Temperature dependent sex determination in reptiles
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. The highlands are colder with a higher magnitude of annual temperature fluctuation and a shorter activity season, delaying maturity, thus GSD is favored so sex ratios are not skewed. 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. The turtles were incubated at temperatures that produce solely males, both sexes, and solely females. Hatchlings from single-sex producing temperatures also had higher first-year survivorship than the hatchlings from the temperature that produces both sexes. Pattern I is found in turtles, e. The extent of the TSP varies a little among species,  and development within the oviducts must be taken into account in species where the embryo is at a relatively late stage of development on egg laying e.
The lizards were female in appearance and were capable of bearing offspring, despite having the ZZ chromosomes usually associated with male lizards. 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. Reptiles in which both incubation temperature and sex chromosomes interact to determine sex may represent "transitional" evolutionary states between two end points: 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. Also, experiments conducted at the pivotal temperature, where temperature is equivocal in its influence, have demonstrated an underlying genetic predisposition to be one sex or the other. However, there is evidence that during climactic extremes, changes in the sex determining mechanism itself to GSD are selected for, particularly in the highly-mutable turtles. For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. Spencer and Janzen found that hatchlings from mixed-sex nests were less energy efficient and grew less than their same-sex counterparts incubated in single-sex producing temperatures. This hypothesis is supported by the persistence of TSD in certain populations of spotted skink Niveoscincus ocellatus , a small lizard in Tasmania, where it is advantageous to have females early in the season. One possible explanation that TSD is common in amniotes is phylogenetic inertia — TSD is the ancestral condition in this clade and is simply maintained in extant lineages because it is currently adaptively neutral or nearly so. In this case—which governs all snake species—males are the homogametic sex ZZ and females are the heterogametic sex ZW. It has been proposed  that temperature acts on genes coding for such steroidogenic enzymes , and testing of homologous GSD pathways has provided a genic starting point. Studying the spotted skink, they observed that the highland population was not affected by temperature, yet, there was a negative correlation between annual temperature and cohort sex ratios in the lowlands. While aromatase is involved in more processes than only TSD, it has also been shown to play a role in certain tumor development. 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. They concluded that this differentiation in climate causes divergent selection on regulatory elements in the sex-determining network allowing for the emergence of sex chromosomes in the highlands. Very near or at the pivotal temperature of sex determination, mixed sex ratios and more rarely intersex individuals. Print Advertisement Alex Quinn, a Ph. 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. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. An alternative hypothesis of adaptive significance was proposed by Bulmer and Bull in  and supported by the work of Pen et al. Temperature pulses during the thermosensitive period are often sufficient to determine sex, but after the TSP, sex is unresponsive to temperature. The highlands are colder with a higher magnitude of annual temperature fluctuation and a shorter activity season, delaying maturity, thus GSD is favored so sex ratios are not skewed. These chemicals block the conversion of testosterone to estradiol during development so each sex offspring can be produced at all temperatures. Other reptiles governed by GSD have a system, similar to one found in birds, with Z and W sex chromosomes. There are certainly many known examples of fish and amphibians with GSD, in which both high and low incubation temperatures can cause sex reversal.
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