GENETIC-DETERMINANTS OF PROTANDRIC SEX IN THE PACIFIC OYSTER, CRASSOSTREA-GIGAS THUNBERG

A unique feature of sex in Crassostrea oysters is the coexistence of p rotandric sex change, dioecy, and hermaphroditism. To determine whethe r such a system is genetically controlled, we analyzed sex ratios in 8 6 pair-mated families of the Pacific oyster, Crassostrea gigas Thunber g. The overall female ratios of one-, two-, and three-year-old oysters were 37%, 55%, and 75%, respectively, suggesting that a significant p roportion of oysters matured first as males and changed to females in later years. Detailed analysis of sex ratios in factorial and nested c rosses revealed significant paternal effects, which corresponded to tw o types of sires. No major maternal effects on sex were observed. Majo r 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 mod el 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 pro tandric females that are capable of sex change. The rate of sex change of FF individuals may be influenced by secondary genes and/or environ mental factors. Strong maternal and weak paternal effects on sexual ma turation or time of spawning were also suggested.