A red- eared slider.
There’s a global pandemic happening on a scale that hasn’t been seen in roughly a century. So we decided it would be the perfect time to talk about turtle sex. Not turtles having sex, which is undoubtedly an interesting geometry problem, but rather the process by which turtles develop as male or female.
That process is interesting because it seems, at least from our XY chromosomal perspective, to be a bit haphazard: turtles and many other reptiles determine their sex based on ambient temperature. In elevated temperatures, most of the eggs will develop as female; at lower temperatures, most of the eggs will develop as males. We don’t really know how they register the temperature and somehow translate it to a complex program of anatomical development. But a new paper in Science fills in some of our gaps.
Sex, of the less interesting sort If pressed, most of us could remember that human sex determination involves the X and Y chromosomes. But it’s often overlooked how things get very complicated downstream of this simple signal. A gene on the Y chromosome turns out to be critical to registering which chromosome combination someone has. A specific tissue interprets the presence or absence of that gene to start a cascade of hormones that reshape how tissues develop and continue to influence things throughout a person’s life.
(This is ignoring all the many things that can happen during this process and produce a non-binary result.) So it’s not enough for a signal to simply indicate which sex to develop as. The signal has to be significant enough that it can trigger a large program of changes in response to it.
Despite that seemingly rigorous requirement, biology has found a bewildering variety of ways
for handling sex. To give just one example, fruit flies also have X and Y chromosomes, but you can get rid of the Y chromosome and males will still develop mostly normally. And rather than having a hormone to get every cell in the body pulling in the same direction, each individual cell in the fly figures out which chromosomes it has without consulting its neighbors.
Compared to chromosomes, which are pretty stable, it’s hard to see how temperature can work to set a binary state. Turtle eggs are left in the environment, where the temperatures vary over time. That variation occurs over the long term, as incubation may take place during changing seasons; medium term, due to day-to-day changes in temperature; and short term, with temperatures rising in the morning and falling at night. Somehow, all that has to be converted to a reasonably binary decision. For turtles, that conversion occurs in a way that does have some strong parallels to humans. The signal is read by cells that go on to form the animal’s gonads, which go on to produce hormones that direct the turtle’s development.