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Global Warming is Wiping out the Males!Malehood gene, sensitive to temperature |
By Stefan Anitei, Science Editor
20th of April 2007, 07:11 GMT
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Hot sex for lizards can mean ... no males!
Because warmer temperatures bias the sex of dragon lizards while inside the eggs, transforming males into females. It seems that high temperatures turn off the maleness gene(s) on their sex chromosomes.
"The sex-reversed lizards look female and have female organs but genetically they are male," said lead author Alexander Quinn of the University of Canberra in Australia.
Biologists have believed that the sex of reptiles is determined by either the sex genes on sex chromosomes or temperature, but this is the first time when both factors are connected. In the case of the Australian central bearded dragon lizards (Pogona vitticeps), gender was thought to be a question of sex chromosomes, but
opposite to humans and mammals. In the case of mammals (and humans), females have two X sex chromosomes (XX), while males have one X and one Y (XY), thus Y is masculinizing.
For the dragons, males have two similar sex chromosomes (ZZ), while females present heterochromosomes (two different sex chromosomes): WZ. Thus, it was thought that WZ embryos would give females and ZZ ones males.
Quinn's team incubated Pogona vitticeps eggs at different temperatures, varying from 20 to 37 degrees C (over or below these temperatures the embryos died). The optimum proved to be between 22 and 32 degrees C, resulting in an roughly 1:1 sex ratio between males and females. But over 34 degrees C hatched much more females, pointing to a temperature biased sex determination. Then the researchers connected the "phenotypic" gender with the sex chromosomes.
At intermediate temperatures, there was an almost 100 % match between the sex of the offspring and their sex chromosomes. But at high-temperature incubation, there was a high percentage of ZZ females.
"High temperature during the development of the embryos prevented the male DNA triggering testis development. By default, they developed instead as females with ovaries." said Quinn.
The researchers suppose that a gene on the Z chromosome could be masculinizing and its encoded protein sensitive to temperature. "At most temperatures the protein is working at its best, but high temperatures make it less effective, making it unable to trigger male development," Quinn said.
Till now, temperature was known to influence sex determination in other reptile groups, like alligators and marine turtles, a worrying process due to the current global warming.
"But now our study opens up the possibility that many [genetically sex-determined] reptiles might face the same risk as well, if they show reversal to 100 % of one sex at high temperatures, like the bearded dragon," Quinn said.
"Obviously, reptiles with temperature influences in their sex determination must have persisted through many climatic fluctuations throughout their evolutionary history but a concern is that the current rate of climate warming might be too fast for these animals to adapt to it." he said.
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