Plant & Animal Genome VI Conference

W9

GENETIC MAPPING IN OYSTERS: PROBLEMS WITH NON-MENDELIAN SEGREGATION AND COMPLETE INTERFERENCE

1. Bodega Marine Laboratory, University of California, Davis, Bodega Bay, CA USA 94923-0247
2. Ecole Normale Superieure de LYON, 46 allee d'Italie , 69 007 LYON , France
3. Laboratoire Genome et Populations, UPR 9060, Universite de Montpellier II, Place E. Bataillon, 34095 Montpellier, cedex 5, France

Inheritance studies with marine bivalve molluscs, including the American and Pacific oysters Crassostrea virginica and C. gigas, often detect departures from Mendelian segregation. In controlled, pair-crosses, about half of the genetic markers, whether protein or DNA polymorphisms, show significant discrepancies between observed and expected genotypic proportions in the resulting progeny. Discrepancies are evident during larval development but are especially marked at the juvenile or adult stages; they arise in random-bred progenies but are striking in inbred families. The loci affected vary from cross to cross. These deviations from normal Mendelian segregation, whatever their cause, necessitate modifications of standard methods of linkage or QTL mapping. While statistical corrections are available for cases of zygotic or gametic selection, another approach is to examine segregation in very early life stages, in haploid embryos, even in gametes directly. It is possible to make haploid oyster embryos, which survive for 6-8 hrs, producing enough template for 20-40 PCR reactions. We successfully amplified microsatellites from 6 hr old haploid and diploid Pacific oyster embryos, and preliminary data for one microsatellite locus suggest that segregation, in both male and female parents, and genotypic proportions of progeny are normal in three families. One female parent is hypothesized to carry a null allele. Further studies involving more loci and parents are needed to confirm that segregation distortion is consistently absent in early haploid or diploid stages. A second problem in genetic mapping is the complete interference that has been reported in studies of gene-centromere recombination in triploids made by inhibition of the second polar body. Whether large blocks of genes remain linked because of stereotypical sites for crossing-over remains a concern in constructing a linkage map for oysters.