In a revolutionary new study from Duke University, researchers have found that not only do warmer temperatures make turtle eggs more likely to hatch as females, but these females may also be better equipped to reproduce.
The findings, published in the journal Current Biology, hint at why many animals have temperature-dependent sex determination, despite the inherent risks of such a strategy in the face of fluctuating climates. Moreover, these findings could also predict the future of these species in a rapidly warming world.
The study was led by senior author Professor Blanche Capel and postdoctoral researcher Boris Tezak. According to the results, the number of “germ cells” or pre-eggs that an embryo carries is heightened by higher incubation temperatures. Intriguingly, the researchers discovered that these germ cells themselves contribute to the embryo’s feminization.
“Sex determination by temperature isn’t just one mechanism,” explained Professor Capel. “Higher temperatures seem to affect sex determination in incremental ways through multiple cell types in the embryo.”
These germ cells, in their increased numbers, appear to fuel the feminization process, explained Tezak. “The temperatures that produce females are also the temperatures that increase germ cell number,” he said.
This process isn’t limited to turtles, either. Professor Capel pointed out that higher numbers of germ cells also govern female development in fish. To validate their theory that a larger number of germ cells leads to female turtles, the researchers removed some germ cells from red-eared slider embryos incubated at a temperature expected to produce equal proportions of males and females. The result was a surprising increase in male hatchlings.
The enigma of temperature-dependent sex development, known to scientists for decades and apparent across various parts of the tree of life, is thought to have evolved in diverse ways. “It popped up everywhere,” said Tezak. The question then arises: “Why would this seemingly risky strategy, especially in the context of weather variations and climate change, persist?”
The Duke team believes the answer lies in the reproductive advantage this system provides. “A female that hatches with more germ cells is presumably more reproductively fit – it increases her reproductive potential to carry more eggs,” said Tezak.
“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 warming continues to escalate, concerns emerge about the fate of turtles and other temperature-sensitive breeders. Tezak expressed the need to evaluate the effects of further temperature increases on the pool of germ cells. “Will it produce less-fit females?”
To investigate, Tezak nurtured clutches of red-eared slider eggs in a controlled environment. Each clutch, sourced from a Louisiana breeder, was incubated in a plastic box filled with moist medium and maintained at a constant temperature.
An interesting phenomenon emerged when eggs incubated at different temperatures were compared. The embryos incubated at higher temperatures (31 degrees Celsius, optimal for female production) were noticeably larger and more active than those incubated at a cooler 26 degrees Celsius, which yields more males.
“We are hypothesizing that there’s a temperature ‘sweet-spot,’” said Professor Capel. “There is a short range where you get a large number of germ cells, and beyond that you start to see declines.”
This theory was further underscored by Tezak, who reported peculiar results when eggs were incubated at 33.5 degrees Celsius, only a mere two and a half degrees higher than the optimal temperature for females. “It created some really wonky embryos – there were cyclops and two-headed embryos. We haven’t counted their germ cells yet.”
To expand their research, the Capel lab is gearing up to receive a delivery of alligator eggs. Notably, alligators exhibit a different pattern of temperature-dependent sex determination compared to the red-eared slider turtle. They produce females at low temperatures and males at high ones.
Intriguingly, the low temperature in alligators aligns with the high temperature in turtles, leading both species to produce females at 31 degrees Celsius.