Temperature-dependent sex determination

Temperature-dependent sex determination (TSD), also called environmental sex determination, is where the surrounding temperature determines the sex of an organism. It is most prevalent among amniote vertebrates such as the Amphibolurus muricatus (the Australian Jacky Lizard). Within the mechanism, two distinct patterns have been discovered and named Pattern I and Pattern II. Pattern I is characterized by possessing a single transition zone where eggs incubated below this temperature zone result in predominantly male hatchlings and above results in predominantly female hatchlings. Pattern II has two transition zones, at intermediate temperatures males dominate and females dominate at both extremes.

Sex determining mechanisms in extant reptilian groups differs from the conventional chromosomal methods seen in most vertebrates. The eggs are affected by the temperature at which they are incubated during the middle one-third of embryonic development. This critical period of incubation is known as the thermosensitive period (TSP)

In turtles with TSD, males are generally produced at lower incubation temperatures than females (TSD IA), with this change occurring over a range of temperatures as little as 1-2 °C At cooler temperatures ranging between 22.5 and 27 degrees Celsius mostly male turtles are produced and at warmer temperatures around 30 degrees Celsius only female turtles arise. . In lizards and crocodilians, this pattern is reversed (TSD IB). Very near or at the pivotal temperature of sex determination, mixed sex ratios and, more rarely, intersex individuals are produced. Several authors resolved histological chronology of sex differentiation in the gonads of turtle with TSD, and so the specific time of sex-commitment is known.

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. The reverse experiment, males produced at female temperatures, only occurs when a nonaromatizable testosterone or an aromatase inhibitor is administered, indicating that the enzyme responsible for conversion of testosterone to estradiol, aromatase, plays a role in female development.

Interestingly, 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. Yet, the genetic sexual determination pathway in TSD turtles is poorly understood and the controlling mechanism for male or female commitment has not been identified.

Advantages over genotypic sex determination
A 1977 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.