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Thursday, March 25, 2004
Male-killers on the loose
By ROWAN HOOPER
Things are never what they seem. Men certainly aren't, according to the American writer Marilyn French: "Whatever they may be in public life, whatever their relations with men, in their relations with women, all men are rapists, and that's all they are."
French said that in 1977. It wasn't good PR for the feminist movement. Maybe some feminists did, as detractors claimed, have something against men -- but it was nothing compared to what a group called Wolbachia have against them.
Wolbachia are male-killers, though sometimes they only castrate males. Yet they are not radical feminist guerrillas: They are bacteria. Wolbachia quietly infect at least a million species of insects, spiders and crustaceans, and for males with a dose, the outcome is bleak.
Wolbachia are obligate symbionts, which means they can only live in the cells of a host animal. But they don't like males. In some species the bacteria kill the male outright (no males mean more resources for females), while in others Wolbachia hijack the male's reproductive apparatus and makes it female. In some insects Wolbachia cause parthenogenesis (egg development without sperm fertilization) and thus do away with the need for males altogether.
Biologists, naturally, find Wolbachia fascinating. It is only one group of bacteria yet it infects a huge variety of different hosts (no one knows how it can infect so many), and sometimes has profound biochemical and physiological effects. (There can't be many things more profound than suddenly turning into a female halfway through your life.)
And studying the bacteria offers insights into that most intriguing of subjects: sex. In some cases infection may even accelerate evolution (more on this later). There are potential medical benefits too. Biologists looking at all these problems will be aided by the publication last week, in the Public Library of Science, of the complete genome sequence of a Wolbachia species.
Sequencing has been difficult because until now scientists haven't been able to grow enough Wolbachia outside of the host insect. But an international team led by Jonathan Eisen from the Institute for Genomic Research, Maryland, succeeded by using a Wolbachia species that lives in fruit flies.
The researchers found that the Wolbachia genome is overrun with "mobile genetic elements." As their name suggests, mobile genetic elements move around the genome, allowing it to change rapidly. This might explain why the bacteria can infect so many different hosts and have so many different effects.
One of its most subtle effects is the way the parasite enters the sperm cells of its host male, and biochemically alters them, making the male infertile unless it mates with a female infected with the same Wolbachia strain. If this happens, the altered sperm are "rescued" by the Wolbachia in the female.
John Werren, a professor of biology at the University of Rochester, N.Y., who works on Wolbachia, likens the alteration to encryption. The bacterium encrypts the sperm cell, rendering it useless unless it encounters and is "de-coded" by bacterium from another infected host. The result is that infected males can only impregnate other infected females, not uninfected ones, and this makes it difficult for uninfected females to find a compatible mate.
Encryption and decoding: It sounds like science fiction, but it's quite likely that any butterfly or beetle you see outside -- peaceful as it may seem -- is infected.
The system ensures that as many hosts as possible will pass Wolbachia on to the next generation. Infected males can have offspring only with infected females, and since the bacteria hitch a ride down the generations in a female's eggs, infected females automatically pass the infection on to all their offspring. The only possibility for producing uninfected offspring is for two uninfected hosts to mate.
"This is the best evidence of a parasite contributing to speciation that we've seen," said Werren. "Splitting a species in two is probably just a side effect of the bacteria's reproductive method, of their way of eliminating noninfected hosts."
Darwin was the first to suggest (in "On the Origin of Species") that when a species is divided (say by a river or a mountain range), then they will evolve differently. Eventually, if brought back together again, they would be unable to breed: They would have become different species. Wolbachia infection may have the same effect.
And as when any new organism's genome is sequenced, so with Wolbachia there are also practical applications. Wolbachia, it has been proposed, could be used to control insect pests. Also, in 2002, a paper in Science showed that river blindness -- the second leading infectious cause of blindness in the world -- was caused by Wolbachia.
Eighteen million people a year are infected when parasitic worms burrow into the skin and release millions of offspring that spread throughout the body. But the worms themselves are parasitized by Wolbachia, and it is the bacteria, scientists found, that trigger the signature inflammatory response. That can then lead to blindness and serious skin disorders (such as elephantiasis). Now the Wolbachia genome is known it is hoped that new ways will be found to control the worm.
The latest genome to be sequenced might be a mere bacterium, one that until recently was considered obscure. But it is now recognized as a kind of godfather, a puppet master of millions of insects, a force driving evolution, and a male-killer supreme: Things are never what they seem.
A book of Natural Selections columns translated into Japanese, "Nou to sekkusu no seibutsugaku (Evolution, Sex and the Brain)," is published by Shinchosha. Rowan Hooper is a researcher at Trinity College Dublin. He welcomes readers' comments at firstname.lastname@example.org