For humans and other mammals, sex is neatly determined by the X and Y chromosomes. If you have a Y you are male, and without it you are female. Reptiles however, use a variety of strategies, and the mammalian X/Y system is just one of them.
In some species, the female is the one with different chromosomes, in this case Z and W, and the male has two Zs. And some reptiles ignore sex chromosomes altogether. For them, an individual's sex is determined by the temperature that their eggs were incubated at.
Scientists had long believed that these strategies were mutually exclusive with each species choosing one of the other.
But Alexander Quinn and colleagues form the University of Canberra have found that an Australian lizard, the central bearded dragon (Pogona vitticeps), flouts this rule. It has become the first animal known to use two separate methods to determine the sex of individuals.
The dragon uses the Z/W system, where the males carry two Z chromosomes and the females have a Z and a W. But Quinn found that these genes are only the dominant influence on gender if eggs are incubated between 20 and 32 degrees Celsius.
At higher temperatures, males ignore their genetic heritage and become females instead. When Quinn incubated broods of eggs between 34 and 37 degrees Celsius, the hatchlings were almost invariably female. And as predicted, about half of these sisters were genetically male. For dragons at least, when the heat is on, the men turn into women
Quinn believes that the key to the manliness of boy dragons lies in a temperature-sensitive protein produced by the Z chromosome. The protein's activity needs to surpass a certain threshold before a dragon can become male. For that, there need to be two copies of Z, and the temperature must be just right.
Reptiles that use temperature to assign gender must have fine-tuned their systems over time to cope with an ever-changing environment. But Quinn fears that the current pace of climate change may be too rapid for these animals to adapt to.
If temperatures rise far enough to bias an entire species over to a single gender, extinction would be all but inevitable. These warnings have been sounded before, and Quinn's work suggests that they should be shouted a little bit louder.
Reference: Quinn, Georges, Sarre, Guarino, Ezaz & Graves. 2007. Temperature sex reversal implies sex gene dosage in a reptile. Science 316: 411.
- Log in to post comments
That's fascinating. I wonder what the adaptive benefit of gender/temp is.
This is awesome, Ed. I always knew that women were better under fire...heh.
Well, the lizardy varieties of women anyway, Dr. Isis.
i know that there are other reptilian species where sex is determined at least in part by temperature (eg, crocs, some turtles). but aren't some (lizards as well as others) influenced in the opposite direction, ie, heat makes more males instead of females?
and another 'dragon' species (specifically, komodo) also uses the ZW system, but (as far as i know) if not temperature-dependent for sex determination. are there any notable evolutionary patterns here to speak of?
oops! that should be "...is not temperature-dependent..." (obviously).
Ed, you seem very certain that these lizards are headed for extinction if global warming continues, but do we know anything about their evolutionary history?
The globe has been both colder and warmer in the past. How did they cope then? Is the temperature-linked gender something (relatively) new or has it been part of their genome for many millions of years? Under what circumstances did they evolve this trait?
It seems to me that there are some more questions to be answered before we can have a very good idea of what will happen to these bearded ladies!
perhaps nit-picky, but shouldn't the references to gender refer to sex instead? I thought that gender was the human psychological construct and sex was the biological one.
I wonder how this trait changes with overall temperature... I wonder if a long-term study between different incubation broods would show an ability to adapt to rising temperatures?
This is very interesting. Some clades, such as Kinosternids (Mud and Musk Turtles) have some species with TSD (temperature dependent sex determination) and some with genetic sex determination (GSD). I would like to know whether the ancestral state for these Beardies is GSD or TSD. Are they evolving towards or away from TSD (or is a mixture best in their situation?). If they are evolving towards it, does their situation apply universally to past intermediates in TSD. Is this (protein denaturing at high temps) the mechanism by which all or most TSD has evolved?
The Charnov-Bull model is the dominant theory about the adaptive value of TSD. It basically states that the fitness of each sex depends on their temperature during development and differs between the sexes. There is evidence that temperature during development does have important effects on phenotype for several reptiles including Eastern Ratsnakes (Elaphe spiloides aka Elaphe obsoleta perhaps due to be reassigned to Pantherophis). In this situation TSD is adaptive because it allows the embryos to develop into the sex best suited to the embryonic temp regime that they find themselves in. The Charnov-Bull model had not really been tested until recently because most of the species with TSD are slow growing and late maturing with a long lifespan and so don't lend themselves to lab work within a grant cycle. However, it was recently tested with fast maturing Aussie lizards, Jacky Dragons (Amphibolurus muricatus) and a paper has been published in Nature. Looks good for Charnov Bull.
TSD potentially comes in four flavours. Type I is the pattern in which one sex is produced at high temps and the other at low temps. Type IA (MF) occurs when warmer = females, cooler = males. Type IB (FM) is the opposite and is very rare (almost mythological but now thought to occur in Tuataras). Type II has one sex at intermediate temps and the other produced at warmer or cooler temperatures. As far as I know all reptiles with the Type II system produce males at the intermediate range (FMF). Crocodilians are all/mostly Type II and turtles, depending on species, can be Type I or II.
Generally sex determination seems to be very adjustable within non-avian reptiles (birds are mostly (all?) ZW GSD). Within a lineage the sex determining mechanism seems to be switched rather easily.
I actually blogged the Jacky dragon paper back when it came out. It's on the old Wordpress site.
The best bit about it was that Eric Charnov himself sent me an email saying he liked the post!
That's excellent. Do you assign points to yourself when the study authors comment? If so what do you do when one of the authors of the theoretical underpinning of the experiment leaves a comment?
I thought sex in mammals was defined by gamete size, ie. the female is the one that produces the larger gamete?
very nit-picky, but i'd considered nearly the same thing. very roughly, gender is a sociosexual construct, while sex (female, male, both) is biological or physiological fact.
thx for the overview of sex-determination systems in "reptiles"! just what i was looking for.
when did dinosaurs (especially theropods + birds) become 'temperature-independent' reproducers? is there anything obvious about homeothermy that lends itself to a non-TSD system? do you think it likely that earlier dinosaurs and dinosauromorphs were largely Type II TSD reproducers (as with crocs) or that there was wide variation even throughout the Archosauria?
I think that works for all animals (and plants). Males have sperm and females have eggs. That is how we define which is female but it doesn't explain how development to one sex or another is accomplished and why.
when did dinosaurs (especially theropods + birds) become 'temperature-independent' reproducers? is there anything obvious about homeothermy that lends itself to a non-TSD system?
Yes, by definition homeotherms are at a constant temperature and this would apply to eggs in most cases as well. So birds keep their eggs at a particular temperature so there is little opportunity for TSD and no adaptive advantage to it anyway. Now, I'm not sure when, how, or which lineages of Dinosaurs evolved endothermy. (Homeothermy can be accomplished w/out endothermy through things like thermal inertia). I would guess that there was no TSD in endothermic lineages after endothermy evolved. The really pertinent question is, how did dinosaurs keep their eggs at constant temperatures? Or did they?
"Yes, by definition homeotherms are at a constant temperature and this would apply to eggs in most cases as well. So birds keep their eggs at a particular temperature so there is little opportunity for TSD and no adaptive advantage to it anyway."
i deserve a big ol' DUH! for that one, heh. but thanks for your response anyway.