Why do bucks sport antlers, and other males equally extravagant but useless body parts? One theory holds that such traits are signs of genetic vigor; a peacock that can grow a giant tail must look healthy and strong, a virile symbol of malehood, to a peahen. Now a study of male flies with grotesquely long eye stalks, described in tomorrow's issue of Nature, supports that idea--but in an unexpected way: Females may be after a male's ability to withstand killer genes that would prevent females from conceiving male offspring.
A team of biologists led by Gerald Wilkinson of the University of Maryland, University Park, studied a type of Malaysian fly, called Cyrtodiopsis, with a strange genetic battle being waged inside its body: To perpetuate themselves, genes on the fly's X chromosome code for proteins that kill sperm carrying a Y chromosome. As a result, flies with these selfish genes produce mostly XX-offspring--daughters, that is. As a countermeasure, some males carry a Y chromosome with genes that can suppress the attack.
Wilkinson and his colleagues noticed that this skewed sex ratio showed up only in Cyrtodiopsis species in which females seek out long-stalked mates. To probe the side effects of this favoritism, they carried out experiments with C. dalmanni, in which only the males at the extremes--those with either the shortest or longest stalks--were allowed to breed. After 22 generations, males in the long-stalk lineage fathered more male offspring than previously, while ones from the short stalk camp sired predominantly daughters.
The explanation, the researchers say, must be that the genes for long eye stalks are found only on Y chromosomes carrying elements that suppress the selfish X genes. So while females may seem to have a crush on long eyes, what they really hanker for are sons. In that sense, the female is acting in the best interest of her genes: A son will, on average, produce more grandchildren for her than would a daughter, because males have more opportunities to mate in a female-dominated population.
While sexy, the theory may have a fatal flaw, says Rolf Hoekstra, an evolutionary geneticist at Wageningen Agricultural University in the Netherlands: The stalk-size genes and the suppression genes could wind up on different chromosomes during the normal process of recombination, in which the X and Y chromosomes exchange DNA. The researchers didn't address this possibility, Hoekstra says. And a previous study by Wilkinson showed that daughters of short-stalk fathers prefer short-stalk mates themselves. "That doesn't fit in with this model," say Hoekstra.