Life Lines by Dr. Dolittle

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Hot Chicks, Cool Dudes: How sex can be shaped by temperature

Rosario-Marroquin-Flores-300x300This guest blog entry was written by the 2020 Dr. Dolittle Award recipient, Rosario Marroquín-Flores, a Biology doctoral student currently studying at Illinois State University. The Dr. Dolittle Award is given to a trainee in the Comparative and Evolutionary Physiology section of the American Physiological Society who submits the best blog entry describing their research project. Congratulations on a well-deserved award for this guest blog:

 

Hot Chicks, Cool Dudes: How sex can be shaped by temperature 

You are pregnant and you really want to have a girl, so you’ve decided to take things into your own hands. Every day, you head out to the beach and sunbathe, hoping that your little one gets warm enough to make the big shift and become a female. Obviously, this is not going to be an effective strategy for you, but it would be for a turtle. This is because humans have sex chromosomes, tightly wound threadlike structures that contain genetic information. By the time you are pregnant, your child has sex chromosomes and will become male or female. But this is not the case for all animals.

Many turtles don’t have sex chromosomes. Instead, they become male or female based on the temperature that they experience as an egg. This is called Temperature dependent Sex Determination, or TSD. Turtles are sensitive to temperature during the middle third of development, something akin to the second trimester of pregnancy. In many species, during this time, warm temperatures will result in female development and cooler temperatures will result in males. Just think, hot chicks, cool dudes.

I identify as a developmental biologist and eco-physiologist. I currently work in a research lab and we use turtles to study the role of temperature on the development of ovaries and testes. Researchers have been studying TSD for over 50 years, but we still don’t really understand how temperature drives sex. In humans, genetic signals from sex chromosomes determine whether a child will become male or female. So, if turtles don’t have sex chromosomes, where do these genetics signals come from? In order to figure this out, we have to identify genes that are responsive to temperature.

Some of this work has already been done. Researchers have incubated eggs at constant warm and cool temperatures, then used gene sequencing technology to see what genes are responding. The problem with this approach is that temperatures are more variable than that in nature. Temperatures are highest in the day, then they come down at night. Every now and again, you get a heatwave, where it gets really for hot multiple days in a row, then it comes down again. So, previous work using these constant incubation conditions don’t really capture what’s happening in nature.

My research uses temperatures that match those found in an incubating turtle nest. We use fluctuating conditions that mimic daytime and nighttime temperatures and use heatwaves to induce female development. Then we use gene sequencing technology to figure out what genes are responding to the heatwave. In doing so, we have found some surprising results.

Over the last two years, there has been some debate among researchers about a temperature-responsive gene that might be binding to and regulating other genes associated with female development. Under constant warm temperatures, it increases its activity, leading researchers to suppose that it may be an initial trigger that results in females. Our research found that this is not the case. Under fluctuating temperatures, this gene does not increase its activity, which means that it probably has nothing to do with female development.

Along the same lines, previous work using constant temperature incubation has found thousands of genes responsive to these temperature treatments. Our work found only 176. Of these temperature-responsive genes, only 22 were found to overlap between constant temperature and fluctuating temperature incubations.

So, what does this mean? TSD researchers have long used constant temperature incubation to identify genes that regulate sex and development. While this may be useful in narrowing the field, it does not tell us what is actually happening in the body. Our use of temperature fluctuations and heatwaves are more ecologically relevant because they mimic what turtles are naturally experiencing in the nest. By using these incubations, we may be able pin down some of the temperature-responsive genes that lead to males and to females.

We use the red-eared slider turtle for our research, but many other turtles, including all sea turtles, have TSD. We study TSD because climate change is altering our environment. In this altered environment, we expect heatwaves to become more frequent and to be longer in duration. Animals with this sex-determining strategy may be more at risk because they are developmentally sensitive to temperature changes. If we want to come up with management strategies to mitigate this risk, it is important to understand how temperature shapes sex in these animals.

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Categories: Climate Change, Most Popular, Physiology on the Road

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