Sex change: hot days turn boys into girls, reptile study finds

Edward O'Daly

1 May 2007: Hot weather can turn male dragons into females, according to research published in the prestigious international journal Science by a team from the University of Canberra, working with colleagues from The Australian National University.

A study into Australia's bearded dragon, which, like humans, other mammals and birds, has sex chromosomes to determine sex genetically, has found this genetic sex can be reversed by high temperatures. Sex is determined in some reptile species by temperatures experienced in the egg, but it was previously thought that sex was determined either by genes or temperature but not a combination.

Female dragons can be genetically male, the researchers found Photo: Alex Quinn

The discovery brings scientists a step closer to understanding the biological processes behind determining sex, but alarmingly reveals that more species are potentially under threat from climate change.

The discovery is the exciting outcome of an Australian Research Council-funded collaboration between the study's lead author Alex Quinn, ecologist Professor Arthur Georges and geneticist Dr Stephen Sarre from the Institute for Applied Ecology at the University of Canberra and chromosome experts Professor Jenny Graves and Dr Tariq Ezaz from the Comparative Genomics Group at the ANU Research School of Biological Sciences.

Hot incubation temperatures of 34 degrees or more during the middle third of development caused the genetically male dragons to develop as females, researchers found. Even though the dragons then hatched as fully formed females, they retained their male chromosomes.

“These findings dramatically change what was traditionally thought about sex determination,” Mr Quinn said.

“Traditionally scientists have regarded the mechanisms of sex determination, genetic and environmental, as fundamentally different. We've now found the two mechanisms can coexist in one individual.

“This discovery has given us vital clues in unravelling the mystery of sex determination in all living creatures, which is very exciting. However a worrying implication is that more species than previously thought could be impacted by the rising temperatures associated with climate change.”

The bearded dragon has Z and W chromosomes. A male has two Z chromosomes and a female has one Z and one W. In humans, females have two X chromosomes and males one X and one Y.

“What we have found is the dragon equivalent of a fully formed human male with no Y chromosome,” Mr Quinn added.