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Sex-change lizards settle a hot topic

An Australian research team has provided what they say is the "Holy Grail" of evolutionary biology - proof a 30-year-old theory about reptile sex and survival is right.

The findings explain the evolutionary advantage of a reptile's sex being determined by temperature.

The University of Sydney's Professor Rick Shine and his former student Dr Daniel Warner, now of Iowa State University, report that the temperature at which a reptile egg is incubated not only determines sex but optimises the number of offspring in future generations.

Their findings, published online this week in the journal Nature, provide the first "unequivocal" demonstration that incubation temperatures affect the reproductive success of males and females.

In mammals and birds sex is determined by genotype at fertilisation resulting in roughly equal numbers of sons and daughters.

However in many reptiles and some fish, sex is determined after the egg is laid and is dependent on the environment, most commonly the temperature.

Shine, from the School of Biological Sciences, says in some species of reptiles only males or females will be born at certain temperatures.

He says about 30 years ago US biologists Professor Ric Charnov, now of the University of New Mexico, and Professor Jim Bull, of the University of Texas, suggested this "environmental sex determination" was not just a quirk of nature.

Rather they believed males or females incubated at certain temperatures had an evolutionary advantage - specifically, that they would have an optimised number of offspring.

But proving this theory was another matter and has since been the "Holy Grail" for evolutionary biologists, Shine says.

Experimental challenges

Shine says one challenge in proving the ideas of Charnov and Bull was finding a species with a short enough life span so it was practical to measure the entire number of off-spring it had over its life.

Most species with environmental sex determination have life spans of more than 60 years and reach sexual maturation late.

But Shine says the short-lived Jacky dragon (Amphibolurus muricatus), a common species of lizard found on the east coast of Australia, helped overcome this challenge.

The Jacky dragon produces off-spring within one year of hatching and lives no longer than four years.

Wrong sex

The other experimental challenge was to artificially produce the "wrong" sex at a given temperature.

Shine says female Jacky dragons are produced from eggs incubated at low (23°C-26°C) and high (30°C-33°C) temperatures whereas males are produced at intermediate temperatures.

In their Australian Research Council-funded study the researchers hormonally manipulating eggs to produce males and females at temperatures they would not normally be produced at.

He says, the hormonal manipulation of the eggs had no effect on the health and survival of individual hatchling Jacky dragons.

But the natural males were five to 10 times better in terms of mating and producing offspring, while the natural females produced four to five times more offspring, says Shine.

"Thus reproductive success of each sex was optimised by the incubation temperature that produces that sex in nature as predicted by the [Charnov-Bull] model," the researchers say.

Shine says he is "astounded" at how "beautifully" his team's data matches the Charnov-Bull model.

He has received an email from a "delighted" Bull congratulating him on proving the theory.