Temperature-dependent sex determination
As temperature-dependent Geschlechtsdetermination , TGD (temperature-dependent sex determination, TSD) a form of forming the is sex named in animals, among other things, at different egg-laying reptiles occurs, such as at turtles , lizards and crocodiles .
With temperature-dependent sex determination, the sex of the offspring is not determined by sex chromosomes , but by the temperature that prevails in the nest during a specific period of time. This so-called thermosensitive phase lies in the middle third of embryonic development and lasts e.g. B. in turtle species with temperature-dependent sex fixation and at constant incubation temperatures about 12-15 days. Three types of temperature-dependent sex determination are known: Type 1a produces females at high temperatures and males at low temperatures; Type 1b males at high temperatures and females at low temperatures; in type 2, low and high temperatures favor females, while medium temperatures favor males. The type of nest, which in many crocodiles is built from fermenting plants, for example, and in the case of buried eggs, the depth at which the eggs were laid, usually determines the sex-determining temperatures.
The enzyme responsible for temperature-dependent sex determination is aromatase . It converts the male sex hormone testosterone into the female sex hormone estrogen . The activity of the aromatase increases with increasing temperature. At higher temperatures z. For example, turtles tend to have a female gender phenotype, and low turtles a male one. With alligators, however, it is exactly the opposite. If the eggs of these animals are exposed to high temperatures (around 34 ° C) in the 2nd or 3rd week, males develop, while females develop at only around 30 ° C.
TGD is also widespread in fish, where, in addition to temperature, water-chemical factors or stress can also play a role in gender expression.
Individual evidence
- ↑ Claude Pieau et al .: Temperature-dependent sex fixation in pond , water and land turtles , Marginata 1 (4) 2004