Temperature-dependent sex determination

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Temperature-dependent sex determination. Sex determining mechanisms in extant reptilian groups challenge the conventional chromosomal methods seen in most vertebrates. Most prevalent is temperature-dependent sex determination, or TSD, where individual sex is determined by the temperature at which eggs are incubated during the middle one-third of embryonic development (Wibbels 1991b).

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 degrees C (Bull 1980). 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 (Bull 1980). Several authors (e.g. Wibbels et al 1991b) 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. By administering estradiol at male-producing temperatures, Wibbels et al. (1991a) generated 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 (Crews 2003). 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. Likewise, Crews (1996) proposes that temperature acts on genes coding for such steroidogenic enzymes, and testing of homologous GSD pathways has provided a genic starting point (Miller et al. 2003). 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.

[edit] References

Bull, J. J. 1980. Sex determination in reptiles. Quart. Review of Biology 55: 3-21.
Crews, D. 1996. Temperature-dependent sex determination: the interplay of steroid hormones and temperature. Zoo. Sci. 13:1-13.
Crews, D. 2003. Sex determination: where environment and genetics meet. Evolution and Development 5: 50-55.
Miller, D., J. Summers, and S. Silber. 2004. Environmental versus genetic sex determination: a possible factor in dinosaur extinction? Fertility and Sterility 81: 954-964.
Wibbels, T., J.J. Bull, and D. Crews 1991a. Synergism between temperature and estradiol: A common pathway in turtle sex determination. The Journal of Experimental Zoology 260: 130-134.
Wibbels, T., J.J. Bull, and D. Crews 1991b. Chronology and morphology of temperature dependent sex determination. The Journal of Experimental Zoology 260: 371-381