Structural differences between XX and ZW sex lampbrush chromosomes in Rana rugosa females (Anura: Ranidae)

In this study we investigated the morphology and pairing behavior of sex lampbrush chromosomes of XX and ZW females of Rana rugosa from five localities in Japan. Whereas lampbrush chromosomes of XX females from Hiroshima and Isehara had subterminally located centromeres and showed remarkable similarity, those of XX females from Hamakita had the centromeres in the middle. Analysis of landmark configurations revealed that chromosome Xq of Hamakita females closely resembled a part of Xq of Hiroshima and Isehara females, whereas Xp of Hamakita females was inverted compared with the other part of Xq of Hiroshima and Isehara females. Z chromosomes from Kanazawa and Niigata closely resembled the Hiroshima X, whereas the W closely resembled the Hamakita X. XX pairings from Hiroshima, Isehara, and Hamakita were found to be joined by one to four chiasmata at various points all along the axis in both the short and long arms, whereas chromosomal pairs from Kanazawa and Niigata showed only one chiasma between Zp and the distal region of Wq. From these findings we conclude that (1) both the W and the Hamakita X must have evolved from the more primitive Hiroshima and Isehara X chromosomes by a series of pericentric inversions; and (2) females distributed in Hamakita possess two X chromosomes similar to the W, suggesting that either sex-determining or sex-modifying genes on the Hamakita X are clearly different from those on the Kanazawa and Niigata W chromosome.