How Temperatures Determine Whether A Turtle Is A Cool Dude Or A Hot Chick

Warmer temperatures make more female turtle hatchlings, but new research shows that those females also have a greater capacity for egg production, even before their sex is set

A recent study has found that turtles exposed to warmer ambient temperatures have increased numbers of germ cells — specialized cells that develop into eggs — leading to the development of more female turtles. Curiously, the researchers found that the presence of greater numbers of germ cells themselves influence whether an individual turtle develops into a female.

Some reptiles, including turtles, have a biological strategy known as temperature dependent sex determination. This is a molecular process whereby the sex of embryos is determined by the incubation temperature that embryos are exposed to during a critically important stage of their development. In the case of turtles, warmer temperatures lead to increased numbers of female hatchlings whereas cooler temperatures result in greater numbers of male hatchlings.

Embryonic turtles start out with a bipotential gonad that has the potential to develop into either male or female reproductive organs (Figure 1). Previous research has found that, at lower incubation temperatures, non-reproductive (somatic) cells of the gonad differentiate into special support cells for testis function and for sperm cell development, whereas higher temperatures trigger these somatic cells to differentiate into special support cells for ovarian development and function. It is likely that hormones are involved in this.

“Sex determination by temperature isn’t just one mechanism,” developmental biologist Blanche Capel, senior author of the study and the James B. Duke Distinguished Professor of Cell Biology in the Duke University School of Medicine, said in a statement. One of Professor Capel’s several areas of expertise lies in studying temperature dependent sex determination in red-eared slider turtles, Trachemys scripta elegans, by investigating the biology of the germ cells in its testis and ovary.

“Higher temperatures seem to affect sex determination in incremental ways through multiple cell types in the embryo,” Professor Capel said.

Significantly, this study found that, as incubation temperature increases, germ cells became more abundant, and their greater numbers appear to drive feminization. This, according to marine biologist Boris Tezak, lead author of the study. Dr Tezak’s speciality is temperature dependent sex determination in sea turtles, and he now studies this phenomenon as a postdoctoral researcher in Professor Capel’s group.

Dr Tezak is currently investigating how temperature increases will impact the reservoir of germ cells in turtle embryos and whether it will lead to the development of less physically fit females. To better understand this, he is incubating clutches of red-eared slider eggs at constant temperatures in the lab. These eggs were obtained from a turtle breeder in Louisiana.

One incubator is kept at 26° Celsius, and it produces more males. Another is at 31° C and it produces more females. As these turtle embryos develop, it is possible to visually inspect them by shining a bright light through the eggs. Using this method, Dr Tezak and his collaborators found that embryos incubated at the warmer temperature are noticeably larger and more active inside the egg.

“The temperatures that produce females are also the temperatures that increase germ cell number,” Dr Tezak noted (Figure 2). This phenomenon is also seen in other unrelated groups of animals. For example, increased numbers of germ cells are also known to regulate the development of female fishes.

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

To test their hypothesis that more germ cells lead to more female turtles, Dr Tezak and his collaborators removed some germ cells from red-eared slider turtle embryos raised at an intermediate temperature that typically yields 50:50 male:female ratios. As they predicted, removing germ cells increased the number of male hatchlings.

In the case of red-eared slider turtles, temperature-dependent sexual dimorphisms in germ cell numbers optimizes female reproductive fitness when the embryo incubates at the female promoting temperature of 31°C. But what happens when these turtle eggs are incubated at higher temperatures?

“We have incubated some eggs at 33.5 degrees, only two and a half degrees higher than the optimal temperature for females,” Dr Tezak reported. “It created some really wonky embryos — there were cyclops and two-headed embryos.”

“We haven’t counted their germ cells yet,” Dr Tezak added.

Evolutionarily speaking, temperature sensitive sex determination seems particularly dangerous, especially because global temperatures are warming so rapidly, so why does this strategy persist? Why has it evolved so many times and in so many animals?

“A female that hatches with more germ cells is presumably more reproductively fit — it increases her reproductive potential to carry more eggs,” Dr Tezak explained. “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.”

So one could say that evolution does not follow a set plan.

Temperature dependent sex determination is not new to science; it has evolved many different times and in many different ways across the tree of life.

“It popped up everywhere,” Dr Tezak asserted.

But other animal lineages that also show temperature sensitive sex determination may have evolved this mechanism for their own ecological or evolutionary reasons. For example, it is proposed that temperature sensitive sex determination may be advantageous when environmental conditions differentially influence the reproductive fitness of males and females. Although this is a straightforward concept, it is challenging to test, because the majority of vertebrates with temperature sensitive sex development have long lifespans and delayed sexual maturity, so it is challenging to measure lifetime reproductive fitness amongst these animals.

In view of these challenges, what does the team anticipate they might find in other reptiles that have the opposite pattern of sex determination (hot males and cool females) from red-eared slider turtles? And what about reptiles with different optimal temperatures for developing each sex? Already, Dr Tezak and his team are investigating this question in alligators, which fulfil both conditions.

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

As climate change pushes global temperatures ever higher, it is obvious this environmental situation will impact temperature sensitive species, which raises the question: what may lie in the future for turtles and other temperature-sensitive animals?

Source:

B. Tezak, B. Straková, D.J. Fullard, S. Dupont, J. McKey, C. Weber, and B. Capel (2023). Higher temperatures directly increase germ cell number, promoting feminization of red-eared slider turtles, Current Biology | doi:10.1016/j.cub.2023.06.008

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