Genetic sex determination is common to many species. For example, in most mammals and various insects, the XX/XY chromosome system is employed. In this system, individuals inherit a pair of chromosomes that determine their sex. Females usually have two X chromosomes, while males usually have one X and one Y chromosome.
Turtles seem to be an exception to this general rule, as genetics do not play a significant role in sex determination. Rather, research has shown that turtles exhibit temperature-dependent sex selection. The mechanism behind this is not fully understood, but laboratory studies found that incubating eggs at temperatures above 35°C results in primarily female offspring while incubation at a cooler temperature of 25 °C results in primarily males.
This makes turtles highly sensitive to climate change, as extreme changes in temperature have the potential to shift the offspring sex-ratio and thus negatively impact turtle populations. Turtles have withstood several periods of extreme climatic conditions, suggesting that there is another factor that contributes to the determination of turtle sex.
Previous studies conducted at the Chinese Academy of Sciences reported that turtles are able to move around in the egg for thermoregulation. A recently published follow-up study aimed to investigate if this movement has any effect on sex determination.
To investigate this, the researchers coated turtle eggs with Capsazepine, a chemical that when applied, inhibited turtles from being able to sense temperature. Unlike uncoated eggs, these embryos did not move within the egg and developed to be all male or all female depending on the external temperature. Embryos that were uncoated were able to move in response to changing temperatures. Half of these embryos developed as males and half developed to be female.
While these results are promising, the control that turtles have over their own sex determination likely has limitations, as described by the study author, Wei-Guo Du:
"The embryo's control over its own sex may not be enough to protect it from the much more rapid climate change currently being caused by human activities, which is predicted to cause severe female-biased populations." However, the discovery of this surprising level of control in such a tiny organism suggests that in at least some cases, evolution has conferred an ability to deal with such challenges."