Moon snails' mysterious egg cases

Temperature determines gender in reptiles

Posted May 30, 2018 at 6:22 AM

Updated May 30, 2018 at 6:22 AM

I am not superstitious. I believe in uncanny coincidences, not mystical connections, but still, I’m always excited when it feels like the universe is talking to me...as it was last week. An article came out in Science magazine about how temperature determines gender in a variety of reptiles, snapping turtles for example. The next morning, I saw a huge snapping turtle next to the road by my house. If that isn’t the universe telling me to educate myself about reptile gender determination, I don’t know what is.

The Science article in question “The histone demethylase KDM6B regulates temperature-dependent sex determination in a turtle species” (by Chutain Ge et al) concerns some complicated genetics that made me think about turtles in a whole new way.

Temperature-dependent sex determination occurs in a large number of reptiles - from alligators to turtles. Instead of different ‘X’ and ‘Y’ chromosomes determining gender (as is the case for humans) the gender of these species is determined by temperature. So male and female reptiles don’t have different chromosomes (like we do) that tell them whether to be male or female, the environment does.

We’ve known about temperature-dependent sex determination (TSD) for over 50 years but, until this study, didn’t have a clear idea of how it happens at the molecular level. For quite some time it has been supposed that the molecular mechanism for TSD lies not in the code of our DNA but in an epigenetic mechanism. Epigentics involves chemicals or molecules outside of the DNA that can control or regulate the expression of genes within our chromosomes. They aren’t part of the DNA sequence but are on or attached to the DNA. The way I understand this, in the context of sex determination in reptiles, is that genetic differences don’t determine gender - instead, environmental cues (in this case temperature) influence epigenetic mechanisms that turn gender-determining genes off or on.

This particular study involved the red-eared slider, a turtle that is native to the southeastern United States but has become invasive in this area - it is a popular pet species and has been released all over the country. When eggs are incubated at warmer temperatures, female turtles are produced, cooler temperatures produce male turtles.

This gets complicated but bear with me. Researchers manipulated the expression of an epigenetic modifier (the KDM6B gene) that produces a protein that interacts with a different gene that acts as a master switch - that can turn female embryonic turtles into males. Cooler temperatures increase activity of the KDM6B gene - more protein is produced, more males are produced. When the researchers suppressed the activity of the KDM6B gene embryos destined to be males turned into females. What’s more they were able to identify how this happened. DNA is wound around proteins called histones. The histones are covered with methyl groups that keep the genes in the DNA inactive. Colder temperatures increase KDM6B expression, methyl groups are removed from the histones allowing the genes to be more active - in this case making it easier for the master switch to function, producing more males. Warmer temperatures suppress KDM6B expression so more females are produced. Try reading the title of that research paper now and see if it makes at least a little more sense.

Climate change can pose problems for animals with temperature-dependent sex determination. One worry is that in a rapidly warming world all of the turtles of a given species could be born female. Research bears this out - studies of loggerhead turtles in Florida found that many nests were producing 90-100 percent female hatchlings. Research into painted turtles, (the ones we see sunning themselves on logs around here) showed that a temperature increase of a little over 1 degree Celsius would produce all female hatchlings.

I have no idea whether the snapping turtle I saw by the roadside was male or female. I’m guessing female since this is the time of year they are out and about digging holes and laying eggs. Let’s just hope that temperatures stay cool enough for her to have both sons and daughters.