Temperature Sex Determination in Reptiles: The Third Strategy.

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Considering gamete distribution of sex heteromorphic chromosomes, three groups of vertebrates can be distinguished: i. Heterogametic males with X:Y chromosome pairs; ii. Heterogametic females with Z:W chromosome pairs and, iii. Species without heteromorphic sex chromosomes. Whereas in species with heteromorphic sex chromosomes, sex is determined at fertilization, in the other species sex determination occurs later. Among this group, there are species where temperature may act as sex-determining factor. In turtles, higher temperatures are female-promoting whereas in crocodiles they are male-promoting. Regulation of steroid hormones levels by temperature is suggested as the physiological mechanism of sex determination. Several genes related to sex determination in mammals and birds are also expressed in gonads of reptiles with temperature sex determination (TSD). However, the diverse sequence and timing of their expression indicates that different genetic pathways of sex determination have evolved. Determination of Sertoli cells is a critical stage in developing gonads of vertebrates for sex determination. Sox9 is a Sry-related gene implicated in differentiation of Sertoli cells in reptiles, birds and mammalian embryos. In the present review, Sox9 expression in the sea turtle Lepidochelys olivacea, a species with TSD, is compared with Sox9 expression in other vertebrate species with temperature or genetic sex determination.

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