Heterozygosity and body size in triploid Pacific oysters, Crassostrea gigas Thunberg, produced from meiosis II inhibition and tetraploids

Abstract

Triploid molluscs grow significantly faster than diploids in most species studied so far, a phenomenon that has been referred to as triploid gigantism. Three hypotheses have been proposed, attributing triploid gigantism to sterility, increased heterozygosity, or cell size. Testing the heterozygosity hypothesis, the authors measured the body size and allozyme heterozygosity in three replicates of a normal diploid group (2 n), an induced triploid group (3 nCB) and a mated triploid group produced from diploid×tetraploid mating (3 nDT). Body size measurements at 1 year of age showed that both 3 nDT and 3 nCB triploids were significantly bigger than normal diploids, by 26% and 14%, respectively, in meat weight. The 3 nDT triploids were 10% bigger than the 3 nCB triploids, although the difference was not statistically significant. Heterozygosity at five polymorphic loci averaged 0.48 for 3 nCB and 0.57 for 3 nDT triploids, which were 37% and 63% higher, respectively, than that for normal diploids (0.35). Differences in heterozygosity were highly significant among all three groups. Among the three groups, there was a strong and positive correlation between meat weight and heterozygosity. The correlation was weak or undetectable within groups or at the individual level. These results support the heterozygosity hypothesis, but do not negate the cell size and sterility hypotheses. It is possible that the cell size is the fundamental cause for triploid gigantism, which is better expressed in 3 nDT (than 3 nCB) triploids because of further increases in heterozygosity or other genetic factors.