Genes versus heat – a reptile sex trigger

High temperatures can make an Australian lizard that is genetically male develop into a female. The finding throws new light on how sex is determined in reptiles.

For most reptiles, a gene on a sex chromosome triggers an embryo to develop as either a male or a female. In some species, males have an X and a Y chromosome, while females are XX, as in mammals. In other species of lizards, males are ZZ while females are ZW, as in birds.

But for a third group of reptiles, which includes all crocodiles, alligators and marine turtles, temperature, rather than a gene on a sex chromosome, triggers either male or female differentiation. Extreme low or high temperatures generally lead to more females.

Now a team led by Alex Quinn at Canberra University in Australia has found that the central bearded dragon (Pogona vitticeps) is susceptible to both types of sex trigger, and that temperature can override its genetic gender.

Transitional form

When the team incubated eggs at relatively high temperatures – between 34°C and 37°C – the majority of embryos that had ZZ sex chromosomes (genetically male), hatched as females. The team thinks the bearded dragon represents a transitional form, in evolutionary terms, between the two main methods of sexual determination.

The research shows that, for the bearded dragon at least, the W chromosome is not necessary in producing a female. The team suspects that a double dose of a particular gene on the Z chromosome is instead crucial for maleness, and that this gene is inactivated by high temperatures.

“The possibility that there is a male-determining, dosage-dependent gene on the Z chromosome of bearded dragons is an important insight,” says Quinn, “because to date, scientists have discovered the master sex-determining gene only in mammals and a single species of fish.”

The team plans to hunt for that master gene in the bearded dragon. They also want to investigate how widespread the phenomenon of temperature sex reversal really is in reptiles.

If many other reptiles with sex chromosomes are also susceptible to temperature, this would broaden the number of species that could be vulnerable to climate change.

“The concern is that the current rate of climate warming could be too rapid for these species to adapt to, and this could potentially result in heavily skewed sex ratios, and even population crashes in some cases,” Quinn says.

Journal reference: Science (vol 316, p 411)