GENETIC DETERMINANTS OF PROTANDRIC SEX IN THE PACIFIC OYSTER,CRASSOSTREA GIGASTHUNBERG

Abstract

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.

Production of inbred larvae through self-fertilization using oocytes and cryopreserved sperm from the same individuals after sex reversal in eastern oysterCrassostrea virginica

“…This observation agrees with a previous study of eastern oysters (Needler 1941), and another for Pacific oysters (Lango-Reynoso, Chavez-Villaba & Le Pennec 2006). It has been suggested that sex change may be under some genetic control in the eastern oyster (Haley 1977) and the Pacific oyster (Guo et al 1998). However, no detailed evidence has been reported to show that sex change is controlled by specific factors (e.g.…”

“…In nature, self-fertilization in hermaphrodites is usually avoided through strategies such as asynchronous maturation or outcrossing (Foighil & Eernisse 1987;Rios, Canales & Pena 1996). Due to the free dispersal of gametes, however, most benthic marine invertebrates such as bivalves usually possess a small chance of self-fertilization In oysters of the genus of Crassostrea, hermaphrodites are rare, and occurrence varies with age and environment (Galtsoff 1964;Kennedy, Newell & Eble 1996;Guo, Hedgecock, Hershberger, Cooper & Allen 1998). Therefore, it is not practical to create selfing inbred lines by using natural hermaphrodites.…”

Experimental Effects of Temperature and Photoperiod on Synchrony of Gametogenesis and Sex Ratio in the European OysterOstrea edulis(Linnaeus)

“…Sex of individual oysters may be, to a lesser or greater degree, under genetic rather than environmental control, yet environmental factors clearly play a role in genetic expression and generation of hormonal cues (Guo et al 1998, Baghurst & Mitchell 2002. By manipulating environmental factors such as feed availability and temperature during conditioning, hatcheries may be able to select expression of desired sex in some species of bivalve molluscs.…”

Exposure to 17α-ethynylestradiol causes dose and temporally dependent changes in intersex, females and vitellogenin production in the Sydney rock oyster

“…As S. glomerata exhibits protandric diocey (Asif 1979;Guo et al 1998), it remains unclear whether intersex individuals observed in our initial experiments were functionally intersex or the result of an opportunistic sampling of individuals that were in the process of undergoing an estrogenically mediated protandric transition from male to female. If the latter is the case, we may expect to observe Effects of 17a-ethynylestradiol exposure in the Sydney rock oyster 1441 shifts in the sex ratio towards females with greater exposure durations to estrogens, at later intervals in the gonadal development cycle and/or with greater sampling frequency during such potential transitions.…”