Dragon sex. It turns out it’s complicated.
Whether the Australian central bearded dragon grows up to be a male or a female depends not only on its genetics but also on the temperature of the nest in which it is incubated.
What’s more, the females with male sex genes grow up to display many male behaviours – being bolder and more sociable. And in the lab at least, that appears to produce more offspring.
The findings, published in the Proceedings of the Royal Society B, overturn assumptions that sex is caused either just by genetics or just by the environment and show how, under the right conditions, a species could rapidly and completely shift from one way of determining sex to the other.
Hong Li from the University of Sydney and colleagues carefully manipulated the temperature at which the lizards’ eggs were incubated, and then ran a series of tests on the lizards that hatched.
They found that if eggs with typically male sex chromosomes were incubated in a nest at 32C or greater, they emerged as females. Below that temperature, sex appeared to be completely determined by their genetics.
Despite those sex-reversed females being functionally female – clearly having the required hormones and successfully reproducing – they maintained many male traits, some of which were even exaggerated in the temperature-induced females.
The temperature-induced females developed with a normal body mass for a female – which is bigger than that of a male.
But when it came to behavioural traits, the temperature-induced females were much more male-like, suggesting those traits were determined by the sex genes.
Usually, male dragons tend to be bolder, more comfortable with new situations, and more sociable. In lab tests, the temperature-induced females were more male-like in these traits, and on some measures more so than normal males.
“What seems to happen with the environmental switch is that your reproductive system turns into a perfectly functional female reproductive system but there are aspects of your brain that are not as dramatically affected and they retain some influence of your male genetic heritage,” said Rick Shine, a co-author on the study from the University of Sydney. “So you end up with an individual that really in some ways combines the characteristics of both sexes.”
Shine said it was not clear how these traits would play-out in the wild, but in the laboratory, they meant that the temperature-induced females were more successful than the genetically-determined females.
Shine said he knew of only one other animal – an Australian skink – that had been seen to have its sex determined by both genetic and environmental factors.
It was previously believed that each mechanism was mutually exclusive. Many animals rely on temperature or sex, Shine said only these two lizards were known to rely on both.
However, by examining evolutionary trees, Shine said it was clear species had often shifted back and forth between the two. And these findings could explain how those transitions occurred.
“Under the right circumstances, this could create a very rapid transition favouring the environmental sex determination,” Shine says. Within a generation, he said, all the genetic females could be eliminated, leaving a population where females could only be induced by temperature.
How climate change would impact animals with temperature-dependent sexes was unclear, Shine said, but there were indications that it was shifting their balance.
This article was first published by The Guardian on 08 Jun 2016.