SEX-CHROMOSOME POLYMORPHISM AND HETEROGAMETIC MALES REVEALED BY 2 CLONED DNA PROBES IN THE ZW ZZ FISH LEPORINUS-ELONGATUS

In order to study the divergence of teleost sex chromosomes, subtracti ve cloning was carried out between genomic DNA of males and females of the rainbow trout (XX/XY) and of Leporinus elongatus (ZW/ZZ). Inserts cloned in a plasmid vector were individually tested on Southern blots of DNA of males and females for sex specificity. No sex-specific inse rt was obtained from trout, but two out of ten inserts cloned from L. elongatus showed sex-specific patterns in this species: one correspond s to a sequence present on both Z and W chromosomes, while the other i s W specific. Sequences of these two inserts show neither clear homolo gy with other known sequences, nor an open reading frame. They cross-h ybridize with the genomic DNA of Leporinus friderici, but without sex- specific patterns. Twenty-four L. elongatus adults were sexed by gonad al observation, chromosomed examination and Southern hybridization wit h one or the other insert. Ten males and 11 females had chromosomes an d hybridization patterns typical of their sex. One ZW female was recog nized as a male with the W-specific probe. This was also the case for two unusual ZW males, one having a male hybridization pattern with the other probe. These three atypical individuals may result from single genetic exchanges between four regions of the Z and the W, giving rise to three atypical W chromosomes. Finding males with such atypical het erochromosomes in a female heterogametic species may indicate that a g radual transition occurs between the heterogametic systems.