Chemical treatments with cytochalasin B were used to induce triploidy in the 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 (MI) triploids) or the second (meiosis II (MII) triploids) polar
bodies. These animals, together with their diploid siblings, were divided for two experiments. 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 microsatellite loci
were highly polymorphic, allowing independent confirmation of ploidy status and the unambiguous
identification of parentage for each oyster. Significant differences in parentage were found between
ploidy classes, 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 generally valid. Knowledge of parentage also allowed the
more accurate scoring of allozyme loci. As expected, triploids were found to be significantly more
polymorphic than diploids. However, MI triploids were not significantly more polymorphic than
MII triploids. MII triploid genotypes were used to estimate recombination rates between loci and
their centromeres. These rates varied between 0·29 and 0·71, indicating only moderate chiasma
interference.