Unlike mammals, some reptiles' sex is not determined by inherited chromosomes. Scientists now determine the gene responsible for temperature-dependent sex determination in turtles and other reptiles, unlocking a 50-year-old mystery.

Warmer Nests Result In Female Hatchlings

In humans and other mammals, sex is determined by inherited chromosomes. For reptiles such as turtles, alligators, and lizards, it's actually determined by the environmental temperature during a particularly sensitive stage of development. Warmer temperatures result in female hatchlings, whereas cooler temperatures result in male hatchlings.

For instance, red-eared slider turtle eggs incubated at 32 degrees Celsius yield female hatchlings, while eggs incubated just a few degrees cooler at 26 degrees Celsius yield male hatchlings.

Interestingly, this temperature-based sex-determination is not a new discovery, but it has actually been known to scientists for over 50 years. However, the exact mechanisms behind it remained a mystery.

Genetic Mystery Unlocked

In previous studies, researchers determined that a gene called Kdm6b was one of the first genes to be activated during early development and that it is more active in cooler temperatures but silent at warmer temperatures.

In a new study published in Science, researchers gathered newly laid eggs and incubated them at either 32 degrees Celsius or 26 degrees Celsius. This way, they were able to observe how the genes are turned "on" early in the development before the turtles are determined to be male or female. They also conducted experiments that practically stamped Kdm6b's role in temperature-dependent sex determination.

Using a new technique, researchers suppressed the Kdm6b in a developing turtle embryo and found that doing so resulted in a female hatchling even if the egg was incubated in temperatures that would normally result in a male turtle.

In addition, further experiments revealed that Kdm6b also interacts with the genome region Dmrt1, which is strongly related to testis development. When Kdm6b activity interacts with Dmrt1, it is made easier to access and read.

"It's like removing the brakes off the male pathway," said Ceri Weber of Duke University, coauthor of the study,

Sex Determination In Reptiles

Evidently, this temperature-related shift in Kdm6b activity is also observed in other reptile species whose sex is determined by incubation temperature, suggesting that the mechanisms observed in the turtles are likely present in other reptiles as well.

However, researchers believe that Kdm6b does not detect temperature changes because it does not affect other developing temperatures. As such, the next course of action for researchers is to determine the temperature-sensing triggers.

Why is it important to understand sex-determination in reptiles? For one thing, such creatures are a significant part of ecosystems worldwide, and understanding the mechanisms in which their population's sex ratio is determined is significant to species survival and ecosystem health.

For instance, in the beginning of 2018, researchers found that rising temperatures have turned a majority of the Great Barrier Reef's green turtle population into females. This is rather problematic because such feminization could lead to population collapse or even extinction.

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