15 June 2017: A University of Canberra-led research collaboration has discovered the key to how temperature affects sex determination in reptiles.
The team has discovered what could be the master switch for environmentally influenced male and female development, a temperature-sensitive molecular signature in a family of genes which control the expression of many other genes.
Many reptiles, such as crocodiles and marine turtles, don’t have sex chromosomes like humans and other mammals. Instead, their gender is set by the temperature at which their eggs are incubated.
Scientists have been working to discover how this sex-switching happens for at least 50 years, with the team’s findings published in the international journal Science Advances.
Lead author Dr Clare Holleley, CSIRO Research Scientist and an Adjunct Associate Professor at the University of Canberra’s Institute for Applied Ecology, said the key to the breakthrough lies within Australia’s bearded dragon lizard, which the team has been studying for years.
“The dragon lizard has sex chromosomes similar to birds that determine sex at normal temperatures. But at high temperatures, embryos with male sex chromosomes reverse sex and hatch as females,” Dr Holleley said.
Dr Holleley and her co-lead author Ira Deveson, a PhD student from the Garvan Institute of Medical Research in Sydney examined the sex-reversed female dragons.
“Our work looked at all the messenger RNA molecules that were made by dragons that were functional females, even though genetically they were male. We compared these molecules with RNA made by normal males and normal females,” Dr Holleley explained.
Mr Deveson said their work uncovered a unique difference.
“We found that sex-reversed females produce a unique message, with their RNA retaining a chunk of sequence that is normally spliced out of the message.”
“This means that the gene won’t make a normal protein. Somehow that throws a spanner in the works when it comes to making a male,” he said.
Even more surprising was the finding that the same gene retains this chunk in crocodiles and turtles. This suggests that temperature-determined sex depends on this “alternative RNA” in all reptiles.
“We think our discovery will spark a whole new approach to understanding how to make males and females – in all animals,” La Trobe University Distinguished Professor and University of Canberra Adjunct Professor Jenny Graves added, who collaborated on the research.
“There are many different ways males and females are determined throughout nature, this breakthrough moves us all a step closer to understanding the whole picture of sex.”
Other contributors towards the discovery include University of Canberra Distinguished Professor Arthur Georges, whose previous work with Dr Holleley and others in 2015 identified the first instance of temperature-driven sex reversal in bearded dragons in the wild, which was published in Nature, and Professor John Mattick AO FAA (Executive Director, Garvan), a pioneer in understanding the role of RNA in gene regulation and development.