COLDER INCUBATION MAKES BABY TURTLES MALE, AND NOW WE KNOW HOW

While a genetic coin-flip determines the sex of a human baby, turtles have a more interesting method. A turtle egg hatches male or female depending on the temperature of its nest, and scientists from Duke University in the US and Zhejiang Wanli University in China have found out how that mechanism works.

For a common species called the red-eared slider, eggs incubated at 32°C produce female hatchlings, while those at 26°C hatch as males. This study showed that cooler temperatures turn on a gene called Kdm6b which in turn flicks a biological switch called Dmrt1 triggering the development of testes and resulting in the birth of a male turtle. If that gene is not turned on, testes don’t develop and the turtle hatches as a female.

Kdm6b activates the Dmrt1 switch by modifying histones; proteins that wrap themselves around DNA inside the cell nucleus and stop the genes from working.

“It’s like taking the brakes off the male pathway,” said study co-author Ceri-Weber, PhD candidate.

While it isn’t known exactly why turtles and other reptiles have developed this method of sexual differentiation, it can leave them vulnerable to climate change. Dr Blanche Capel, lead author of the study, said that some nests are already producing very few male hatchlings.

“The males who are born at a higher temperatures likely have a genetic make-up that makes them resistant to switching at high temperatures,” she said. “And since they’re the only game in town, they’re going to be producing the next generation. So we can hope that at a population level, nature has a way of sorting this out.”

Red-eared sliders are referred to as turtles in the US, but in the UK we call them terrapins. They are native to the US, but have been introduced to many other countries due to their popularity as pets

The centre image is a developing turtle gonad that was incubated at a temperature that should have turned it into a testis (left). But when the researchers knocked down its Kdm6b gene, it started turning into an ovary (right) instead