Researchers from the universities of Queensland and Canberra and our own Australian National Wildlife Collection have been studying sex-swapping bearded dragons and the formation of genitalia. They discovered that what happens in the egg doesn’t necessarily stay in the egg. Published recently in the journal EvoDevo, they discovered something unexpected about females, adding a curiously-shaped piece of the puzzle to the evolution of sex. But before we get there, let’s hit up some basics.
Let’s talk about sex, baby
Reader, we’re about to whisk you up in a genetic whirlwind, inverting and scrambling your preconceptions of sex around the animal kingdom—not the ‘sexy’ sex (you can scramble your own preconceptions about that), but the ‘female XX, male XY’-type sex. Sexy.
In fact, from here on in, whenever we say ‘sex’, imagine we’re talking about the concept relating to femaleness and maleness. With that out of the way, let’s talk pre-conceptions. We humans have a tendency to create our own definitions based on our own experience and get confused when reality is different. Being warm-blooded and constantly needing food? Normal. Having four limbs? Normal. Seeing a rainbow of colours? Normal. As it happens these things are not normal—only the absolute minority of animals on Earth share these traits. We humans are in fact the abnormal ones. But we have the dictionaries and pens, so there.
How your sex is determined is a great example. You might imagine that our XX, XY system—where you’re a female unless you carry a Y chromosome— is a universal system, but not so. In fact, our XX, XY system has ‘only’ been around for about 166 million years. Before that, the X and Y chromosomes were basically mirror-images of one another and didn’t control sex. So how was sex determined before then?
It’s getting hot in here, so hot, so take note of sex ratios
Consider humans. In all of your cells you have the instructions to construct toes, hair, a heart, a brain, your whole body, but over time and as you grow, these instructions get locked away and aren’t accessed anymore. This is part of normal development; once you’ve grown one brain you don’t want your body trying to construct duplicates—it’d be a mess. We still have the instructions to build brains, but they’re turned off.
Where human males and females differ is the instructions we have for building genitals, primary sex-defining traits and secondary sex-defining characteristics. We have one or the other and these are locked-in, genetically, at the moment of conception when either an X-bearing or Y-bearing sperm meets your mother’s egg. Unless you have the male-defining Y chromosome, you don’t have the genetic instructions to construct testes, a penis, or produce sperm.
And here’s where things are different for reptiles. All crocodiles, a bunch of turtles, and certainly a lot of lizards have genetic instructions to build toes, skin, a heart, a brain, like us, but they also carry the instructions to build either type of genitals and become either a male or female. Their genomes hold information to be either sex.
So, unlike us humans, their sex is not locked in at conception.
If not by chromosomes, how is it decided? Mostly by temperature, it turns out. While they’re in their eggs the sex of baby reptiles is determined at the whim of the ambient temperature. For some species of turtles coldness turns you male, whereas heat turns you female. For other species, such as lizards and crocs, extreme temperatures at either end turn you female, and intermediary ones, male. Your sex is finally locked in about half-way through development (before hatching) and after that there’s no going back. Just like you can’t make two brains, they can’t make both sexes.
Temperature isn’t the only other way sex is determined in the animal kingdom either. There’s an array of other factors that influence different species’ sex such as the amount of nutrients in the environment, how long the sun is out, the acidity of the water, and, for some fish, how dark the background of their fish tank is. It’s just nuts. (Just imagine the institutions and wacky traditions we humans would have if our sex determination were as wild and diverse as this!) But, as with everything on planet Earth, it gets more complicated than that.
Enter, the dragon
So, you can have your sex determined by genes (like us) or by temperature (like crocs), but here be dragons, where the plot thickens. The central bearded dragon, our very own Australian native, is one of just two reptile species that is known to have its sex defined by genes BUT can have its sex flipped by temperature afterwards. If exposed to hot temperatures while developing in the egg, a chromosomal, genetic male can hatch as a female. It is able to access female genetic instructions and not only appear as a female but have baby dragons as well. They are female dragons with the genome for a male.
Is there some benefit to flip-flopping? Perhaps. Not only do males-turned-females perform as mothers, they perform better, producing up to twice as many eggs! These new females also have different personalities; they’re more brazen than either males or females. This suggests sex-reversal may have ‘adaptive’ benefits for a population, letting the environment inform which sex hatches to take advantage of the changing conditions when hatching. But we all knew all of this from a series of papers dating all the way back to 2007.
What has recently been discovered is that all dragons start life off in their eggs with a pair of ‘hemipenes’, the male sexual organ. Before hatching, genetic females and heat-induced females (even those with two genetic male parents) all do the same thing. They resorb their hemipenes to become regular ol’ females and have similar experiences in the egg.
What does this mean? It means that whether the sun tells you to be a female or your genome does, it doesn’t matter too much when it comes to development. As long as there are hormones telling your body what to do, your body doesn’t care where that kick-start comes from.
When temperature flips the sex of a dragon, its developmental sequence doesn’t change but its adult behaviour and fecundity (fertility) does. Why? We’re not sure yet.
Since this is one of the few animals that has two types of sex-determination and is flexible enough to have its genome over-ruled by the weather—all while still in the egg—it’s a perfect ‘model organism’ to study the evolution of sex. It may even answer the question of how a group of animals can transition, evolutionarily, from being genetic-determined to temperature-determined, and vice versa.
Further, female genitalia in lizards and snakes are far more diverse than are males’, in terms of shape and size, from simple organs, to larger, more muscled hemipenis-like structures. Since all individuals start life with a (male) hemipenis, evolution may have used this hemipenis ‘blank canvas’ to draw all the wondrous diversity we see in female genitalia.
Hot winds of a changing climate and the battle of the sexes
What if the sex of our unborn children were determined by the ambient temperature, say if consistent maxima of >35 oC resulted in all babies becoming female, and a resultant loss of a generation of males? (Rhetorical question, but stick with me.)
Needless to say, climate change would be getting a touch more attention. For the central bearded dragons, not only might we lose a sex (males) in certain populations, but we might also lose a chromosome—you know, that irreplaceable collection of genetic information kept safe inside an unbroken line of custodians that stretches back to the beginning of life?
This, of course, is an issue for countless other species whose sex is influenced by temperature, even those we don’t know about. And that’s the problem: the study of sex determination as affected by temperature is embryonic and our understanding is limited. The rate of climate change is outstripping our progress at understanding the effects and their severity on an unknown number of species.
Our researchers are working to better understand the dynamics of temperature-dependent sex-determination, how it might have evolved, and, how it might evolve.