Female flies pick mates with sexy eyes

Among some species of stalk-eyed flies, the guy with the longer eye stems gets the girl.

 That alluring length has turned out to be one of the few documented cases in which a flamboyant male ornament, like the peacock's tail, indicates superior genes, says Gerald S. Wilkinson of the University of Maryland, College Park. In the Jan. 15 Nature, his team suggests that females of two tropical species in the Diopsidae family may not be so silly when they yield to the dazzle of stalks that can separate a male's eyes by up to one and a half times his body length. 

 In the wild, the flies stop grazing in the evening, and males stake out rootlets dangling from stream banks. On a disputed rootlet, males face off eye to eye, and the one with the shorter span usually flees. Challengers with similar eye stalks wrestle for the root. Females cruise the bank of fighting males, and a particularly enticing male can mate with as many as 24 partners in half an hour.

 The hidden benefit of going for the long-eyed males, Wilkinson suggests, is that these heartthrobs carry a tough Y chromosome that increases a female's chances of having sons, which are XY. In these fly populations, more sons means more grandchildren.

 In the wild, female flies outnumber males by roughly 2 to 1. Wilkinson attributes the skew to a so-called selfish form of the X chromosome, which sabotages regular Y's during sperm formation. The preponderance of X-bearing sperm results in an abundance of daughters.

 However, he predicted that the Y chromosome in long-eyed males can resist the selfish X and so create an unusual number of sons. In the largely female fly world, a mom who produces males has the best chance of spreading her genes wide in future generations.

 In the laboratory, the researchers allowed only the males with the longest eye stalks to breed. After 22 generations, the males had even more impressive stalks, and the sex ratio had shifted to a surplus of males -- suggesting a link between stalk length and more persistent Y chromosomes.

"Whether this is a freaky thing or common is too soon to say," Wilkinson notes.

 Michael J. Ryan of the University of Texas at Austin calls the notion that outrageous male ornaments signal good genes "a very intuitively pleasing argument," but, he laments, "there's only a handful of demonstrations."