Why Sex is Good

It is time-consuming, behaviourally and developmentally exhausting, and the genetic jumbling involved means that favourable combinations of genes are forever at risk of being broken up.

On the face of it, it is much easier to reproduce asexually — so why do we bother with sex?

In the nineteenth century, scientists suggested that sex was sustainable because it allowed faster evolution. However, to the embarrassment of evolutionary biologists there has never been a simple and general verification of why sex is good for us — until now.

Now, reports Dr Matthew Goddard and colleagues in Nature, when the going gets tough, yeast that have sex are much better at adapting to conditions than their non-sexy counterparts.

Dr Goddard, who is now at the University of Auckland and who carried out the work while at University from Imperial College, London, says when yeast cells are supplied with sufficient nutrients they reproduce asexually, but deprivation of nutrients triggers sexual reproduction.

However, it has been difficult to study the net effect of sex on the organism’s rate of adaptation, he says, because the yeast cells have been subject to different environmental conditions. Using new knowledge about the molecular mechanisms of cell division in yeasts, Dr Goddard developed a new asexual strain of yeast, which continues to reproduce asexually even when deprived of nutrients (as opposed to the usual switch to sexual reproduction).

In turn, this enabled Dr Goddard to directly compare the rate of adaptation of the new asexual yeast population with a wild yeast population placed under the same environmental conditions.

This study showed that sex does in fact increase the rate of a species adaptation to a harsh environment, but has no measurable effect on fitness in a benign environment.

“[This study is] an advance on earlier studies because by manipulating the sexual status of the yeast, we have mimicked the natural situation more closely and excluded a variety of possible confounding factors,” says Dr Goddard.

Dr Goddard says his results suggests that sex does come in handy when it comes to survival of the fittest, as the genetic shuffling may allow a greater chance that a useful suite of genes will come together.

The work by Dr Goddard and his team, says Rolf Hoekstra in a related article in Nature, is “exemplary” with the yeast system allowing “clear-cut comparisons” between sexual and asexual reproduction. “The paper by Goddard and colleagues is exemplary in this respect, owing to the excellent possibilities offered by their yeast system,” writes Hoekstra.

But while the Nature paper has lent weight to the ideas first put forward by August Weissman back in the nineteenth century, there are still many unanswered questions about sex.

For instance, yeast has no male-female distinction, so preventing a straightforward generalisation of the conclusions from this study to most animals and plants.

The Nature paper acknowledges that: “Population experiments with fast-replicating microorganisms have been very valuable in testing different ideas about the maintenance of sex, and a challenge now is to understand the nature of the mutations that underlie adaptation and to extend these techniques to larger plants and animals.”

Writes Rolf Hoekstra, “We are still far from a definitive answer to the question of why sexual reproduction is so common.”

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