Australian "dragon." If it's hot, eggs hatch with female genitalia but male genetics.
As if climate change doesn’t bring enough to worry about, now comes word that it's affecting the sex of newborn lizards in Australia, and could even make males extinct.
"A boy or a girl?" is the classic post-partum question. In humans and other mammals, the choice is made by the sperm that penetrates the egg. If it carries a Y chromosome, "It's a boy!" Otherwise, "It's a girl!"
This is called genotypic sex determination.
We've just learned it's a different story in reptiles. In some, the sex of the offspring depends on the egg's incubation temperature. So-called temperature sex determination (TSD) means that reptiles with female genitals carry male (called ZZ) chromosomes.
That transformation has been seen in several species in the lab. Now, for the first time, scientists have found the phenomenon in the semi-arid outback of Australia, in the Australian central bearded dragons (Pogona vitticeps).1
Leapin' lizards: Where have all the guys gone?
This real-world find comes with a twist of major ecological significance: If temperatures remain high, ZZ females could predominate. In other words, guys could eventually go extinct, and with them, the entire population of these dragons.
It's not clear whether TSD confers an evolutionary advantage, but the authors found signs that the process could become self-perpetuating: the offspring of ZZ females are more likely to become ZZ females. And those sex-reversed mothers lay nearly twice as many eggs per year as normal mothers, leading to more feminized populations.
Should we worry that the population will expire for lack of males? "That is true," corresponding author Arthur Georges of the University of Canberra wrote us.
When genes determine sex, ZZ equals male and ZW equals female. "Our research shows a transition from a ZZ/ZW population to a pure ZZ population over relatively few generations," Georges said. "The W [female] sex chromosome is jettisoned."
But that's not necessarily a dead end. "What rescues the population is that some of these ZZ individuals would be reversed to females," he added. "So there would still be both males and females even if the W chromosome is lost to the population." But if temperatures continue to rise, "We could find that only females are produced and the population would crash" for lack of males, even though they carry the ZZ genes.
To test whether TSD was taking place in the wild, the researchers captured 131 dragons between 2003 and 2011 and found that 20 percent them were ZZ females. The numbers showed a non-significant trend toward feminization that paralleled rising temperatures. Another possibility – perhaps also linked to warming – the loss of vegetation may expose the nests to more sunlight, leading to more poached (heh heh!) eggs.
Reproductive reptiles: It's a real zoo!
We confess that we'd given the subject little thought, but Georges wrote to say that "Reptiles have an astonishing number of ways of determining the sex of their offspring." Some use the human system (relying on father's chromosomes), others rely on the mother's. He mentions many alternatives:
Parthenogenic reptile species "have done away with males altogether, with the females engaging in virgin birth" from an unfertilized egg;
Facultative parthenogenesis is an optional variant: females deprived of males can spawn male offspring, "a great benefit to sole invaders of isolated islands;"
"Many turtles, all crocodiles, some reptiles and New Zealand’s tuatara, have dispensed with genetics and incubation temperature alone determines offspring sex."
"Just when scientists thought they had a handle on all this, the Australian Central Bearded Dragon has delivered another surprise," Georges says, a single species can show that there is more than one system for controlling sex.
“The emerging picture is that reptilian sex determination is more flexible on an evolutionary scale than could ever have been imagined," commented2 James Bull of the University of Texas in Nature: “... chromosomal and environmental sex determination can both be highly functional, adaptive systems ... that are alternative states that could, in theory, evolve back and forth."
The new study shows “how quickly such a substantial evolutionary change can occur in response to a rather modest shift in climate," Georges wrote to us. “We are used to hearing that evolutionary change is too slow to be an effective response to the rapid and unprecedented climate change we are currently causing on Earth. It seems that there are after all opportunities for very rapid evolutionary responses, such as changing one’s mode of sex determination."
– David J. Tenenbaum