Inquiring scientists want to know: Why is sex so popular?

Thursday, February 14th 2002, 12:00 am
By: News On 6

Don't let this dampen your Valentine's Day, but the fact is, scientists are still trying to figure out what's so great about sex.

They're looking beyond candy hearts and romantic dinners to the fact that in the cold light of biology, sex is a pretty inefficient way to reproduce.

But it's so widespread among plants and animals that there must be some payoff. After more than a half-century of debate and some 20 published theories, scientists are still trying to pin down just what the payoff is.

``It's clearly one of the most fundamental questions in evolutionary biology,'' says William Rice of the University of California, Santa Barbara. He has pursued the question by giving fruit flies bogus chromosomes, then checked the eye color of thousands of descendants.

It is easy to forget that sex is not the only game in town for the plant and animal kingdoms. In fact, there are thousands of asexual species now, Rice said.

Many species of the whiptail lizard, for example, are all-female and just clone themselves with unfertilized eggs. Scientists believe the asexual lizards arose from sexual species.

Microscopic creatures called bdelloid rotifers have reproduced through asexual cloning for tens of millions of years. That greatly annoys scientists who say no creature should be able to keep doing this so long.

Rotifers, in fact, have been called an evolutionary scandal. Imagine that: a no-sex scandal.

Actually, it is rare for an asexual species to persist a very long time in evolutionary terms, suggesting again there's something beneficial about sex.

Yet, at first blush, it makes some sense to have self-reproducing females and just dispense with males altogether, even before you consider things like singles bars. After all, if your job is to pass on your genes to future generations _ and according to evolutionary biology, that is your job _ sex just gets in the way.

Consider the notion of two mating sexes. Each female passes only half her genes to each offspring, rather than all of them. And about half her brood on average ends up without a womb, which chops the next generation's productive capacity in half.

What's more, sex breaks up the successful gene combinations found in the parents and gambles on new, untried mixes in the next generation. Does that make any sense?

Maybe so. It's pretty clear, scientists say, that the evolutionary lure of sex has something to do with that gene-mixing. While clones merely pass along their genetic endowment in a chunk, sexual species shuffle the deck.

To understand that, remember that the genetic makeup of an organism is somewhat like a baseball team. Everybody has a full team, with all the positions covered, but who plays at each spot differs. And there are good players and bad players in the same way there are good genes and bad genes.

Clones essentially pass their own rosters on to their offspring, while sexual species create new rosters.

Why tinker with a successful genetic lineup?

Currently, most scientists offer two general theories about why sex is so good: It helps a species get rid of harmful mutated genes, or, alternatively, it helps the population take advantage of beneficial mutations. Call it bad genes vs. good genes.

Of course, the explanation could be some combination of those two ideas, although scientists suspect one or the other is probably the major factor.

The bad-gene idea says sex can make the faulty genes sitting ducks for elimination by natural selection, by separating them from good genes as they pass through generations. It can group bad genes together so they get wiped out in batches when the unfortunate recipients don't reproduce. Sex can also break up harmful combinations of genes, even when each by itself isn't so bad.

The alternative view says sex can help good genes spread through a species or bring favorable combinations together, speeding up evolution and helping species adapt more quickly to changing environments. One version says sex helps defend populations against evolving parasites and germs, for example.

Rice reported his experiment with the fruit flies in October. It bolstered the idea that sex helps good genes spread through a population. The work showed that a favored gene spread more quickly if it appeared on a chromosome that participates in gene-shuffling than if it did not. Rice's experiment also found evidence that harmful mutations accumulate faster without gene-shuffling.

In fact, Rice said, it appears humans naturally produce harmful mutations so often that we'd go extinct if we didn't use sex to get rid of them. But it's not clear how often that situation occurs in other animals, and therefore how widely the bad-mutations theory of sex can be applied.

That's especially a mystery for non-vertebrates that pump out their generations quickly, said gene expert Peter Keightley of the University of Edinburgh in Scotland.

Most evolutionary biologists probably favor theories about promoting the spread of good genes rather than those focusing on eliminating harmful mutations, Keightley said.

His own pick? ``I'm still open. We need better data to decide.''

Alexey Kondrashov of the National Center for Biotechnology Information said he favors the harmful-mutations idea. But mostly, he too would like to see some definitive research findings _ on whether most animals suffer enough bad mutations to justify sex _ and get the whole debate over with.

``The field is tired about talking about theory,'' Kondrashov said.

``My sense is that it's more or less accepted that there are a couple of coherent ideas, reasonable ideas which may explain what happens,'' he said.

``And also it is accepted that we are still pretty far from knowing what actually happens.''