For humans and other mammals, sex is neatly determined by the X and Y chromosomes. If you have a Y you are male, and without it you are female. Reptiles however, use a variety of strategies, and the mammalian X/Y system is just one of them.
In some species, the female is the one with different chromosomes, in this case Z and W, and the male has two Zs. And some reptiles ignore sex chromosomes altogether. For them, an individual’s sex is determined by the temperature that their eggs were incubated at.
Scientists had long believed that these strategies were mutually exclusive with each species choosing one of the other.
But Alexander Quinn and colleagues form the University of Canberra have found that an Australian lizard, the central bearded dragon (Pogona vitticeps), flouts this rule. It has become the first animal known to use two separate methods to determine the sex of individuals.
The dragon uses the Z/W system, where the males carry two Z chromosomes and the females have a Z and a W. But Quinn found that these genes are only the dominant influence on gender if eggs are incubated between 20 and 32 degrees Celsius.
At higher temperatures, males ignore their genetic heritage and become females instead. When Quinn incubated broods of eggs between 34 and 37 degrees Celsius, the hatchlings were almost invariably female. And as predicted, about half of these sisters were genetically male. For dragons at least, when the heat is on, the men turn into women
Quinn believes that the key to the manliness of boy dragons lies in a temperature-sensitive protein produced by the Z chromosome. The protein’s activity needs to surpass a certain threshold before a dragon can become male. For that, there need to be two copies of Z, and the temperature must be just right.
Reptiles that use temperature to assign gender must have fine-tuned their systems over time to cope with an ever-changing environment. But Quinn fears that the current pace of climate change may be too rapid for these animals to adapt to.
If temperatures rise far enough to bias an entire species over to a single gender, extinction would be all but inevitable. These warnings have been sounded before, and Quinn’s work suggests that they should be shouted a little bit louder.
Reference: Quinn, Georges, Sarre, Guarino, Ezaz & Graves. 2007. Temperature sex reversal implies sex gene dosage in a reptile. Science 316: 411.