Sex‐determining mechanisms in squamate reptiles

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Abstract
Two modes of sex determination occur in squamates: genotypic sex determination (GSD) and temperature‐dependent sex determination (TSD). Within each of these two major modes, there are many different variants, or mechanisms. Male heterogamety, female heterogamety, multiple sex chromosome systems, and homomorphic sex chromosome systems are all types of GSD found in squamates. Two patterns of TSD have been reported.Only three snakes have been investigated for their sex‐determining mechanisms, each having GSD, although incubation temperature does cause differential mortality and affects post‐hatching physiology. Less than 50 lizard species have been investigated, but there is considerable diversity in the sex‐determining mechanisms reported thus far. Apparently, TSD (and/or GSD) has evolved multiple times within a given taxon. Presently, both GSD and TSD are found in the Agamidae, Eublepharidae, and Gekkonidae, and possibly in the Iguanidae, Lacertidae, and Varanidae as well. Only GSD has been reported for the Scincidae and Teiidae.Correlations within the Eublepharidae suggest an adaptive explanation for the evolution of sexdetermining mechanisms; a shift from male‐larger dimorphism to female‐larger (or no sexual size dimorphism) is accompanied by a shift in sex‐determining mode. These shifts are in agreement with similar correlations observed in turtles.