Dragon lizard embryos change sex when temperature rises

Hotter temperatures are messing with the gender of bearded dragon lizards, a new study finds.

Dragons that are genetically male hatch as females and give birth to other lizards. And the way the lizards' gender is determined is getting changed so much that the female sex chromosome may eventually disappear entirely, say the authors of the study, done in Australia.

• Weed killer causes male frogs to lay eggs
• Fish feminizing in Alberta rivers

"This is the first time we have proved that sex reversal happens in the wild in any reptile at all," said Clare Holleley of the University of Canberra, lead author of the study, published Wednesday in the journal Nature.

The research, she said, "is showing that climate extremes can very rapidly fundamentally alter the biology of an organism."

To understand what's happening, it helps to a have a quick lesson in the birds and the bees and the bearded dragons and other reptiles. Some reptiles, like alligators and some turtles, have their genders determined not by sex chromosomes, like humans and other mammals, but by temperature during incubation.

Until now, bearded dragons had their gender based on chromosomes. Like birds, their sex chromosomes are Z and W instead of X and Y. Males are ZZ. Females are ZW.

In humans, everyone has an X and the presence of Y makes a person genetically male. In bearded dragons, everyone has a Z and the presence of a W makes them a genetic female.

In the past, scientists have shown in the lab that hot temperatures can switch that natural chromosome-based gender.

• Read landmark 2007 paper on sex reversal in bearded dragons

Holleley and colleagues examined the genetic sex markers of 131 wild-caught bearded dragons in Queensland province in northeastern Australia and found that 11 of them were female outwardly even having offspring but had the ZZ chromosomes of a genetic male. Their sex determination was "switched into overdrive," Holleley said.

Holleley concedes 11 dragons is a small sample size, so she and colleagues will continue and expand their research.

Males are better moms, in a way

The genetic-male-into-female dragons not only laid eggs, but in a way were better mothers than genetically determined females, laying more eggs, said study co-author Arthur Georges, chief scientist for the Institute for Applied Ecology at the University of Canberra.

The team also found that the offspring of these dragons no longer have their gender determined by chromosomes, but by temperature.

This is happening in an area that is one of the fastest warming places in Australia over the past 40 years, Holleley said. Lab tests show that the switch from genetic sex determination to temperature sex determination seems to start at about 32 degrees Celsius and occurs 100 per cent of the time at about 36 degrees, Holleley said.

Scientist Frederic Janzen at Iowa State University, who was not involved in the research, praised the work as thorough and convincing. He said in an email that the study can help "to better understand these remarkable animals in a rapidly changing world."