microsatellites sexed_by: Sex established by phenotype for adults, microsatellites for tadpoles, dissection for juveniles [marker name].1: One of two alleles for a given marker. For microsatellites, number corresponds to fragment length. For PCR-RFLP markers, 0 or 1 for absence/presence of restriction site. For sequence markers, balearicus or siculus haplotype. [marker name].2: Second of two alleles for the marker.Bufo_genotypesGenotypes of parents and offspring for backcross used to infer sex linkage. Each file contains 6 descriptive columns plus 2 per marker, and one row per individual. Missing data value: empty cell. species: Species cross: Collective identifier for 2 parents and their offspring indiv: Unique identifier for an individual stage: Adult, juvenile, or tadpole sex: Sex, observed or inferred from microsatellites sexed_by: Sex established by phenotype for adults, microsatellites for tadpoles, dissection for juveniles [marker name].1: One of two alleles for a given marker. For microsatellites, number corresponds to fragment length. For PCR-RFLP markers, 0 or 1 for absence/presence of restriction site. For sequence markers, balearicus or siculus haplotype. [marker name].2: Second of two alleles for the marker.Rana_genotypesGenotypes of parents and offspring for crosses used to infer sex linkage. Each file contains 6 descriptive columns plus 2 per marker, and one row per individual. Missing data value: empty cell. species: Species cross: Collective identifier for 2 parents and their offspring indiv: Unique identifier for an individual stage: Adult, juvenile, or tadpole sex: Sex, observed or inferred from microsatellites sexed_by: Sex established by phenotype for adults, microsatellites for tadpoles, dissection for juveniles [marker name].1: One of two alleles for a given marker. For microsatellites, number corresponds to fragment length. For PCR-RFLP markers, 0 or 1 for absence/presence of restriction site. For sequence markers, balearicus or siculus haplotype. [marker name].2: Second of two alleles for the marker.Hyla arborea transcriptomeAdult male H. arborea transcriptome, from brain, eye, heart, liver, muscle, and testis. 2.2 Gbp raw reads assembled with Trinity. When multiple splice variants assembled, only longest retained. Scaffolded with SSPACE. Scaffolds corresponding to genes or gene fragments used in this study: CHD1: scaffold12406 ARL8A: scaffold8654 CHERP: scaffold390 CSDE1: scaffold1308 DMRT1: scaffold6154 DOCK8: scaffold31469 FRYL: scaffold80 KIAA0232: scaffold192 LOC100494802: scaffold13879 MAP1B: scaffold574 MTUS1: scaffold1029 NDRG2: scaffold355 RAD23B: scaffold2417 WDR1: scaffold1232H_arborea_transcriptome.fa

Comparative genomic studies are revealing that, in sharp contrast with the strong stability found in birds and mammals, sex determination mechanisms are surprisingly labile in cold-blooded vertebrates, with frequent transitions between different pairs of sex chromosomes. It was recently suggested that, in context of this high turnover, some chromosome pairs might be more likely than others to be co-opted as sex chromosomes. Empirical support, however, is still very limited. Here we show that sex-linked markers from three highly divergent groups of anurans map to Xenopus tropicalis scaffold 1, a large part of which is homologous to the avian sex chromosome. Accordingly, the bird sex determination gene DMRT1, known to play a key role in sex differentiation across many animal lineages, is sex-linked in all three groups. Our data provide strong support for the idea that some chromosome pairs are more likely than others to be co-opted as sex chromosomes, because they harbor key genes from the sex determination pathway.