In this species, high incubation temperature during egg development reverses genotypic males (ZZ) into phenotypic females so females can be ZZ or ZW, but males are always ZZ. A slightly different example of this temperature-induced sex reversal is found in an Australian dragon lizard, which has the ZW system of sex chromosomes. Therefore, in this species, there are both XX and XY males, but females are always XX. A low incubation temperature during the development of this lizard's egg reverses some genotypic females (XX) into "phenotypic" males-so that they have only functioning male reproductive organs. For example, there is an Australian skink lizard that is genotypically governed by X and Y sex chromosomes. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. This paradigm, though, has been recently challenged, with new evidence now emerging that there may indeed be both sex chromosomes and temperature involved in the sex determination of some reptile species. This model indicates that there is no genetic predisposition for the embryo of a temperature-sensitive reptile to develop as either male or female, so the early embryo does not have a "sex" until it enters the thermosensitive period of its development. In crocodilian species-the most studied of which is the American alligator- both low and high temperatures result in females and intermediate temperatures select for males.Ī widely held view is that temperature-dependent and genotypic sex determination are mutually exclusive, incompatible mechanisms-in other words, a reptile's sex is never under the influence of both sex chromosomes and environmental temperature. For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. This thermosensitive period occurs after the egg has been laid, so sex determination in these reptiles is at the mercy of the ambient conditions affecting egg clutches in nests. In temperature-dependent sex determination, however, it is the environmental temperature during a critical period of embryonic development that determines whether an egg develops as male or female. In this case-which governs all snake species-males are the homogametic sex (ZZ) and females are the heterogametic sex (ZW). Other reptiles governed by GSD have a system, similar to one found in birds, with Z and W sex chromosomes. Males, on the other hand, are "heterogametic," with one X chromosome and one Y chromosome. Many species-such as several species of turtle and lizards, like the green iguana-have X and Y sex chromosomes (again, like mammals), with females being "homogametic," that is, having two identical X chromosomes. Species in the genotypic group, like mammals and birds, have sex chromosomes, which in reptiles come in two major types. Sex-determining mechanisms in reptiles are broadly divided into two main categories: genotypic sex determination (GSD) and temperature-dependent sex determination (TSD). candidate at the Institute for Applied Ecology at the University of Canberra in Australia, sorts this quandary out for us.
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