Comparison of genetic variability and parentage in different ploidy classes of the Japanese oyster Crassostrea gigas

Chemical treatments with cytochalasin B were used to induce triploidy in th e progeny of a mass fertilization of 3 male and 7 female Crassostrea gigas parents. Triploids were produced either by retention of the first (meiosis I (M1) triploids) or the second (meiosis II (MII) triploids) polar bodies. These animals, together with their diploid siblings, were divided for two e xperiments. One set was used to compare physiological performance, and the other set deployed to compare growth in two different natural environments. For both experiments, genetic variability in different ploidy classes was estimated using three microsatellite loci and eight allozyme loci. The micr osatellite loci were highly polymorphic, allowing independent confirmation of ploidy status and the unambiguous identification of parentage for each o yster. Significant differences in parentage were found between ploidy class es, despite the fact they originated from the same mass fertilization. This indicates that the assumptions of a common genetic background among random samples of animals taken from the same mass fertilization may not be gener ally valid. Knowledge of parentage also allowed the more accurate scoring o f allozyme loci. As expected, triploids were e found to be significantly mo re polymorphic than diploids. However, MI triploids were not significantly more polymorphic than MII triploids, MII triploid genotypes were used to es timate recombination rates between loci and their centromeres. These rates varied between 0.29 and 0.71, indicating only moderate chiasma interference .