Cytogenetic mechanism for reversion of triploids to heteroploid mosaics in Crassostrea gigas (Thunberg) and Crassostrea ariakensis

Triploid oyster induction has been very successful. However, during field culture, a certain proportion of triploids apparently loses one set of chromosomes, 'reverting' to diploid-triploid mosaics. The revertants will lose their aquaculture advantages due to the recovery of reproductive ability. Mitotic chromosomes were observed in mosaics of Crassostrea gigas and Crassostrea Ariakensis to elucidate the cytogenetic mechanism of reversion. Most individuals showed a modal number of 30 in triploid cells and varying numbers of chromosomes in other cells. From all 15 mosaics of C. gigas and 16 mosaics of C. ariakensis individuals, high proportions of aneuploid cells with various numbers of chromosomes were observed. Most of the aneuploid cells were hypotriploid cells with 26-29 chromosomes. Notably, in 10 out of 15 C. gigas and 9 out of 16 C. ariakensis, some cells consisted of 20-26 well-spread chromosomes and 3-10 chromosome 'clumps'. Individuals with more chromosome clumps tended to have higher percentages of hypotriploid cells. The correlation between these two categories of cells was significant (P < 0.05) in C. gigas and highly significant (P < 0.01) in C. ariakensis. Thus, we conclude that some chromosomes are eliminated during mitotic divisions because they clump together and are thus unable to undergo normal segregation.