Most sexually reproducing species maintain an equal number of males and females in their population. Now scientists have unlocked a genetic mechanism behind this balance--at least for fruit flies. The find could lead to new insights about the evolution of genomes and the formation of new species.
Although fruit flies maintain a 50/50 gender ratio in the wild, some lab lines produce more females than males. Scientists suspected that the disparity had to do with how the X and Y sex chromosomes duke it out for dominance in being passed on to offspring, but the genetic mechanisms remained elusive. When a team led by evolutionary geneticist Tao Yun of Emory University in Atlanta, Georgia, went searching for the responsible gene, it got two for the price of one.
The researchers compared the DNA of male fruit flies that produce 90% female offspring to the DNA from a line that yields equal numbers of each gender. They traced the difference to a gene located on the X chromosome that they dubbed Dox. Flies with Dox produce sperm carrying the X chromosome, and these sperm somehow disable Y chromosome-carrying sperm, resulting in more female than male offspring. But in a line bred to lack Dox, the Y-carrying sperm maintained their ability to fertilize eggs and produce male offspring, leading to an equal sex ratio among offspring, the researchers report in a pair of papers in the November issue of PLoS Biology.
In a separate experiment, the team also identified a gene called Nmy on another chromosome that suppresses Dox. Flies carrying Dox and Nmy produced equal numbers of each sex, but lines bred to lack Nmy yielded more females. Although it's not clear whether these genes are present in other species, Tao says that the finding could explain a widespread phenomenon--known as meiotic sex chromosome inactivation--in which the X and Y chromosomes are turned off during the production of sperm. This shutdown could be a method of thwarting Dox and thereby preventing the X chromosome genes from dominating the show, he says.
The findings are "groundbreaking," says Daven Presgraves, an evolutionary geneticist at the University of Rochester in New York state. They raise the possibility that Dox and Nmy may lead to the accumulation of genetic changes on the X chromosome, he says. These changes could result in new species by generating individuals that are unable to successfully reproduce with the rest of the population. "The origin of new species could be an incidental byproduct of evolutionary arms races between selfish genes and their suppressors," says Presgraves.
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