What's the point of sex?

Why do we bother with sex? I don’t, of course, mean just the physical act. Artificial insemination made that dispensable several centuries ago.

The point of sex — artificial or natural — is thought to be its mixing of parental genes to generate novel progeny.
But many organisms — for instance, greenfly — reproduce by parthenogenesis, whereby females produce clones of themselves. Similarly, bdelloid rotifers, a group of tiny animals that live in freshwater pools, haven’t had sex for at least 100-million years.

Yet sex remains the dominant reproductive strategy for higher organisms. But why? We expend a lot of time and energy finding, seducing and mating with our mate, and as a method of making babies it just doesn’t seem to make economic sense.

Asexuality is twice as efficient, in terms of making babies, as sex, essentially because the cloning family doesn’t squander its resources on wasteful males. So why haven’t more animals abandoned sex?

The view that dominated for many years is that sexual reproduction, and the consequent gene-mixing, makes our genomes more adaptable. The problem with this theory is that the one thing you do know about your set of genes is that they all work pretty well together. It seems foolhardy to jumble them up with genes from a total stranger in the hope that this new combination just might work better. Sex might occasionally bring together successful gene partnerships, but it’s more likely to break them apart.

The most popular explanation for sex is named after the Red Queen in Lewis Carroll’s Through the Looking-Glass, who had to keep running just to stay on the same spot. So, the theory goes, do organisms in the evolutionary race.

Nowhere is the endless race more deadly than in the battle between parasite and host. To defend us against parasites, evolution has endowed our cells with sturdy locks. Yet the parasites are continually evolving to pick those locks. Our genes countermove by evolving new combinations. But with their short life cycles, parasites can evolve faster. Enter sex to the rescue. By mixing our genes with those of our partners, our children will inherit a new set of locks to resist the invaders.

The Red Queen has many supporters, but the theory is not backed by much evidence. Diseases such as tuberculosis and malaria infect generation after generation, while the theory predicts that sexual offspring should be relatively resistant to their parents’ parasites.

A rival theory, Muller’s ratchet, suggests we have sex to eliminate damaged genes. Mutations drive evolution but are mostly harmful, and in asexual organisms they accumulate: each mutation is a turn of the ratchet. Asexual organisms are condemned to the ratchet until their genome becomes so full of mutations they can’t survive. But as long as the partners in sexual reproduction are unrelated (unlikely to have the same mutations), sex can reshuffle the gene pack. Some offspring will get all the damaged genes but others will escape with mostly good (unmutated) genes from each parent. By reshuffling the gene pack, sex can reverse the ratchet to purge the genome of harmful mutations.

The ratchet theory languished for many years after it was realised that sex was a cumbersome way of ridding the genome of simple mutations that could more easily be repaired. But Matthew Meselson of Harvard University recently pointed out that there is another, more damaging class of mutations, caused by transposons (chromosomal parasites that inactivate genes), which cannot be repaired. Sex may have evolved as a way of purging the genome of transposons.

But what about those hardy bdelloid rotifers? Meselson discovered that they are highly unusual in having transposon-free genomes. Unlike other animals that travelled along the asexual path, the rotifers were not cranked into genome catastrophe by Muller’s ratchet.

So sex is a chromosomal purge, a kind of Epsom salts for your genome. That’s something to think about. —

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