Saturday, July 21, 2007

Selectively Killing Off Males

MSNBC writes up a fascinating story about one of the uncountable number of skirmishes in the struggle for survival. What if a parasite (e.g. the bacteria wolbachia ) figures out the best way into the next generation of a host is to sneak in through genetically female eggs? What if the parasite (with wisdom greater than your normal germ) concludes that killing off genetically male eggs is, therefore, good for the cause.

Butterflies fast forward evolution: Sylvain Charlat of the University of California, Berkeley, and the University College London, along with colleagues, studied the sex ratios of Hypolimnas bolina butterflies on the Samoan islands of Upolu and Savaii, where males had dwindled to 1 percent of the populations in 2001.

The likely culprit was a male-killing parasite, Wolbachia, which lives inside the butterfly's reproductive cells, preferably female sex cells. With a female host, Wolbachia can hitch a ride to the next generation aboard the mother's eggs. Since males are "useless" for the bacteria's survival, the parasite kills male embryos.

But the male butterflies found a way to stealthily overcome the parasites. At the beginning of 2006, the scientists found the males made up about 40 percent of Upolu's butterfly population.

99 females for every guy! This is the story line of thousands of science fiction fantasies. Butterflies as a group have about 25 million years of specific and 450 million years of insect evolutionary strength to draw upon and the boys make a comeback.

Hosts and parasites have a long intimate history of battling each other and the key to parasite survival is to not kill off all the hosts. Within 10 generations the percentage of male butterflies has rebounded from this brink of extinction episode. UW Madison Anthropologist John Hawks notes this culling of most male genomes serves to amplify the specific mix of the survivors as the species moves forward in time.

Escaping the male-killer: The rapid response to the high infection rate is quite expected based on the mechanism of male-killing, as long as the few remaining males are necessary, their genes will be represented in all the next generation. So it's an enormous reproductive advantage, in this case causing an allele to go from near zero to 40 percent in only 10 generations.

Creating a living organism from chemicals is a complex task, miraculous given the number of steps that must go correctly. Millions of years have fine tuned ways to keep the assembly process viable but every step is subject to breakdown or attack. Fortunately, the over all process is remarkably resistant and adaptable to change. Change, after all, is constant.