SEX DETERMINATION STUDIES IN 2 SPECIES OF TELEOST FISH, OREOCHROMIS-NILOTICUS AND LEPORINUS-ELONGATUS

Genetic analyses of sex determination have identified sex chromosomes in many teleost fish species. However, there are several cases for whi ch sex ratios do not fit perfectly with the expectations of heterogame tic systems, suggesting the influence of either minor sex determining genes or environmental influences on the process of sex differentiatio n. The frequent absence of sex chromosome markers makes the identifica tion of minor sex-determining genes very difficult. It is easier to te st first the hypothesis of environmental sex determination (ESD) by st udying the temperature effect, since temperature-dependent sex determi nation has been demonstrated to occur in several vertebrate groups inc luding 1 fish species. To contribute to a better understanding of fish sex determination, we have tested the effects of high temperatures on sex ratios of Oreochromis niloticus, and have attempted to isolate se x chromosome molecular markers in Leporinus elongatus. Treatments of O . niloticus fry at 36 degrees C applied for 10 days and more, and star ting 1 week after fertilization markedly increased the proportion of m ales, and progeny-testing these males confirmed that some of them are sex-reversed genetic females. Two non-coding sequences of L. elongatus Z and W chromosomes were cloned by genomic subtraction. They cross-hy bridized with the genome of a close species without providing sex-spec ific patterns. A collection of L. elongates individuals was subjected to gonadal and chromosomal sexing, and DNA hybridization with both seq uences. These analyses revealed 3 individuals having atypical W chromo somes. Interestingly, 2 of these were males having a ZW karyotype. We assume that these atypical sex chromosome arise by exchanges between Z and W chromosomes, and that a transition between female and male hete rogamety is underway in this species.