Central bearded dragons, which grow up to 60 cm in length, have a reverse system of sex chromosomes to humans.

Credit: Science/Alex Quinn

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CANBERRA: Sex determination in an Australian lizard depends on both genetics and temperature, say biologists who've found that heat makes genetic males develop as females.

The find challenges the traditional views of sex determination and may have implications for the affect of climate change on reptiles.

Experts had thought that the sex of reptiles was determined either by genes on sex chromosomes, which instruct the embryo to develop as male or female; or by the temperature at which embryos are incubated.

"For a long time it was thought that sex chromosomes were incompatible with temperature-determined sex," said geneticist Alex Quinn from the University of Canberra, lead author of a study revealing the find today in the U.S. journal Science.

But Quinn and his team thought there might be more to it, and looked a little deeper into sex determination in the central bearded dragon (Pogona vitticeps), a lizard with sex chromosomes that is found in much of central Australia.

Bearded dragons have a reverse system of sex chromosomes to humans. In humans, males have two different sex chromosomes, X and Y, and females have a pair of X chromosomes. The gene that determines maleness is found on the Y chromosome.

However, in bearded dragons, males have a pair of 'Z' chromosomes while the females have two different sex chromosomes, Z and 'W'.

Their experiments showed that incubating eggs at temperatures of 34°C or more, causes all genetically male dragons to develop as fully formed females, even though they retained their pair of male chromosomes. The researchers found that at lower temperatures, sex was completely determined by genetics.

"The simplest way to explain [our results] is that bearded dragons have a male-determining gene and it is carried on the Z chromosome," Quinn said. Developing bearded dragons need a high enough 'dose' from certain proteins on the Z chromosome to become male, he said. For embryos with only one Z chromosome, this dose is not reached, so it develops into a female.

The researchers speculate that temperature must somehow interfere with proteins formed from the Z chromosomes, thus lowering the dose, and allowing genetic males to develop as females.

"What we achieved was the first really conclusive demonstration that temperature can also determine sex in a species with sex chromosomes," Quinn said. "This is really exciting because it opens up more possibilities for sex determination not only in reptiles, but also in fish and amphibians."

"The more we find out about nature, the more our simple ideas… blur into much more complicated issues," commented evolutionary biologist Rick Shine from the University of Sydney. "I suspect that we'll find these multiple-factor systems to be very common, and this report is just the tip of the lizard sexual iceberg."

Scientists already new that species with temperature-dependent sex determination, like crocodiles and turtles, were vulnerable to climate change – as increasing temperatures may skew sex ratios. But this study indicates that reptiles with sex chromosomes, previously thought to be safe from this affect, could also be at risk.