SEX-DETERMINING MECHANISMS IN SQUAMATE REPTILES

Two modes of sex determination occur in squamates: genotypic sex deter mination (GSD) and temperature-dependent sex determination (TSD). With in each of these two major modes, there are many different variants, o r mechanisms. Male heterogamety, female heterogamety, multiple sex chr omosome systems, and homomorphic sex chromosome systems are all types of GSD found in squamates. Two patterns of TSD have been reported. Onl y three snakes have been investigated for their sex-determining mechan isms, each having GSD, although incubation temperature does cause diff erential mortality and affects post-hatching physiology. Less than 50 lizard species have been investigated, but there is considerable diver sity in the sex-determining mechanisms reported thus far. Apparently, TSD (and/or GSD) has evolved multiple times within a given taxon. Pres ently, both GSD and TSD are found in the Agamidae, Eublepharidae, and Gekkonidae, and possibly in the Iguanidae, Lacertidae, and Varanidae a s well. Only GSD has been reported for the Scincidae and Teiidae. Corr elations within the Eublepharidae suggest an adaptive explanation for the evolution of sex-determining mechanisms; a shift from male-larger dimorphism to female-larger (or no sexual size dimorphism) is accompan ied by a shift in sex-determining mode. These shifts are in agreement with similar correlations observed in turtles. (C) 1994 Wiley-Liss, In c.