EVIDENCE FOR MITOTIC INSTABILITY IN TRIPLOID PROGENY OF THE AMERICAN OYSTER, Crassostrea virginica, PRODUCED BY TETRAPLOID X DIPLOID CROSSES  

Polyploid induction, more specifically the commercial production of triploids and the creation of tetraploid brood stock to support it, has become an important and successful technique in aquaculture of the eastern oyster, Crassostrea virginica. Triploid oysters are valued for their sterility that generates several advantages for oyster culture, such as reduced gonadal development, allowing for higher growth rates and superior market quality during the reproductive season. Nevertheless, tetraploids undergo chromosome loss sometimes losing entire sets of chromosomes to become heteroploid mosaics. The loss of chromosomes from tetraploids are of major scientific interest and the possible effects of using these mosaic tetraploids for triploid production are a practical concern for commercial oyster culture. Until now, flow cytometry (FCM) was the principal research tool for detecting reversion, however, FCM provides little information about aneuploidy. The objective of this study was to determine the extent of aneuploidy in the triploid progeny produced from both stable (mosaic) and unstable (non-mosaic) male and female tetraploids using cytogenetic techniques.

Three non-mosaic and three mosaic tetraploid males were crossed with a single diploid female creating six half-sib groups. In another experiment, three non-mosaic and three mosaic tetraploid females were crossed with a single diploid male. Samples were taken and fixed when most embryos had developed to the 2 to 4 cell stage, and at 6h PF. The level of aneuploidy was estimated by counting 30 metaphases per group showing a similar chromosome spread.

Embryos at 2 to 4 cells are mostly euploid with the expected 30 chromosomes with only 12% aneuploidy, but by 6 hours - based on cells from dividing embryos - aneuploidy has reached, on average, 66% among all test crosses. Non-statistically significant differences for chromosome loss in triploid embryos produced from mosaic and non-mosaic tetraploid x diploid crosses were observed, produced from either male or female tetraploids. With these results we hypothesize that chromosomes in triploids results from mitotic, rather than meiotic, errors.