The objective of this study was to clarify the course of inversions by which a ZW sex chromosome dimorphism has become established in Rana rugosa. Fortunately, R. rugosa preserves three different forms of sex chromosomes in the several isolated populations. In both males and females, the homomorphic sex chromosomes from Hiroshima were closely similar to Z, while those from Isehara were slightly different from the Z. Females from Hirosaki demonstrated heteromorphic sex chromesomes. In this study, the configuration and pairing behavior of sex lampbrush chromosomes were examined in the female offspring produced from a cross between a female from Hiroshima and a male from Isehara, as well as the female offspring of a female from Hirosaki and the male from Isehara. For the sex lampbrush chromosomes from Hiroshima and Isehara, chiasmata were exclusively formed between the distal regions of the long arms of one sex chromosome and the terminal regions of the short arms of the other. As a result, landmarks arranged in reverse order were observed in the achiasmatic regions of these chromosomes. For the sex lampbrush chromosomes from Isehara and Hirosaki, on the other hand, chiasma formation was mainly confined to the lower half of the chromosomes corresponding to the long arms, and the landmarks in the achiasmatic regions of these chromosomes were disposed in the opposite direction to each other. These results seem to indicate that in the primitive sex chromosomes of the Hiroshima type two pericentric inversions occurred, leading to the differentiation of the W chromosomes. This is the first report to substantiate the process of sex chromosome differentiation experimentally.
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