Sex shouldn't be widespread in nature, but a new study shows why it is

It is not just an emotional mystery; its very existence poses a deep question

GUILT-FREE intercourse may, as Philip Larkin wrote, have begun in 1963, but sexual reproduction has been around a good deal longer than that. About two billion years ago single-celled organisms began exchanging and mixing up genetic information in ways modern biologists recognise as rudimentary forms of sex—though it would be well over a billion years more before animals evolved nervous systems that would let them derive any pleasure from it. Yet the question of why sex exists at all remains troublesome. A creature which reproduces asexually passes on all of its genes to each of its progeny. One that mates with another, by contrast, passes on only half of them. On the face of things that is a huge selective disadvantage. There must therefore, evolutionary biologists believe, be equally huge compensating benefits.

Two ideas exist about what these might be. One is that the genetic variety sex creates prevents parasites and pathogens evolving a single, optimum technique for exploiting a host species. This is the “Red Queen” hypothesis, an allusion to a character in “Through the Looking-Glass” who had to run as fast as she could to stay in the same place. The other idea is that the continual mixing of genes from generation to generation separates good and bad mutations, permitting the bad ones to be purged by natural selection without taking the good ones along for the ride. This process was described by Joel Peck, one of its progenitors, as plucking rubies from rubbish.

“Plucking rubies” and the “Red Queen” are not mutually exclusive. Both could be true. But, while the queen has experimental evidence to back her up, rubies have had little such validation. Until now. For Michael Desai of Harvard University believes he has demonstrated such plucking experimentally in brewer’s yeast. This is a well-understood experimental organism and one ideal for Dr Desai’s purpose because it can reproduce both sexually and asexually. Studying the switch between the two modes, he hoped, might illuminate the purging process. And, as he and his team write in Nature, it has.

Despite having two sexes, known as mating types a and alpha, yeast’s default mode of reproduction is asexual, so Dr Desai’s first task was to work out a way to turn his yeast cells on to sex, as it were. He did this by adding to their DNA genes for resistance to two antibiotics, hygromycin and G418, and arranging for this resistance to be turned on only when the gene for mating was also active. Adding the antibiotics to the yeast’s growth medium meant only sexually active yeast cells could survive.

This done, he and his team set up 24 lines of this modified strain (12 of mating type a and 12 alpha) and let them grow for six months, a period that corresponds to about 1,000 yeast generations. Six lines of each mating type were forced to undergo sexual reproduction every 90 generations, by mixing the sexes together and adding the antibiotics. Others were left to carry on cloning themselves. At these 90-generation break points the researchers also sampled each line to look for any genetic mutations that had arisen in the intervening period. Such mutations are the stuff of evolution, and Dr Desai hoped they might tell the story of why, in an evolutionary sense, sex works.

They did. The researchers found, as predicted, that when a beneficial mutation appeared in a few of the asexually reproducing cells, it would spread only if its positive effects outweighed the negative effects of any deleterious mutations that appeared in the same cells. Even if a good mutation prospered, it did so slowly, as any bad mutations associated with it came along for the ride when the genome it was in passed from one generation to the next.

In the sexual yeast population however, good and bad mutations often went their separate ways when the parent cell’s genome was chopped up and mixed around during reproduction. This permitted different combinations of good and bad mutations to pass to the genomes of different offspring of the same parent cells. That made it easier, in an evolutionary sorting of wheat from chaff, for the good mutations to spread, even if they first appeared in bad company. So, as evolutionary theory predicts, over the course of the experiment genomes containing deleterious mutations disappeared and positive mutations accumulated in the genomes of cells that remained.

The crucial test, though, came at the end of the experiment, when Dr Desai compared the asexual to sexual strains. In every case, the descendants of sexually reproducing yeast cells bested their asexual rivals in the competition for food and resources. His experiments thus confirm that the ruby hypothesis works—at least, in a laboratory. That puts it on an equal footing with the Red Queen. What goes on in the wild, though, has yet to be determined.