The Australian central bearded dragon Pogona vitticeps, a favourite of amateur herpetologists, has a reproductive trick that might have appealed to a mythic female firedrake just a breath away from extinction. Just heat your eggs above 34°C and turn some or all of the male embryos into females.

Like birds, P. vitticeps has a ZW sex-determination system that, all things being equable, produces a 50:50 ratio of male to female offspring. But in torrid times, incubation temperatures between 33 and 37 degrees can override the as yet-unidentified Z-chromosome gene that determines maleness, skewing sex ratios increasingly towards females until, at 36°C, all hatchlings are female.

In the April 20, 2007, issue of Science, University of Canberra molecular ecologist Associate Professor Stephen Sarre, his UC colleagues Professor Arthur Georges and PhD student Alexander Quinn, the Australian National University's Dr Tariq Ezaz and internationally renowned sex-determination expert Professor Jenny Graves, reported temperature-induced sex reversal in P. vitticeps embryos under laboratory conditions.

But they have also found a lone, phenotypically female but karyotypically male (ZZ) central bearded dragon, in the Australian arid zone, confirming that temperature-induced sex reversal can indeed occur in nature.

The Canberra researchers chose P. vitticeps as their model species because it breeds readily in the captivity and because it was known to exhibit genotypic sex determination (GSD). But they are also interested in two sister species of Amphiboluris bearded dragons.

A. norrisi and A. muricatus exhibit GSD and Temperature-dependent Sex Determination (TSD) respectively. Sarre says genetic comparisons of two species may yield a potential entry point into the sex-determination pathway.

In A. muricatus, incubation temperature is the key determinant of sex ratios - at both higher and lower temperatures, the sex ratio skews towards females. Sarre believes that, in the GSD species P.vitticeps, the "maleness" gene - or genes - determine sex at incubation temperatures up to about 32°C, but is overridden by temperatures beyond 32°C. When the heat is on, boys will be girls.

Could the interaction of two modes of sex determination in P.vitticeps confer a survival advantage through the manipulation of sex ratios in Australia's unpredictable climate?

"It's a difficult thing to demonstrate, especially when you've got a long-lived species, because it suggests the females are making judgments about which is going to be the advantageous sex some years down the track," Sarre says. "It would be more likely in a short-lived species.

"Finding a sex-reversed individual in nature throws a spanner in the works, because it shows that some natural P.vitticeps nesting sites have the potential to alter the frequency of sex chromosomes, as well as changing the phenotypic sex ratio."

X, Y, Z

In all birds and some reptiles - particularly lizards - females are the chromosomal "odd couple" with a lone Z chromosome, pigeon-paired with a W chromosome. Males have a ZZ karyotype. This system differs from the XY sex-determination system of eutherian and marsupial mammals, where males are the heterogametic sex, and carry a male-determining gene on their truncated Y chromosome. In XX embryos, development defaults to the female pathway.

All crocodilians - crocodiles, alligators, caimans and gavials - have TSD; all snakes have ZW sex chromosomes, while turtles and lizards have plumbed the full range of possibilities: some species have ZW, others use a mammal-like XY system, while many others have temperature-determined sex determination.

A rare few, like isolated populations of the WA gecko Heteronotia binoei, have even resorted to parthenogenetic procreation, producing female clones of themselves - possibly a consequence of dipping into the gene pool of a closely related species, consigning their male conspecifics to local oblivion, according to Sarre.

In the late 1990s, Sarre worked on temperature-determined sex determination in Sphenodon punctatus, one of New Zealand's two species of tuatara, a "living fossil" of uncertain affinity to lizards and snakes. In the chilly maritime environment of several offshore islands, tuatara embryos normally incubate at temperatures of 20-21°C. At a maximum of 25°C - the minimum incubation temperature for dragons - tuatara sex ratios skew towards female.

He returned to Australia in 2001, enticed by the unrivalled richness of the Australian reptile fauna, and the variety of their sex-determination systems, which had clearly evolved independently on multiple occasions in different taxa.

Sarre reasons that, if natural selection has found essentially the same solutions in both unrelated and closely related taxa, all reptiles must share a highly conserved suite of sex-determining genes and hormones. Both genetic and environmental mechanisms may be superimposed on this underlying system early in embryonic development.

Although some reptiles have a mammal-like XY sex-determination system, and presumably a "maleness" gene, like the mammalian SRY, no SRY homologue has been found in lizards.

"Obviously, some genetic switch other than SRY is operating in reptiles, and that's what we're trying to find," he says.

"We want to understand how sex is primarily determined in reptiles, and how the relationship between temperature and genes work."