New Insights Into How and Why Temperature Determines the Sex of Turtles

Scientists have known about temperature-dependent sex development for decades and have found it in many different parts of the tree of life, apparently because it evolved multiple times in multiple ways.

“It popped up everywhere,” Tezak said. “It seems like a really risky strategy, especially in the context of weather variations and climate change, so why would this system persist?”

They think it’s because temperature-dependent sex development creates a reproductive advantage.

“A female that hatches with more germ cells is presumably more reproductively fit -- it increases her reproductive potential to carry more eggs,” Tezak said. “We’ve linked the female pathway to the increased number of germ cells, and if that does make her more reproductively fit, that would go a long way toward explaining why temperature-dependent sex development persists.”

As global temperatures continue to rise, the question becomes: what will happen to the turtles and other temperature-sensitive breeders? “We’ll be looking at how further increases in temperature will affect the pool of germ cells,” Tezak said. “Will it produce less-fit females?”

To answer these questions, Tezak carefully nurtures clutches of red-eared slider eggs obtained from a Louisiana breeder in plastic boxes filled with moist medium and kept at a constant temperature in the lab. One incubator runs at 26 degrees Celsius, producing more males. Another is at 31 degrees, the optimum temperature for producing more females.

When he takes out one of each to check on their progress with a very bright light, the embryo that was incubated warmer is markedly larger and more active inside the egg.

“We are hypothesizing that there’s a temperature ‘sweet-spot,’” Capel said. “There is a short range where you get a large number of germ cells, and beyond that you start to see declines,” Capel said.

“We have incubated some eggs at 33.5 degrees, only two and a half degrees higher than the optimal temperature for females,” Tezak said. “It created some really wonky embryos -- there were cyclops and two-headed embryos. We haven’t counted their germ cells yet.”

The lab is also about to take delivery on some alligator eggs to continue the temperature experiments. Alligators are known to produce females at low temperatures and males at high temperatures, the opposite pattern from the red-eared slider turtle. However, the low temperature in alligators is the same as the high temperature in turtles, so both species produce females at 31 degrees Celsius. “The interesting question is whether we see more germ cells in both species at this temperature,” Tezak said.

Reference: Tezak B, Straková B, Fullard DJ, et al. Higher temperatures directly increase germ cell number, promoting feminization of red-eared slider turtles. Curr Biol. 2023. doi: 10.1016/j.cub.2023.06.008

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