1- URA 1137, Equipe "Formations Squelettiques", 2 place Jussieu, 75251 Paris cedex05, France
KEYWORDS: Sex determination, TSD, consanguinity depression, neutralcharacter, evolution
Temperature-dependent sex determination (TSD) in sauropsids is well documented inmany turtles, some lizards, all crocodilians and both living rhyncocephalianspecies, but its adaptive significance remains controversial. It has has proposedthat consanguinity depression could be the selective factor explaining itsacquisition as two individuals from the same nest cannot mate if they are of thesame sex.
We have developed models to determine the necessary conditions for theconsanguinity depression explaining the occurrence of TSD in sauropsids. Thein sauropsids. The mainfactor influencing the frequency increase of thermosensitive-alleles forsex-determination is the number of surviving offspring of each nest to the adultstage divided by the number of individuals in the population. However, thefluctuation of the mean primary sex ratios appears to be a factor preventing theinvasion of thermosensitive alleles. For values allowing thermosensitive-allelesto invade a population, more than 50% of the polymorphic loci should exhibit asignificant statistical difference of frequencies among both sexes. Thisobservation is not encountered in population genetic studies of sauropsids withTSD: consanguinity depression does not appear to play a major role in theestablishment of TSD. On the contrary, the invasion of thermosensitive alleleshas been observed in a model with high longevity without any other advantage forthem. This suggest that TSD could be a quasi-neutral character compared togenotypic sex determination (GSD) in long-lived animals such as turtles orcrocodilians.
ocodilians.