After Crain and more One of the best-studied reptiles is the European pond turtle, Emys obicularis. The threshold temperature at which the sex ratio is even is The developmental period during which sex determination occurs can be discovered by incubating eggs at the male-producing temperature for a certain amount of time and then shifting the eggs to an incubator at the female-producing temperature and vice versa.
In Emys, the last third of development appears to be the most critical for sex determination. It is not thought that turtles can reverse their sex after this period. The pathways toward maleness and femaleness in reptiles are just being delineated. Unlike the situation in mammals, sex determination in reptiles and birds is hormone-dependent. In birds and reptiles, estrogen is essential for ovarian development. Estrogen can override temperature and induce ovarian differentiation even at masculinizing temperatures.
Similarly, injecting eggs with inhibitors of estrogen synthesis will produce male offspring, even if the eggs are incubated at temperatures that usually produce females Dorizzi et al. Moreover, the sensitive time for the effects of estrogens and their inhibitors coincides with the time when sex determination usually occurs Bull et al. It appears that the enzyme aromatase which can convert testosterone into estrogen is important in temperaturedependent sex determination.
The estrogen synthesis inhibitors used in the experiments mentioned above worked by blocking the aromatase enzyme, showing that experimentally low aromatase conditions yield male offspring. This correlation is seen to hold under natural conditions as well. Temperature-dependent aromatase activity is also seen in diamondback terrapins, and its inhibition masculinizes their gonads Jeyasuria et al.
One remarkable finding is that the injection of an aromatase inhibitor into the eggs of an all-female parthenogenetic species of lizards causes the formation of males Wibbels and Crews It is not known whether the temperature sensitivity resides in the aromatase gene or protein itself or in other proteins that regulate it.
One hypothesis is that the temperature is sensed by neurons in the central nervous system and transduced to the bipotential gonad by nerve fibers see Lance Another hypothesis is that aromatase activity may be regulated by Sox9. This sex-determining gene is seen throughout the vertebrates, where its expression in gonads correlates extremely well with the production of testes.
When two species of turtles were raised at female-promoting temperatures, Sox9 expression was down-regulated during the critical time for sex determination. While TSD has been observed in many reptile and fish species, the genetic differences between sexes and molecular mechanisms of TSD have not been disclosed. The eggs are affected by the temperature at which they are incubated during the middle one-third of embryonic development.
The thermosensitive, or temperature-sensitive, period TSP is the period during development when sex is irreversibly determined. It is used in reference to species with temperature-dependent sex determination, such as crocodilians and turtles. 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.
Temperature pulses during the thermosensitive period are often sufficient to determine sex, but after the TSP, sex is unresponsive to temperature. After this period, however, sex cannot be reversed see sex reversal. Very near or at the pivotal temperature of sex determination, mixed sex ratios and more rarely intersex individuals.
The distinction between chromosomal sex-determination systems and TSD is often blurred because the sex of some species — such as the three-lined skink Bassiana duperreyi and the central bearded dragon Pogona vitticeps — is determined by sex chromosomes, but this is over-ridden by temperatures that are tolerable but extreme. 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.
A study found that hot temperatures altered the expression of the sex chromosomes in Australia's bearded dragon lizards. The lizards were female in appearance and were capable of bearing offspring, despite having the ZZ chromosomes usually associated with male lizards. Synergism between temperature and hormones has also been identified in these systems.
Administering estradiol at male-producing temperatures generates females that are physiologically identical to temperature-produced females. Hormones and temperature show signs of acting in the same pathway, in that less hormone is required to produce a sexual shift as the incubation conditions near the pivotal temperature. 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.
While sex hormones have been observed to be influenced by temperature, thus potentially altering sexual phenotypes, specific genes in the gonadal differentiation pathway display temperature influenced expression. While aromatase is involved in more processes than only TSD, it has also been shown to play a role in certain tumor development.
The adaptive significance of TSD is currently not well understood. 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.
Consequently, the adaptive significance of TSD in all but the most recent origins of TSD may have been obscured by the passage of deep time, with TSD potentially being maintained in many amniote clades simply because it works 'well enough' i. Other work centers on a theoretical model the Charnov — Bull model ,   predicted that selection should favour TSD over chromosome -based systems when "the developmental environment differentially influences male versus female fitness";  this theoretical model was empirically validated thirty years later  but the generality of this hypothesis in reptiles is questioned.
In the absence of estrogen a testis develops. In TSD reptiles, embryos will develop as females when treated with estrogen even if eggs are incubated at male-inducing temperatures, and conversely, will develop as males when estrogen synthesis is blocked in eggs incubated at female-inducing temperatures. A number of other genes have also been shown to be important in mammalian sex determination.
One of these genes, Sox9, which is expressed in differentiating mouse testis, has recently been found to be expressed in embryonic reptile testis.
Current research is now focused on how the gene that produces the enzyme necessary for estrogen synthesis aromatase is regulated in the embryos of reptiles with genetic or environmental sex determination. Controversial issues in reptilian sex determination are 1 the role of the brain in gonadal sex determination, and 2 the role of steroid hormones in the yolk prior to sex determination.
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Sex-determining mechanisms in reptiles are broadly divided into two main categories: genotypic sex determination (GSD) and temperature-dependent sex determination (TSD). Species in the genotypic group, like mammals and birds, have sex chromosomes, which in reptiles come in two major types.
Temperature-dependent sex determination in reptiles. While the sex of most snakes and most lizards is determined by sex chromosomes at the time of fertilization, the sex of most turtles and all species of crocodilians is determined by the environment after fertilization.
Temperature-dependent sex determination (TSD), where the temperature of the embryo’s environment influences its sex development, is a widespread non-genetic process of sex determination among vertebrates, including reptiles. Two factors in reptile sex determination have been studied: (1) the presence or absence of heteromorphic sex chromosomes, and (2) the influence of temperature. Recognizable sex chromosomes are common in snakes and lizards, but are apparently rare in turtles and absent in crocodilians and the tuatara.
In reptiles, sex differentiation appears to be similar to mammals (i.e., the same genes and hormones act ina similar manner), but sex determination is clearly very different. Ovarian differentiation in placental mammals can occur in the absence of estrogen or an estrogen receptor. Ovarian differentiation in reptiles requires the presence of estrogen. Sex chromosomes are unknown in any animals in which temperature sex determination occurs, but they occur in most animals which do not show any sex determination response to temperature. Among reptiles, sex chromosomes are missing in the tuatara, crocodiles, most tortoises, turtles and terrapins, and a few snakes and lizards.