EVIDENCE FOR 2 SUCCESSIVE PERICENTRIC INVERSIONS IN SEX LAMPBRUSH CHROMOSOMES OF RANA-RUGOSA (ANURA, RANIDAE)

The objective of this study was to clarify the course of inversions by which a ZW sex chromosome dimorphism has become established in Rana r ugosa, Fortunately, R. rugosa preserves three different forms of sex c hromosomes in the several isolated populations. In both males and fema les, the homomorphic sex chromosomes from Hiroshima were closely simil ar to Z, while those from Isehara were slightly different from the Z. Females from Hirosaki demonstrated heteromorphic sex chromosomes. In t his study, the configuration and pairing behavior of sex lampbrush chr omosomes were examined in the female offspring produced from a cross b etween 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, chiasma ta were exclusively formed between the distal regions of: the long arm s of one sex chromosome and the terminal regions of the short arms of the other. As a result, landmarks ar ranged in reverse order were obse rved in the achiasmatic regions of these chromosomes. For the sex lamp brush chromosomes from Isehara and Hirosaki. on the other hand, chiasm a formation was mainly confined to the lower half of the chromosomes c orresponding to the long arms, and the landmarks in the achiasmatic re gions of these chromosomes were disposed In the opposite direction to each other. These results seem to indicate that in the primitive sex c hromosomes 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.