Plant & Animal Genome VII Conference

W14

GENETIC LOAD CAUSES SEGREGATION DISTORTION IN OYSTERS: MAPPING AT 6 HOURS

Bodega Marine Laboratory, University of California Davis, Bodega Bay, CA 94923-0247 USA

Inheritance studies with marine bivalve molluscs often detect departures from Mendelian segregation. Discrepancies appear in late larval stages but are especially marked at juvenile or adult stages; they occur in random-bred progenies but are striking in inbred families. The loci affected vary from cross to cross. These deviations from normal Mendelian segregation interfere with standard methods of linkage or QTL mapping. One hypothesis for widespread distortions of Mendelian inheritance is background selection against deleterious mutations, which could result from the very high fecundity of bivalves. If so, we expect minimum expression of genetic load, thus minimum segregation distortion, at the earliest developmental stages. We examine segregation of 22 microsatellites loci in early swimming trochophore larvae (6 hrs old) from nine F₂ or F₃ hybrid families of Crassostrea gigas. For several microsatellite loci, there is evidence for non-amplifying ("null") alleles; but taking null alleles into account, we find that segregation ratios are Mendelian in 95% of the cases. Juveniles (2-3 months old) from the same crosses show segregation distortion in several cases. In the most striking case, a homozygous genotype is seen only once when it is expected 20 times. We thus confirm that segregation distortion increases with age, as predicted from cumulative expression of numerous genetic defects during development. This evidence for large genetic load in bivalves supports Zouros' associative overdominance hypothesis for correlations between multilocus allozyme heterozygosity (or somatic aneuploidy) and growth rate. Given the absence of segregation distortion in 6-hour-old larvae, it will be possible to identify linkage groups using microsatellites and other PCR-based markers. We will start a QTL mapping analysis of growth, based on shell height at 3 months in 3 of these families, which exhibit large coefficients of variation in size (CV=35.99, 31.75 and 50.35 respectively).