Sex Determination: Genetic Models for Oysters

In oysters, sex is determined partly by environment, but previous studies employing controlled crosses suggest that genetic factors are also important. Sex ratios in both full- and half-sib families of the Pacific oyster show paternal control of sex ratio and suggest that a single major gene with 2 genotypes controls sex in the Pacific oyster, with FM oysters being male and FF oysters maturing as male or female. Here, we show that such a model does indeed produce a stable polymorphism for either single or multiple age-class populations, though under limited ranges of f, the probability that an FF individual matures as a female. However, this 2-genotype model cannot explain observed heterogeneity of sex ratios among progeny from different dams within half-sib families. We propose an alternative 3-genotype model that also produces a stable polymorphism, for either single or multiple age-class populations, but over all values of f between zero and one. This model accounts for sex ratio heterogeneity among male half-sib families because it features 2 types of females, a protandric FM and a fixed female FF. Furthermore, the 3-genotype model, accounts for the frequencies of mating types inferred from the observed sex ratios of families more closely than the 2-genotype model. Although the mechanism of sex determination may ultimately prove more complex, simple genetic mechanisms can account for the broad features of sexual maturation in oyster families and the stability of sex ratios in populations.