Why have sex? Cells shed light

April 01, 2005|By Bryn Nelson | Bryn Nelson,NEWSDAY

Why do we have sex?

It's a dumb question for most people, perhaps, but a frustratingly difficult one for evolutionary biologists. After all, sex is a time-consuming, exhausting, and genetically risky affair, and yet most animals and plants, from dogs to dogwoods, do it.

Thanks to the sex lives of yeast cells - or lack thereof - scientists may have a better answer for why sexual reproduction arrived so early in our evolutionary past and pays off so handsomely in the long run.

Yeast cells that engage in sexual reproduction, a new study suggests, boost their genetic variation and adapt better to harsh conditions than those consigned to an asexual existence.

In an e-mail, study co-author Matthew Goddard from the University of Auckland in New Zealand said the research "tackles one of the oldest theories concerning the maintenance of sex" and shows that sexual populations can adapt more rapidly than asexual ones.

By definition, sexual reproduction delivers re-shuffled genetic material from parents to their children. Virtually all animals must have sex to reproduce, but some organisms such as microbes, fungi and yeast often have a choice, depending on the circumstances.

Researchers in the late 19th century suspected that sexual reproduction won out over asexual reproduction early on and now dominates in the animal world because it allows for faster adaptation in a pinch.

But no one could prove it.

In yesterday's issue of the journal Nature, Goddard and colleagues provide a new boost to the old idea by taking advantage of a quirk in yeast reproduction: well-fed yeast cells are content to reproduce asexually. But if starved, they have sex instead, yielding four spores that are functionally equivalent to two sperm and two eggs. The spores pair, fuse, and produce the next generation of yeast.

Goddard's team mutated yeast cells, leaving them sexually impotent, and compared them to their normal counterparts. When both sets were starved and made rather uncomfortable by rising temperatures, the sexually competent cells grew significantly faster.

Rolf Hoekstra, a geneticist at Wageningen University in the Netherlands, credits the "elegant and rigorous experiment" in an accompanying commentary for supporting the idea that under duress, those populations that do it, do better.

Plus, as he writes, there are other perks: "In a world without sex, there would be no males and females, no flowers, no insects specialized in pollinating them, no extravagant colour and form like the peacock's tail; and much animal behaviour aimed at finding and selecting mates would not exist."

A dire world, indeed, but the new results still leave unanswered the question of whether sex is handy because it frees good genes from bad genetic neighborhoods, or because it brings together mutations to work for the common good.

Newsday is a Tribune Publishing newspaper.

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