Sea turtles may be more resilient to global warming than we thought

Sea turtles can cope with climate change better than we thought. Biologists fear the reptiles are at risk of extinction as warmer conditions encourage more turtle eggs to develop into females. But it turns out animals have a genetic safety net that could help them maintain a more even balance between the sexes even as temperatures rise.

“We believe we have uncovered the ability of turtles to adapt to the environment they are in,” he says Chris Eizaguirre at Queen Mary University of London.

The sex of sea turtle hatchlings is determined not by a sex-determining chromosome—as is the case in many animals, including humans—but by the temperature inside the nest. Laboratory studies have shown that at cooler nest temperatures more males will hatch and at higher temperatures more females, leading to concerns that global warming will cause more turtles to hatch as females.

For example, 2018 genetic study found that about 99 percent of young green turtles (Chelonia mydas) aged about 4 to 20 years from the warmer breeding grounds of the Great Barrier Reef in Australia were females. Modeling based on these results led to concerns that without sufficient numbers of males, sea turtles the population will collapse.

However, the true state of affairs after hatching is a mystery, as you cannot tell what the sex of a turtle is until it is several months old unless you kill it to check, so there is little field data on the sex of hatchlings.

To prevent this, Eizaguirre and his colleagues conducted laboratory and field experiments with turtles (Caretta caretta).

In the first part of their work, they collected a total of 240 eggs from seven nematode nests on beaches in Palm Beach County, Florida. They placed the eggs in artificial incubators at one of three temperatures: 27 °C (81 °F), the temperature that favors males; 30 °C (86 °F), “the key temperature for equal numbers of males and females; and 32 °C (90 °F), which should lead to females.”

When the hatchlings were 1 to 3 days old, the team took blood samples and then kept the turtles in captivity for months until they were large enough to verify the sex using keyhole surgery and a laparoscopic camera.

Comparing genome sequencing data obtained from sex-identified blood samples revealed that regardless of the temperature at which the eggs were incubated, male and female turtles had different patterns in the activity of hundreds of genes due to an epigenetic process known as DNA methylation. About 383 genes were hypermethylated in women – meaning they were less active than expected – and 394 were hypermethylated in men. Many of these genes have documented roles in sexual development. This meant that scientists could determine the baby turtle’s gender from a blood sample alone.

The team used this knowledge in a field study by collecting 29 newly laid loggerhead turtle eggs on the beaches of Sal Island in Cape Verde off the coast of West Africa. They divided each clutch, buried one half in a protected area 55 centimeters deep – where it would be cooler – and the other half 35 centimeters down, where it would be warmer, and monitored the temperatures.

When the researchers sequenced blood cell samples from 116 hatchlings, half from “cold” depths and half from “warm” depths, they found more males than expected given the temperatures the eggs had experienced. In fact, models based on incubation temperature overestimated the production of female hatchlings by 50 to 60 percent.

This suggests that in addition to providing a tool for determining the sex of turtles, the work shows that there are molecular mechanisms that help turtles cope with climate change by changing how sensitive their genital development is to temperature, Eizaguirre says.

“We’re not saying there’s no feminization because there is, and we’re not saying climate change doesn’t exist because it’s there and it’s accelerating,” she says. “We say that if the populations are large enough, if there is enough diversity, then it looks like a species [can] evolve in response to the climate in which they live.”

The job backs up recent evidence including the team Graeme Hayes at Deakin University in Australia shows that more male sea turtles hatch than predicted if temperature is assumed to be the sole driver of sex determination. These results show how the key temperature at which the turtle’s sex ratio is 50:50 can be adapted to local conditions, Hays says.

Turtles also have other mechanisms to mitigate the effects of warming, he says. They include nesting earlier in the year and patterns of migration to breeding areas reducing the impact of feminization. “Female turtles generally don’t breed every year, but males travel to nesting sites more often than females,” says Hays. “So the breeding sex ratio is more balanced than the actual adult sex ratio.”

Such behavioral adaptations are good, says Eizaguirre, but hatchlings are still exposed to extreme heat, which leaves permanent DNA methylation changesso signs of molecular adaptation are even better news for these vulnerable reptiles.

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