Sex and meiosis in autotetraploid pacific oyster, Crassostrea gigas (Thunberg)

Sex and meiosis were studied in induced autotetraploids of the Pacific oyster (Crassostrea gigas Thunberg) and were compared with sex and meiosis in autotriploids and normal diploids. Tetraploid oysters reached sexual maturity at 1 year of age in an approximately 1:1 sex taro. In contrast with the abnormally high frequency of hermaphrodites among triploids, tetraploids had about the same level of hermaphrodites as normal diploids. Fecundity of tetraploids was comparable to that of normal diploids, differing from the greatly reduced fecundity of triploids. Homologous chromosomes synapsed predominantly as trivalents in eggs from triploids and as quadrivalents in eggs from tetraploids. After fertilization, eggs from tetraploids and triploids went through two meiotic divisions, as normal eggs did. The average gamete chromosome number was 10.0 for diploids and 19.9 for tetraploids. The distribution of gamete chromosome numbers from triploids suggested that the extra chromosome in the trivalent segregated randomly during anaphase I. In tetraploids, however, the two extra chromosomes in the quadrivalents did not segregate independently and, instead, they preferentially cosegregated to opposite poles producing balanced gametes. These results suggest that mechanisms may exist to weigh, balance, and equally distribute quadrivalents, possibly through mitotic force and tension. Errors in chromosome balancing in normal meiosis may result in nondisjunction, which is the primary cause of human aneuploidy.