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Budding Daughters: Sex offers no advantage to yeast living in mildly difficult conditions

Evolution of Sex

WHY did sex evolve? Experiments suggest that this kind of reproduction has different advantages for an organism, depending on how challenging its environment becomes. Sex is expensive. Besides expending energy in the generation and use of the necessary organs, sex permits an individual to pass on only half of its genetic material to its offspring. Theoretically, evolution should favour asexual organisms that pass on their entire genetic complement. There are two main theories about why such a costly strategy would evolve. One says that sex creates more variability in each generation and so improves a species' chances of survival in an ever-changing environment. The other suggests that sex maintains the overall fitness of a species by clearing out harmful mutations-the least healthy offspring die, taking the damaged DNA with them. "There's a lot of theory and virtually no data," says Clifford Zeyl of Wake Forest University in North Carolina. But in this week's Nature (vol 388, p 465), Zeyl and Graham Bell at McGill University in Montreal say they have got to the bottom of the mystery experimentally-at least for yeast. They found that sexually reproducing yeast clear out bad mutations more efficiently than yeast that don't have sex, but are no better at adapting to a mildly challenging environment. Zeyl and Bell grew brewer's yeast, Saccharomyces cerez?isiae, in two sorts of growth medium. One contained glucose, a sugar which the yeast finds easy to eat; the other contained galactose, a sugar to which it is poorly adapted. They allowed the yeast in some of the lab cultures to reproduce sexually, while the others reproduced asexually. At first, all the cultures in the galactose medium grew more slowly, a reflection of the less suitable conditions. But by the end of the experiment, eight months later, both the sexual and asexual yeast cultures were doing much better on the unfamiliar sugar, showing that sex was not an important factor in their ability to adapt. In glucose, on the other hand, sex made a difference. Whereas the asexual yeast was still growing at the same rate after eight months, about 600 generations, the sexually reproducing populations were healthier on both glucose and galactose, growing 22 per cent faster than they had at the start of the experiment. Since they were fitter than the asexual cultures on both types of food, Zeyl attributes the improvement to the loss of harmful mutations rather than to more rapid adaptation to glucose. "Selection can pick out the few that were lucky and had fewer deleterious mutations," Zeyl says, while the "genetic scape goats" die off. However, recent experiments have shown that a species of green algae adapts more rapidly to a very tough new environment with sex than without it. "We've found that both these mechanisms work," Bell says. Commenting on the yeast experiments, evolutionary theorist Alexey Kondrashov of Cornell University in Ithaca, New York, says "that's pretty good evidence". But he cautions that Zeyl and Bell only infer the clearing of deleterious mutations from the relative fitness of the yeast cultures-they do not demonstrate it directly. "I would like to see the deleterious mutation rate reliably measured for at least one organism," he says. Jonathan Knight