Renaissance lizards

By | Science & Technology
The Australian Central Bearded Dragon is a species of agamid lizard found in arid regions of Australia. Credit@Tambako The Jaguar/Flickr

The world’s first example of climate-induced sex reversal in the wild was discovered last week. The finding was published on the cover of the international journal Nature and demonstrates how male Australian central bearded dragons (Pogona vitticeps) have the unique ability to undergo climate-induced sex changes.

The researchers at the University of Canberra who have been studying Australia’s bearded dragons for a long time were able to show that a reptile’s sex determination process is able to switch rapidly from one determined by genetics, to one determined by temperature.”We had previously been able to demonstrate in the lab that when exposed to extreme temperatures, genetically male dragons turned into females,” said Lead author Dr. Clare Holleley, a postdoctoral research fellow at the University of Canberra’s Institute for Applied Ecology. “Now we have shown that these sex reversed individuals are fertile and that this is a naturally occurring phenomenon.”

In humans, females have two X chromosomes, while males have one X and one Y chromosome. Bearded dragons are little different – females have a Z and a W chromosome; males have two Z chromosomes. Furthermore, while ZW lizards are always female, ZZ lizards have the ability to become either sex. Previous research has shown that higher incubation temperatures may “feminise” lizards who are genetically male to produce “sex reversed” females (ZZf). However, when Dr. Holleley and colleagues observed this phenomenon in the wild, the question was: how might several sex determination systems have arisen in these reptiles and what prompts the switch between them? “[Nobody] knew whether this was going to contribute to the population, or if it was just some freaky weird thing that happens when it gets hot,” says Dr. Holleley.

To begin answering the question, molecular analyses were performed on tail snips of 131 adult lizards. The team soon discovered that some warmer lizards had the male ZZ chromosomes, however were biologically female.

Adults can reach up to two-feet in length and can vary wildly in colour. Credit@Frank Paul Silye/Flickr

Adults may reach up to two-feet in length and may vary wildly in colour. Credit@Frank Paul Silye/Flickr

In addition, the team found that temperature began to override chromosomal sex determination “normal” ZW females and cause sex reversal from above 32 degrees Celsius. This appeared to produce an increasing proportion of female hatchlings as temperatures rose. It was also observed that sex-reversed ZZf mothers are more likely to reverse and even lay more eggs per year as regular ZW mothers, leading to an excess of females in a population. Therefore, rising temperatures might lead to a sex change cascade that drives populations, or even an entire species, towards being totally female.

“By breeding the sex reversed females with normal males, we [aimed to] establish new breeding lines in which temperature alone determined sex. In doing so, we discovered that these lizards [might] trigger a rapid transition from a genetically-dependent system to a temperature-dependent system,” Dr. Holleley explained. “We also found that sex-reversed mothers (females who are genetic males) laid more eggs than normal mothers. So in a way, one [might] actually argue that dad lizards make better mums.”

Distinguished professor at University of Canberra and senior author of the paper emphasised the importance of this research to the broader understanding of sex determination in animals. “The mechanisms that determine sex have a profound impact on the evolution and persistence of all sexually reproducing species. The more we learn about them, the better-equipped we’ll be to predict evolutionary responses to climate change and the impact this [may] have on biodiversity globally.”

Overall, the research indicates that climate extremes may be responsible for modifying lizard genomes. With climate change, the sex reversal may increase exponentially, especially since ZZf mothers lay more eggs with a higher chance of growing up to be female, leaving few eligible lizard bachelors for the picking. Nevertheless, the team suggests that this flexibility in sex determination may prove useful, as the climate grows warmer.

What productive benefits might there be in the ability to naturally switch between sexes?

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