GENETIC CONTRIBUTION TO SEX DETERMINATION IN TURTLES WITH ENVIRONMENTAL SEX DETERMINATION

In many reptiles, sex determination is temperature-sensitive. This phe nomenon has been shown to take place in the laboratory as well as in n ature, but its effect on natural populations remains questionable. In the turtle Emys orbicularis, the effects of temperature override a wea k mechanism of genetic sex determination which is revealed in incubati on at pivotal temperature. At this temperature, the sexual phenotype i s concordant with the expression of the serologically defined H-Y anti gen (H-Ys) in non-gonadal tissues; males are H-Ys negative (H-Y-) wher eas females are H-Ys positive (H-Y+). To estimate the importance of se xual inversion (sexual phenotype and H-Ys expression discordant) in po pulations of Brenne (France), the frequencies of male and female sexua l phenotypes among H-Ys phenotypes were determined. The frequencies of sex reversed individuals are low, only 6% of phenotypic females being H-Y- and 11% of phenotypic males being H-Y+. According to these data, two theoretical models have been constructed to estimate the contribu tion to sex determination of individuals in relation to their genotype . The first model excludes any influence of incubation temperature and sexual phenotype on the fitness of individuals. The second one consid ers that these parameters influence fitness because this model has bee n previously shown to favour environmental sex determination. In both models, it appears that sex determination can be viewed as genotypic a nd monogenic with some individuals sexually inverted by the action of temperature. One category of homozygous animals differentiates mainly into one sex, and the heterozygous animals differentiate mainly into t he other sex. The second category of homozygotes has a low frequency i n the populations and can differentiate as male or female without high constraint. Then it is estimated that in Brenne approximately 83% of the eggs are incubated in conditions allowing the genetic component to influence sex determination.