Identity crises are not usually among the problems associated with climate change, but for some reptiles living in a world that's getting increasingly warmer, this might just be one more thing to worry about (if they think about these kinds of things, that is.) Unlike humans, the gender of reptiles such as crocodiles and marine turtles is determined by the temperature during the incubation process. When the mercury rises, their eggs hatch as females. For 50 years, scientists have been trying to figure out how this actually happens and now, scientists in Australia think they might have finally cracked the code.
For a long time, it was believed that environmental and genotypic sex determination were two different and separate things – i.e. that an organism's sex was determined either by environmental cues, such as temperature, after conception or genetic factors at the point of conception. However a few years ago, Australian researchers called this theory into question by showing how both these factors are involved in determining the sex of the bearded dragon lizard. Like humans, they typically inherit sex chromosomes from each parent, though in their case, they receive a combination of a Z and W chromosome instead of an X or a Y. Male lizards have a ZZ combination while females have a ZW one.
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What the researchers found during the study was that they could change the unborn reptile's gender by turning up the temperature dial. Anything above 33 degrees Celsius (91.4 degrees Farenheit) and the resulting hatchling would be female, even if they had ZZ chromosomes. In other words, they were genetically male lizards with functioning lady parts, that could mate with other males and lay eggs. This process also eliminated the W chromosome altogether, meaning that the hatchlings of these sex-reversed females could in turn only produce offspring determined solely by temperature. One potentially disastrous outcome for these reptiles is that climate change could drive them to extinction since higher temperatures would eventually result in an all-female population.
"The dragon lizard has sex chromosomes similar to birds that determine sex at normal temperatures," explains lead author and CSIRO scientist Clare Holleley, who was involved in that study as well as the recently published one. "But at high temperatures, embryos with male sex chromosomes reverse sex and hatch as females."
So what exactly causes this to happen? Hollely and her colleagues believe the key to this genetic mystery could lie in a biological master switch found in the protein-making process.
At the heart of this study is the messenger ribonucleic acid (RNA), which is responsible for carrying the blueprint from the DNA to provide instructions on how to build a particular protein that contributes to a genetic trait, including sex, in the ribosome. The researchers thus decided to look at all the messenger RNA molecules that were made by the sex-reversed females and compare them with those made by normal males and normal females. What they discovered was one crucial 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," explains co-author and researcher Ira Deveson. "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."
What make it even more intriguing is that this chunk is not unique to the bearded dragon but is also found in crocodiles and turtles, which suggests that it could be responsible for temperature-determined sex in all reptiles.
"We think our discovery will spark a whole new approach to understanding how to make males and females – in all animals," says La Trobe University geneticist Jenny Graves, who was part of 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."
The study was published in Science Advances.
Source: University of Canberra