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Sex matters — for worms, at least

by Rowan Hooper

It is perhaps rare for readers of British tabloid newspapers to ponder the same questions as evolutionary biologists, but that may have been the case last week. The tabloids enjoyed themselves at the expense of women suffering from a rare and often debilitating condition: persistent sexual arousal syndrome. (The Boston Globe also carried the story, though it didn’t aim to titillate its readers.)

The syndrome can lead to spontaneous, almost continuous orgasms, leaving the affected woman unable to concentrate on even mundane tasks. Some readers may have wondered what causes the syndrome (unlike the tabloid editors, who didn’t seem to care), but no one really knows. As a consequence, women affected can be offered no instant cure.

From wondering about what causes the syndrome, more thoughtful readers might have been prompted — understandably — to question why males are necessary, and indeed why sex between males and females exists at all. It is this question (the two ask essentially the same thing) that is a favorite of evolutionary biologists. This is not just because evolutionary biologists enjoy thinking about sex, but because it is one of the great mysteries of science.

Why don’t we, like many species of insects, amphibians and reptiles, reproduce by parthenogenesis? That is, simply clone ourselves and ensure that all and not half of our genes go into the next generation? Why bother making males?

There must be a big advantage to sex that outweighs the numerical advantage in genes gained by cloning. (The claim “because it’s more fun” is not one that scientists believe accounts for the evolutionary persistence of sex.) Biologists have suggested that sex prevents harmful mutations from building up in the genome and that it generates novel gene combinations. But they haven’t been able to agree on what the advantage is in the short term. Now, new work suggests one.

There are two sexes in the nematode worm Caenorhabditis elegans: males and hermaphrodites. The latter bear both sperm and eggs, and can self-fertilize; they can reproduce without males. So why do males exist at all?

Because, say a team of scientists from the United States and Canada, sexual reproduction enhances the developmental flexibility of offspring. It allows baby nematodes to change their sex and genetic makeup after birth, and in so doing they gain a critical survival advantage when times are hard.

Like human females, the hermaphrodite nematode worm bears two X chromosomes. The male nematode has only a single X (there is no Y chromosome in this species). The team, led by University of Wisconsin professor of genetics Elizabeth Goodwin, showed that the sex ratio — the percentage of males and hermaphrodites — changed depending on the amount of food the animal senses in its environment. When an XX hermaphrodite worm is still too young to display any sexual characteristics, it can judge how much food is likely to be available when it matures.

If there is a lot of food available, it means there is probably going to be lots of food around when the worm is sexually mature: In this case, in times of plenty, the animals keep both their XX chromosomes and remain hermaphrodites. If, on the other hand, food is scarce, or a certain chemical metabolite from bacteria is present in the worms’ environment, a significant number of XX animals will lose one of their X chromosomes and become male.

This sex switch may be carried out, Goodwin’s team suggests, to increase the chances of survival, as males may be better able to withstand the changed chemical environment, and because they can forage for food at greater distances. The group published their results earlier this month in the journal Science.

“We all know that we can alter our behavior, depending on the environment in which we are raised,” said team member David Pilgrim, of the Faculty of Science at the University of Alberta. “But it was thought that our basic genetic makeup is unaltered by these effects. What we have now shown is that our nature, our genes, may be altered by our nurture, the environment.”

The worm progeny produced by hermaphrodite self-fertilization were unable to make the same switch. “What we learned is that sex is good,” Goodwin notes. “The advantage of having boys around is you have more flexibility in development and gene expression. It’s probably not the only reason, but it seems to be one of the reasons C. elegans has kept boys around.”

So can men breathe easy, their role validated by science? Not quite, as clearly, what’s good for nematodes may not be quite the same for humans. All we can say is that having male nematodes about — and therefore sexual reproduction — confers a greater degree of developmental flexibility and may accelerate evolutionary change.

In an evolutionary context the work is exciting, says Goodwin, throwing some crumbs to us poor men:

“The take-home message is that sex matters,” Goodwin says. “There is clearly an advantage to an animal that comes from mating vs. nonsexual reproduction.”

Evolutionary biologists (and tabloid editors) are unlikely to stop being obsessed with sex for some time to come.