Sex‐determining mechanisms in squamate reptiles

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 physiolom. 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. b

Temperature-Dependent Sex Determination in Vertebrates

“…Future research will undoubtedly confirm TSD in some species in these and other lizard families. Genotypic sex determination, however, remains the dominant form of sex determination in lizards, and Viets et al (1994) provide evidence that only GSD occurs in all species so far investigated in a further five lizard families (Anolidae, Basiliscidae, Crotaphytidae, Sceloporidae, and Teiidae). All TSD species in the families Agamidae, Eublepharidae, and Gekkonidae so far investigated over the full range of viable incubation temperatures show a similar sex-determining pattern: only females are produced at high and low incubation temperatures, and varying proportions of males hatch at intermediate temperatures (Tokunaga 1985;Viets et al 42 5…”

“…Within each of these families, genotypic sex determination (GSD) has also been shown to occur in other species (Viets et al 1994). Thus TSD and GSD co-occur within all these families.…”

“…Within the squamate reptiles, TSD has so far only been convincingly confirmed (but see Valenzuela et al 2003) in a few species in two lizard families in the Gekkota (Eublepharidae and Gekkonidae; Viets et al 1994), in one family within the Iguania (Agamidae; Harlow 2001), and in the family Scincidae (Robert and Thompson 2001). Within each of these families, genotypic sex determination (GSD) has also been shown to occur in other species (Viets et al 1994).…”

“…Thus TSD and GSD co-occur within all these families. There are limited data and anecdotal reports to suggest that TSD may also occur in some species in the lizard families Anguidae, Chamaeleontidae, Diplodactylidae, Iguanidae, Lacertidae, and Varanidae, but at present no single species in these families that is suspected of having TSD has been rigorously investigated (Viets et al 1994). Future research will undoubtedly confirm TSD in some species in these and other lizard families.…”

“…The African fat-tailed gecko Hemitheconyx caudicinctus is another eublepharid gecko in which TSD is well documented ( Anderson and Oldham 1988), and it has an FMF pattern of sex determination similar to that of E. macularius (Viets et al 1993(Viets et al , 1994. Three species of the eublepharid genus Coleonyx have been investigated for their sexdetermining mechanism by laboratory incubation (C. brevis, C. mitratus, and C. variegatus), and all clearly show GSD (Viets et al 1994;Bragg et al 2000).…”