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Confused Sex Life of Tuatara

There's two sexes, right? Male and female -- well, perhaps not if you're a tuatara.

Dr Stephen Sarre, a molecular ecologist from Massey University with a special interest in reptile genetics, has a theory that New Zealand's own little Jurassic survivor, the tuatara, might have four sexes, or another two variations on the common male/female theme.

Laboratory research has found that eggs of the common tuatara species, incubated at 22oC, will produce mostly male tuatara. Incubated at 18oC, the eggs will turn into females. At the pivotal point of temperature, it could go either way.

This trait puts them in a category with a number of other reptiles such as crocodiles and some turtles, and is being used in a tuatara captive incubation programme to help increase numbers on outlying islands of New Zealand.

But Sarre says studies of the DNA of tuatara have found there appear to be differences in the genetic make-up of males and females. If the differences are found to be significant, this would indicate the sex of tuatara is determined not only by temperature during incubation, but also by genetics and the interplay between the two.

"A study of tuatara chromosomes published in the 1960s did not show obvious sex chromosomes, like X and Y that humans have, so it has been assumed their sex was not determined by genetics. But if we find that there is a significant difference between male and female DNA, and that sex can be determined by temperature during incubation, then this would imply they could have four sexes," says Sarre.

"Males that are genetically male and look male; males that are genetically male, but look like females; genetic females that look like females; and genetic females that look like males."

Sarre says their findings could have major implications for the tuatara captive incubation programme, especially if they find that genetic males or females that become their opposite because of the temperature in their environment during incubation, are less fit or have problems reproducing.

"Tuatara are unique reptiles, they have been in their own lineage for over 200 million years, since they branched away from lizards and snakes. We know a lot about some of the characteristics of tuataras but little about others. Because they can live a very long time, up to 100 years, researchers have to live a long time to study their life cycle."

While mammals are a long way from tuatara on the evolutionary tree, Sarre says their research might provide a unique model for studying genetic and environmental interactions in the development of males and females.

Sarre, along with colleagues from Victoria and Sydney University, has support from the Marsden Fund to investigate how sex is determined in tuatara. This collaboration between universities brings together expertise in molecular genetics with people who have experience with tuataras in the laboratory and in the wild.