Shared Origin of Y and Z Chromosomes in the Turnover of XY and ZW Systems in the Frog Glandirana rugosa.

The Japanese frog Glandirana rugosa, endemic to Japan, exhibits both XY and ZW sex determination systems in different populations, representing a rare example of sex chromosome turnover within a single species. To explore the genetic basis of this phenomenon, we analyzed the X, Y, Z, and W chromosomes using microdissection followed by next-generation sequencing. All chromosomes originated from chromosome 7, and the sex chromosomal sequences were homologous. Comparative analyses revealed a high degree of sequence similarity between the Y and Z chromosomes. This suggests that the Y and Z chromosomes may have originated from the same ancestral chromosome and remained highly homologous at the genomic sequence level. This relationship supports the idea that transitions between the XY and ZW systems can occur through the reuse of homologous chromosomes. Our findings indicate that G. rugosa provides an informative model for studying the early stages of sex chromosome differentiation and turnover. Understanding these processes in G. rugosa may enhance our understanding of the evolutionary dynamics of sex chromosomes across vertebrates.