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In many reptiles, sex determination is temperature-sensitive. This phenomenon has been shown to take place in the laboratory as well as in nature, but its effect on natural populations remains questionable. In the turtle Emys orbicularis, the effects of temperature override a weak mechanism of genetic sex determination which is revealed in incubation at pivotal temperature. At this temperature, the sexual phenotype is concordant with the expression of the serologically defined H-Y antigen (H-Ys) in non-gonadal tissues; males are H-Ys negative (H-Y−) whereas females are H-Ys positive (H-Y+). To estimate the importance of sexual inversion (sexual phenotype and H-Ys expression discordant) in populations of Brenne (France), the frequencies of male and female sexual 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 contribution 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 considers that these parameters influence fitness because this model has been previously shown to favour environmental sex determination. In both models, it appears that sex determination can be viewed as genotypic and monogenic with some individuals sexually inverted by theaction of temperature. One category of homozygous animals differentiates mainly into one sex, and the heterozygous animals differentiate mainly into the other sex. The second category of homozygotes has a low frequency in 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.
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