How and why do temperatures determine the sex of turtles?

Warmer temperatures improve turtle motherhood.

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Warmer temperatures cause more turtle eggs to hatch into female hatchlings. According to new Duke University research, females also have a larger capacity for egg production even before their sex is determined,

This discovery may explain why many species other than turtles have temperature-dependent sex determination and why the system continues despite appearing to be a dangerous technique.

Higher incubation temperatures increase the number of germ cells (pre-eggs) that an embryo carries, and those germ cells themselves have a part in the embryo becoming female.

Senior author Blanche Capel, the James B. Duke Distinguished Professor of Cell Biology in the Duke School of Medicine, said, Sex determination by temperature isn’t just one mechanism. Higher temperatures affect sex determination in incremental ways through multiple cell types in the embryo.” 

According to Boris Tezak, a postdoctoral researcher in the Capel lab who led this investigation, abundant germ cells appear to cause feminization. 

He said, “The temperatures that produce females also increase germ cell number.”

Higher amounts of germ cells are also thought to influence female development in fish. To demonstrate that more germ cells result in more female turtles, they deleted some germ cells from red-eared slider embryos grown at an intermediate temperature, which should have resulted in 50-50 proportions, and found more males than expected.

Temperature-dependent sex development has been known to scientists for decades, and it has been observed in many different areas of the Tree of Life, presumably because it evolved several times in multiple ways. 

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

The researchers believe this is due to temperature-dependent sex development, which provides a reproductive advantage. 

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

The researcher carefully fosters clutches of red-eared slider eggs received from a Louisiana breeder in plastic boxes filled with moist medium and kept at a steady temperature in the lab. One incubator produces more males by running at 26 degrees Celsius. Another is at 31 degrees, the ideal temperature for increasing female fertility.

What will happen to turtles and other temperature-sensitive breeders as global temperatures rise? The researchers will investigate how further temperature increases affect the pool of germ cells and whether they result in less fit females.

When he removes one of each to examine its progress under a bright light, the embryo incubated warmer is noticeably larger and more active inside the egg. 

The researchers hypothesize that there is a temperature sweet spot,’ where there is a limited range where you have a significant number of germ cells, and then you start to observe reductions.

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

The team will also get more alligator eggs to continue the temperature trials. Alligators produce females at low temperatures and males at high temperatures, which is the polar opposite of the red-eared slider turtle’s habit. However, because alligators’ low temperature is the same as turtles’ high temperature, both species produce females at 31 degrees Celsius. 

The researcher said, “The interesting question is whether we see more germ cells in both species at this temperature.”

This research was funded by the National Science Foundation of the United States and the Czech Science Foundation.

Journal Reference:

  1. B. Tezak, B. Straková, etal.Higher Temperatures Directly Increase Germ Cell Numbers, Which Promotes the Feminization of Red-Eared Slider Turtles. Current Biology. DOI: 10.1016/j.cub.2023.06.008

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