The sexes of many baby reptiles are determined by the temperature that the eggs experience while being incubated in the nest. What’s more, lizards in the lab have been known to switch to this temperature-dependent system from one in which sex is determined by chromosomes (as in humans). And now, researchers have discovered the world’s first naturally occurring case: Australian central bearded dragons (Pogona vitticeps) are susceptible to climate-induced sex reversal in the wild, according to findings published in Nature this week.
Human females have two X chromosomes, while males have an X and a Y. This is flipped with Australian bearded dragons: Females have a Z and a W, while males have two Z chromosomes. But while ZW lizards are always females, ZZ lizards could become either sex. Previous experiments found that high incubation temperatures “feminize” chromosomally male lizards, producing so-called “sex reversed” females (ZZf). How do different sex determination systems coexist in climate-sensitive reptiles, and what triggers the switch between them?
A team led by Clare Holleley from the University of Canberra collected tail snips and blood from 131 wild adult lizards living in widely distributed populations of semi-arid Australia. With molecular analyses, they discovered that 11 of those lizards – found toward the warmer end of the species’ range – had a male set of chromosomes but were female (that is, they lacked the paired genitalia known as hemipenes). These lizards had undergone a rapid transition from a genetically-controlled system to one that’s dictated by temperature.
Then the team mated sex-reversed ZZf females (including both wild-caught as well captive-bred females) with ZZ males. Their offspring were viable and fertile and, as expected, the W sex chromosome was eliminated. Their sex was entirely determined by egg incubation temperature, which ranged from 26 up to 36 degrees Celsius. In the absence of a W chromosome, female offspring are only possible via sex reversal. While low incubation temperatures of 28 degrees Celsius produced all male ZZ hatchlings, by 34 degrees Celsius, the offspring were predominantly female ZZf hatchlings.
Furthermore, with “normal” ZW females, temperature began to override chromosomal sex determination – and cause sex reversal – from above 32 degrees Celsius, resulting in an increasing proportion of female hatchlings as temperatures rose. Additionally, offspring of sex-reversed ZZf mothers are also more predisposed to reverse, reinforcing the transition. Also, sex-reversed mothers lay nearly twice as many eggs per year as control ZW mothers – which may lead to an excess of females in the population.
The findings suggests that climate extremes might alter lizard genomes, although perhaps being more flexible with sex determination could prove to be useful as the climate becomes more unpredictable.
Images: Arthur Georges