Female-biased sex ratio in adults of the turtle Emys orbicularis at the northern limit of its distribution in France: a probable consequence of interaction of temperature with genotypic sex determination

Abstract

Adult sex ratio in the turtle Emys orbicularis was determined in populations from seven ponds in Brenne (Indre, France). In all populations, the sex ratio was biased toward females. Among 290 captured animals, the male:female ratio was close to 0.5. Among different demographic factors that could affect the adult sex ratio, the most influential was probably the sex ratio of hatchlings. In Emys orbicularis, a ZZ male/ZW female system of genotypic sex determination has been postulated. Moreover, gonad differentiation is dependent on temperature and sex-reversed individuals can occur. To evaluate the importance of sex reversal among adult females, the blood of 78 animals was typed for the serologically detectable H-Y antigen, used as a tool to identify sexual genotype. In 73 of them, the H-Y phenotype was positive, conforming with female genotype, but in the other 5 females, it was negative (as in genotypic males), revealing that the sexual phenotype of these animals had been inverted. As the percentage of these sex-reversed genotypic males is low, the influence of temperature would appear not to be the sole cause of the observed unbalanced sex ratio. The female bias can be partly explained by the interaction of temperature with the ZZ/ZW system of genotypic sex determination. Indeed, in this system, sexual inversion under the influence of an epigenetic factor increases the ratio of genotypic females (ZW and WW) in the progeny.