A unique feature of sex in Crassostrea oysters is the coexistence of protandric sex change, dioecy, and hermaphroditism. To determine whether such a system is genetically controlled, we analyzed sex ratios in 86 pair-mated families of the Pacific oyster, Crassostrea gigas Thunberg. The overall female ratios of one-, two-, and three-year-old oysters were 37%, 55%, and 75%, respectively, suggesting that a significant proportion of oysters matured first as males and changed to females in later years. Detailed analysis of sex ratios in factorial and nested crosses revealed significant paternal effects, which corresponded to two types of sires. No major maternal effects on sex were observed. Major genetic control of sex was further indicated by the distribution of family sex ratios in two to four apparently discreet groups. These and other data from the literature are compatible with a single-locus model of primary sex determination with a dominant male allele (M) and a protandric female allele (F), so that MF are true males and FF are protandric females that are capable of sex change. The rate of sex change of FF individuals may be influenced by secondary genes and/or environmental factors. Strong maternal and weak paternal effects on sexual maturation or time of spawning were also suggested.
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