One species of Australian lizard switches sex when temperatures rise in the outback, a new study has found.
Central bearded dragons (Pogona vitticeps) with male chromosomes become females that are capable of producing twice as many eggs as standard females, report researchers in today's issue of the journal Nature.
"Dads make better mums," quips senior author applied ecologist, Professor Arthur Georges of the University of Canberra.
The findings suggest that as global temperatures rise, bearded dragons can quickly change their sex determination process from one governed by chromosomes to one governed by temperature.
Male bearded dragons usually have two Z chromosomes and females usually have a Z and a W chromosome. At lower temperatures, the female determines the sex of her offspring by passing on either a Z or a W chromosome.
Georges and colleagues have previously found evidence in the laboratory that if the reptile's eggs are exposed to temperatures above 32.5°C, ZZ males can reverse their sex and become females, even without a W chromosome.
"What we've shown now is that this isn't just some aberrant laboratory artefact," says Georges. "This actually occurs in the wild, and the animals are [sex] reversing up in Western Queensland."
For their study, Georges and colleagues went to 'Lizard Country', at Eulo in Western Queensland's semi-arid region.
Over a number of years they examined 131 bearded dragons in the field. They compared their genetic sex, determined by blood samples taken from the animals' tails, and their actual sex, as determined by their size, behaviour and gonads.
Eleven dragons turned out to be reversed sex males, in other words, females with ZZ chromosomes.
This is the first case of sex reversal seen in a terrestrial vertebrate in the wild, says Georges.
Viable offspring
In a breeding population back in the laboratory, Georges and colleagues produced ZZ females and mated them with ZZ males and showed they produced viable offspring.
They then carried out a modelling exercise to see what would happen to a population under increasing environmental temperatures.
At low and intermediate temperatures the offspring of ZZ females were all male, but these males were more likely to reverse sex than males born to ZW mothers.
"The tendency was inherited," says Georges.
As the temperature increased more and more of a ZZ female's offspring would develop into females, producing an oversupply of females.
This resulted in a genetic selection against the W chromosome in an attempt to correct the sex imbalance.
"Normal [ZW] females get driven to extinction and you end up with just ZZ animals," says Georges.
When this happens, there is no genetic sex determination mechanism left and only temperature remains as the mechanism by which sex is determined.
At around 35°C, 100 per cent of the offspring would be ZZ females -- a situation which could be potentially "catastrophic", says Georges.
But, he says, it is quite possible that in the field there are other yet-to-be-discovered feedback mechanisms that prevent this scenario.
While producing so many females seems like "a step in the wrong direction", Georges says it may actually help in a situation where climate is shifting habitat.
"If your range is moving in response to climate and you're producing more females than males then you can invade new territory quicker," he says.
This is because females produce so many eggs, which means more individuals to populate new areas. In fact, ZZ females lay twice as many eggs as ZW females, says Georges -- a finding which supports this hypothesis.
http://www.abc.net.au/science/articl...02/4264672.htm
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