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Hot and cold

UND biologist’s research of snapping turtles could yield far-reaching implications

Turk Rhen
At his laboratory in Starcher Hall, UND biologist Turk Rhen studies the temperature-dependent sex determination in snapping turtles – research that could have implications for women’s reproductive health and species at risk from warming climate. Photo by Dima Williams/UND Today.

Standing at the center of the drab room, whose walls have vanished behind shelves, Turk Rhen reached into a grey incubator and pulled out a red party cup. He stuck two fingers in it and retrieved a small egg.

Lightly squeezing it, the University of North Dakota biology professor could tell the little critter inside was alive.

He glanced at the edge of the incubator, searching for the number etched in black ink: 27. At that temperature in degrees Celsius (or around 81 Fahrenheit), the egg would grow into a male snapping turtle. Anything above 29 degree Celsius and it would be a female. At 28 degrees Celsius, the chances for a boy or a girl are even.

It is here, in the hatching room of his lab in the labyrinthine basement level of Starcher Hall, where Rhen’s research on temperature-dependent sex determination (TSD) begins.

In reptiles, the ambient heat – or chill, for that matter – influences the development of embryos’ gonads, the gland that morphs into a testicle or an ovary.

In some ways, the development of the gonad is the same across sundry species. The bipotential gonad can grow into either male or female sex organs.

Rhen has a simple metaphor for the process. Imagine an orange, he said. If the outer layer – the fruit’s skin – expands and overwhelms the inner cluster of cells – the gonad is to become an ovary. In the reverse scenario, where the inner cells grow, a testicle emerges.

In humans, a gene on the Y chromosome controls this differentiation. In the snapping turtles Rhen focuses on, it is temperature.

Since he first began studying the phenomenon as an undergraduate at UND, Rhen has garnered the esteem of a top researcher.

In 2016, together with colleagues in his lab, he identified a distinct gene – the cold-inducible RNA-binding protein (CIRBP) – that impacts embryos’ responsiveness to temperature – a discovery that splashed across national media outlets.

“We are continuing with that line of research,” he said. “We are trying to use genetic approaches and DNA sequence differences to find the genes that are affecting that temperature sensitivity.”

In the summer of this year, Rhen published an article, co-authored with UND graduate student Danielle Roush, that presents a sweeping outline of the experiments and data required to grasp the genetic variations in TSD.

The CIRBP gene accounts for only 25 percent of the variation, and Rhen is looking for the rest.

Turk Rhen
Depending on the ambient temperature, turtle embryos develop into male or female. Photo by Dima Williams/UND Today.

The beginning

Rhen’s fascination with biology harks back to his boyhood, when he would visit his grandmother’s farm outside of Leonard, Minn. There, the East Grand Forks native would chase and catch frogs and snakes.

Rhen dreamed of becoming a veterinarian but his time at UND revealed other pursuits. He did some research in the biology department here and enjoyed it so much that he obtained a master’s degree and then a Ph.D. – never to become an animal doctor.

“I decided that research would be a lot more interesting for me rather than doing the same thing every day, doing clinical stuff with cats and dogs and horses,” he said. “With research you get to discover new things.”

Rhen commenced his career piercing “the big picture” of TSD’s evolutionary significance. Then, he delved into physiology before succumbing to the pull of molecules.

“What I am trying to do with my research now is tie everything together,” he said, “to understand how things at the molecular level influence the way animals develop and how that process is influenced by the environment and how the characteristics of the whole individual can influence their ecology.”

Gene focus

To do so, Rhen is zooming in on three types of inquiries.

Aside from seeking the genetic switches of TSD, he is prodding at epigenetics, or the survey of how the environment spurs gene expression without alteration of the DNA sequence.

Often, biological markers or tags attach to the DNA to trigger an epigenetic change, such as tanning under the intense sun rays in summer.

Rhen is examining the probable existence of such epigenetic gene expressions during the maturation of the gonad at disparate temperatures.

Are there any ecological factors that flick on the genes, which could guide TSD? For now, Rhen and his lab team have piles of data to analyze to arrive at an answer.

They also have the entire snapping turtle genome to assemble for research into how DNA compaction affects the activation and expression of genes important for the development of testicles versus ovaries.

Because of its length, DNA is coiled around proteins called histones, Rhen said, adding that it evokes a fishing line around a reel. The tighter the bundle, the harder it is to read the DNA and rouse various genes.

“It is like having a book,” Rhen said. “If it is closed, you cannot read the information. But if you open it up, the instructions are available.”

Turk Rhen
Researching the relations between genes and ambient temperature, Rhen believes snapping turtles could help shed light on polycystic ovarian syndrome in women. Photo by Dima Williams/UND Today.

Can turtles help women’s health?

Although nationally recognized, Rhen’s research may sound quite esoteric to a layman, especially in a state heavy on oil and agriculture.

After all, how do the biological oddities of snapping turtles, collected from the lakes of Minnesota, wield every-day implications?

But they do – in several striking ways.

Take, for instance, polycystic ovarian syndrome, a condition where a woman has too much androgen or male sex hormones. Its manifestations include over-active development of follicles in ovaries, male-type facial hair and an overweight frame.

The underlying cause is embryonic exposure to heightened levels of androgen. And, there is a similar inducement in snapping turtles.

The application of male steroid hormones – such as dihidrotestosterone – to eggs incubating at male-generating temperatures results in ovaries, instead of testicles, Rhen said.

“Even though the embryos are at a temperature that normally produces males, those embryos will develop ovaries,” Rhen said. “And those ovaries have a polycystic phenotype and they are similar to the polycystic ovaries in humans.”

Working with snapping turtles, Rhen has discovered a gene, Foxl2, that is regulated by androgen and may explain why women suffer from polycystic ovaries.

Too much androgen pushes the gene in overdrive and results in the condition of polycystic ovaries. Too little of it and premature ovarian failure settles, Rhen said, cautioning that further research is needed.

 Help for other species

Another area where Rhen’s snapping turtles could prove instrumental is in understanding the impact of climate change on their cousins, the sea turtles.

Global warming could skew the embryonic sex ratio of the latter, driving already endangered species further off the evolutionary cliff.

Rhen hopes that any further revelations about the genetic subtleties of TSD in snapping turtles could translate to sea turtles and other at-risk animals.

“That is a conservation implication in the long run that our research can shed light on,” he said.

Back in his lab, Rhen placed the egg back in the cup and inside the incubator. He ventured to another room, where tiny turtles up to a year old swam in a metal pool.

Across the hall, saturated with an earthly smell, in yet another chamber, rested adult snapping turtles, grouped in breeding trios. Rhen fetched a plastic stick and retrieved one of the animals as it readied to launch. They are called snapping turtles for a reason, he quipped.

Holding it away from his torso, he looked at it. It may hold more biological secrets and far-reaching benefits than catch the eye.