Who needs sex?

Sex is an evolutionary conundrum. If the point of sex, as most biologists believe, is to propagate your genes, parthenogenesis, in which females' eggs develop into young without fertilization by males, would seem to beat sexual reproduction hands down. Why split your genetic legacy with a second parent?

Yet the cost of sex doesn't stop 99.9 percent of known animal species from reproducing sexually. To understand why, biologists study the evolutionary history of the other 0.1 percent. Four evolutionary biologists, led by Michael Kearney of the University of Melbourne in Australia, studied two parthenogens in the deserts of Australia: a grasshopper (Warramaba virgo) and a gecko (Heteronotia binoei). In both, parthenogenesis evolved as a result of hybridization between sexual ancestors. And in both, parthenogenesis probably arose not once but twice.

Kearney's team sequenced DNA from the parthenogenetic grasshoppers and geckos, then compared it to that of their sexual ancestors, which are still alive today. The team discovered that the first parthenogenetic populations of both grasshoppers and geckos had historically expanded their ranges at the same time and in a similar pattern. So had the second. During the same period--late in the Pleistocene epoch, which lasted from 1.8 million until 10,000 years ago--the Australian desert underwent a series of expansions and contractions. Kearney thinks the shifting desert habitat probably brought isolated populations of the animals' sexual ancestors into contact long enough for their asexual hybrid offspring to become established. Parthenogenetic reproduction might be advantageous in harsh climates where mates are hard to find. In more comfortable environments, however, the genetic diversity that sex bestows may outweigh the costs of sharing your genes. (Molecular Ecology 15:1743-8, 2006)

COPYRIGHT 2006 Natural History Magazine, Inc.
COPYRIGHT 2008 Gale, Cengage Learning