Temperature-dependent sex determination

[11] The specific time of sex-commitment is known due to several authors resolving histological chronology of sex differentiation in the gonads of turtles with TSD.

[12] The TSP typically spans the middle third of incubation with the endpoints defined by embryonic stage when under constant temperatures.

[18] A 2015 study found that hot temperatures altered the expression of the sex chromosomes in Australia's bearded dragon lizards.

[20] Research from 2020 identified the timing of gonadal commitment in the American alligator to understand the effects of estrogen-signaling in TSD.

It has been proposed[26] that temperature acts on genes coding for such steroidogenic enzymes, and testing of homologous GSD pathways has provided a genic starting point.

[27] 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.

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.

The warmth early in the season ensures female-biased broods that then have more time to grow and reach maturity and possibly reproduce before they experience their first winter, thereby increasing fitness of the individual.

[1] In support of the Charnov and Bull hypothesis, Warner and Shine (2008) showed confidently that incubation temperature influences males’ reproductive success differently than females in Jacky Dragon lizards (Amphibolurus muricatus) by treating the eggs with chemicals that interfere with steroid hormone biosynthesis.

[2] Spencer and Janzen (2014) found further support for the Charnov-Bull model by incubating painted turtles (Chrysemys picta) at different temperatures and measuring various characteristics indicative of fitness.

Spencer and Janzen (2014) 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.

TSD may be advantageous and selected for in turtles, as embryo energy efficiency and hatchling size are optimized for each sex at single-sex incubation temperatures and are indicative of first-year survivorship.

An alternative hypothesis of adaptive significance was proposed by Bulmer and Bull in 1982[38] and supported by the work of Pen et al. (2010).

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.

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.

However, there is evidence that during climatic extremes, changes in the sex determining mechanism itself (to GSD) are selected for, particularly in the highly-mutable turtles.

Patterns of temperature-dependent sex determination (TSD) in reptiles. Pattern I is found in turtles, e.g. Red-eared slider turtles ( Trachemys scripta ), Olive Ridley sea turtles ( Lepidochelys olivacea ), or Painted turtles ( Chrysemys picta ). Pattern II has been found in American alligators ( Alligator mississippiensis and Leopard geckos ( Eublepharis macularius ). [ 13 ]
Some reptiles use incubation temperatures to determine sex. In some species, this follows the pattern that eggs in extremely high or low temperatures become female and eggs in medium temperatures become male. [ 14 ]