Aneuploidy in the Pacific oyster, Crassostrea gigas Thunberg and its effects on growth

We previously described the incidental induction of aneuploids in the Pacific oyster, Crassostrea gigas Thunherg, from tetraploid and triploid production. Here we report the intentional production of aneuploids and isolation of trisomic families, with observations on their growth performance. The first generation of aneuploids was produced from triploid x diploid crosses. Two types of crosses were made: diploid female x triploid male (DTA) and the reciprocal triploid female x diploid male (TDA) crosses. DTA crosses were highly effective in producing aneuploids, and 80-95% of the DTA progeny were aneuploids as determined by chromosome counts at one year of age. Aneuploid conditions included 2n + 1, 2n + 2, 2n + 3, 3n - 2, and 3n - 1. TDA crosses produced fewer aneuploids (16-20%) and more triploids (20-53%) than DTA crosses. Aneuploids as a group are significantly smaller in size than normal diploids. DTA progeny with an approximate diploid DNA content were separated using flow cytometry and considered putative trisomics. The putative trisomics were crossed with normal diploids in single-pair matings. Sixty putative trisomic families were produced, and 20 of them were confirmed as trisomic families using chromosome counts of embryos at the 2-cell stage. In most families, the frequency of trisomics sharply declined, from about 50% at the 2-cell stage to 5-25% at one year of age, possibly due to mortality or chromosome loss. In some families, the trisomics remained at 40-61%. Trisomic oysters are smaller on average than normal diploids in most families, but not different from normal diploids in others. Results of this study show that the Pacific oyster can tolerate aneuploidy up to 15% of its genome. Aneuploids as a group have growth retardation, but certain aneuploid conditions grow as well as normal diploids.