In this case—which governs all snake species—males are the homogametic sex ZZ and females are the heterogametic sex ZW. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. Furthermore, the process leading to sex determination under the influence of temperature poses problems that are not encountered by species with genetic sex determination. Although most genes involved in gonadal development are conserved in vertebrates, including TSD species, temporal and spatial gene expression patterns vary among species. 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.
Species in the genotypic group, like mammals and birds, have sex chromosomes, which in reptiles come in two major types. 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. Although most genes involved in gonadal development are conserved in vertebrates, including TSD species, temporal and spatial gene expression patterns vary among species. Reptiles in which both incubation temperature and sex chromosomes interact to determine sex may represent "transitional" evolutionary states between two end points: In addition, transitory gene regulatory networks leading to gonadal TSD have evolved. For example, there is an Australian skink lizard that is genotypically governed by X and Y sex chromosomes. 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. Estrogen levels may influence sex determination or gonad differentiation depending on the species. Despite variation in TSD pattern and gene expression heterochrony, the structural framework, the medullary cords, and cortex of the bipotential gonad have been strongly conserved. Sex-determining mechanisms in reptiles are broadly divided into two main categories: However, manipulation of estrogen levels rarely mimics the precise timing of temperature effects on expression of gonadal genes, as occurs with TSD. 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. It is quite possible that there are other species of reptiles with more complicated scenarios of temperature reversal of chromosomal sex. A 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. Yolk steroids of maternal origin and steroids produced by the embryonic nervous system should also be considered as sources of hormones that may play a role in TSD. There are certainly many known examples of fish and amphibians with GSD, in which both high and low incubation temperatures can cause sex reversal. 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. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. Furthermore, the process leading to sex determination under the influence of temperature poses problems that are not encountered by species with genetic sex determination. Other reptiles governed by GSD have a system, similar to one found in birds, with Z and W sex chromosomes. 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. 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. Higher temperatures can produce either males or females, and the temperature ranges and lengths of exposure that influence TSD are remarkably variable among species. Environmental sex determination mechanisms in reptiles. Males, on the other hand, are "heterogametic," with one X chromosome and one Y chromosome.
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