The origin and differentiation of the heteromorphic sex chromosomes Z, W, X, and Y in the frog Rana rugosa, inferred from the sequences of a sex-linked gene, ADP/ATP translocase

Sex chromosomes of the Japanese frog Rana rugosa are heteromorphic in the m ale (XX/XY) or in the female (ZZ/ZW) in two geographic forms, whereas they are still homomorphic in both sexes in two other forms (Hiroshima and Iseha ra types). To make clear the origin and differentiation mechanisms of the h eteromorphic sex chromosomes, we isolated a sex-linked gene, ADP/ATP transl ocase, and constructed a phylogenetic tree of the genes derived from the se x chromosomes. The toe shows that the Hiroshima gene diverges first, and th e rest form two clusters: one includes the Y and Z genes and the other incl udes the X, W, and Isehara genes. The Hiroshima gene shares more sequence s imilarity with the Y and Z genes than with the X, W, and Isehara genes. Thi s suggests that the Y and Z sex chromosomes originate from the Hiroshima ty pe, whereas the X and W chromosomes originate from the Isehara-type sex chr omosome. Thus, we infer that hybridization between two ancestral forms, wit h the Hiroshima-type sex chromosome in one and the Isehara-type sex chromos ome in the other, was the primary event causing differentiation of the hete romorphic sex chromosomes.