SECTION: Section F; Column 1; Science Desk; SIDE EFFECTS; Pg. 3

LENGTH: 786 words

HEADLINE: A World Without Males: For Insects, Science Fiction Can Be Reality

BYLINE: By JAMES GORMAN

BODY:


What people dream, insects do. Ask Michael Majerus.

Dr. Majerus, a geneticist at the University of Cambridge, studies insects and the male-killing bacteria that live in them.

In his recent book, ''Sex Wars: Genes, Bacteria and Biased Sex Ratios,'' he points out right at the start that the idea of a plague killing all the men is a common science fiction plot, while in insects, the bacteria are already at work.

In science fiction, men often turn up from another planet or place or time. But in some stories, Dr. Majerus writes, there are no men at all. He mentions a short story by the English writer John Wyndham, perhaps best known for his novel ''The Day of the Triffids.'' In ''Consider Her Ways,'' Dr. Majerus writes, ''the human population has become solely female, males having been wiped out by a male-specific pathogenic virus.'' The women survive, with parthenogenetic reproduction of daughters.

Among insects, male-killing bacteria are no fantasy. They have evolved several different times and affect a variety of insects. Apparently it's an attractive evolutionary option for the bacteria, which are simply trying to get along and spread their DNA.

Scientists have known about these bacteria for about 50 years, but how they do what they do is still a puzzle. In the current issue of Nature, Dr. Zoe Veneti and Dr. Gregory D.D. Hurst, both of University College London, and colleagues report on how a particular group of male-killing bacteria figure out which insect embryos are male.

Dr. Hurst, the senior author, said that the bacteria pick up on a biochemical trick that male fruit flies use to compensate for having only one X chromosome. The flies increase protein production by that single X, and the bacteria react to chemical signals of this process.

These bacteria are hitchhikers, passed on from mother to daughters because they live within cells, including the egg cell. Insects don't have a good intracellular immune system, and the female gamete, the egg, is big enough to accommodate freeloaders.

The male gametes have no room to spare. They themselves are looking for a nice large egg to join up with. If an egg is a minivan, then a sperm is a motor scooter. I think there's a children's book on reproduction here: ''The Voyager and the Vespa.''

I'm sure the sperm imagine that they are big, rumbling Harley Davidsons, but that's just male ego.

For bacteria, this is just another way to spread their genes. What we call infection, they call life.

Some bacteria and viruses spread horizontally, infecting other organisms. The egg riders travel vertically, from one generation to the next. One infection moves through space, the other through time.

''Now,'' Dr. Hurst wrote in an e-mail message, ''if male and female siblings compete for food, then the death of males will enhance the survival of their sisters: and this is adaptive from the perspective of the bacterium, as it cannot be transmitted by those males.

A nice example is in ladybird beetles, where the female eats the egg carrying her dead brother, and male-killing by the bacterium then is a clear way of transferring resources from an infected male host (that cannot transmit it) to an infected female host (that can).''

This wouldn't happen with people, for a couple of reasons. Mammals have good intracellular immune systems, and we usually produce children one or two at a time. The bacteria are playing a numbers game, and they need to have a significant group of female embryos each generation.

What the human fantasies are about is a disease that targets men alone. The odds are that more than gamete size drives the imagination.

Or perhaps that really is the source of these fantasies. If you have a minivan, you don't really need a scooter. It's a lot easier to find a way to circumvent the need for sperm than for eggs.

And as reproductive technology develops rapidly, the need for men shrinks. As a friend of a friend said: ''Men are good for only two things. One is taking out the garbage. And I forget what the other one is.''

Dr. Majerus speculates in his book about men dreaming of coming upon an island of women who lost all their men to a plague. What, he wonders, would the female version of this fantasy be?

Hard to say. I don't know of any plots, invented by men or women, in which a plague kills off all the women.

A colleague suggested that this is because oppressors have no desire to get rid of the oppressed.

Perhaps. Or perhaps the reason really is gamete size and not sexual politics. The plot is likely to go on longer with women. Managing to keep reproducing with eggs alone is less of a jump that trying to keep going with male gametes.

Gentlemen, start your Vespas.

URL: http://www.nytimes.com

CORRECTION-DATE: March 12, 2005

CORRECTION:


An article in Science Times on Tuesday about bacteria that kill only male insects misstated the name of the journal that published a report on them by Dr. Zoe Veneti and Dr. Gregory D. D. Hurst and colleagues. The journal was Science, not Nature.