Sex, genes, and heat: Triggers of diversity

In vertebrates, sex is determined by a surprising variety of mechanisms. In many reptiles, the primary testis or ovary-determining trigger is regulate d by egg incubation temperature. This temperature dependent sex determining (TSD) mechanism occurs in all crocodilians and marine turtles examined to date and is common in terrestrial turtles and viviparous lizards (Ewert et al. 1994. J Exp Zool 270:3-15; Lang and Andrews. 1994. J Exp Biol 270:28-44 ; Mrosovsky. 1994. J Exp Zool 270:16-27; Pieau. 1996. Bioessays 18:19-26; V iets et al. 1994. J Exp Zool 270:45-56; Wibbels et al. 1998. J Exp Zool 281 :409-416). In contrast, sex in mammals and birds is determined chromosomall y (CSD). Despite these differences, morphological development of the gonads in all these vertebrate groups appears to have been conserved through evol ution. Therefore, the genetic mechanisms triggering sex determination appea r not to have been conserved through evolution, although the basic genetic pathway controlling the morphological differentiation of the gonads appears to have been conserved. J. Exp. Zool. 290:624-631, 2001. (C) 2001 Wiley-Li ss, Inc.