Reconstruction of female heterogamety from admixture of XX-XY and ZZ-ZW sex-chromosome systems within a frog species

Sex-determining mechanisms change repeatedly throughout evolution, and it is difficult to track this continual process. The Japanese soil-frog Glandirana rugosa is a remarkable evolutionary witness to the ongoing process of the evolution of sex-determining modes. The two geographic groups, designated XY and Neo-ZW, have homologous sex chromosomes, yet display opposite types of sex chromosomes, XX-XY and ZZ-ZW, respectively. These two groups are sympatric at the edges of their respective ranges in Central Japan. In this study, we discovered molecular evidence that the eastern part of the Neo-ZW group (Neo-ZW2 subgroup), which is found near the sympatric area, shares mitochondrial haplotypes with the XY group. By analysing single nucleotide polymorphism (SNP) loci, we have also discovered that the representative nuclear genome of the Neo-ZW2 subgroup shares allele clusters with both the XY group and another part of the Neo-ZW group (Neo-ZW1 subgroup), indicating a hybrid origin of the Neo-ZW2. Further analysis of sex-linked SNP loci revealed that the alleles on the W chromosomes of the Neo-ZW2 were derived mostly from X chromosomes, while alleles on the Z chromosomes originated from the Z chromosomes of the Neo-ZW1 subgroup and partly from the Y chromosomes of the XY group. Our study revealed that admixture of the two opposite sex-chromosome systems reconstructed a female heterogametic system by recycling the X chromosomes into new W chromosomes. This work offers an illustrative example of how de novo sex-chromosome systems can arise by recycling material from ancestral sex chromosomes.