When survival is on the line, sex may be the answer. So says a new study that sheds light on the mystery of why sex evolved as a reproductive strategy, despite the time and energy drain of mating. Biologists have shown sexually reproducing yeast adapt more quickly to stressful conditions than asexually dividing yeast do.
A century-old theory suggests sex evolved because it increases genetic variation in offspring, accelerating natural selection. But this theory remained untested because comparing sexual and asexual reproduction under identical conditions was a tricky experiment to do.
Matthew Goddard of the University of Auckland, New Zealand, and colleagues at Imperial College London solved the problem by genetically altering yeast cells to be incapable of reproducing sexually. Normal yeast can reproduce by dividing either asexually, producing daughter cells with little genetic recombination, or sexually, producing spores with only half of the parent's chromosomes. These spores must "mate" with other spores and combine genetic information to create normal yeast.
Goddard's group found that both the altered and unaltered yeast grew at the same rate under non-stressful conditions. But when the team stressed cultures of each type of cell by increasing the temperature and adding salt to the mix, the sexually reproducing strain grew faster than the asexual strain.
The findings, published 31 March in Nature, confirm the old theory that sex confers advantage by jumbling genes, Goddard says. Sexual reproduction recombines genes more quickly, creating new mutations that might offer an advantage in the new environment. Next the group plans to study how the process works in yeast. "We still don't know about the exact nature of the underlying mutations that have allowed this adaptation to occur," Goddard says.
The experimental technique is a "fantastic move forward," says Sarah Otto, an evolutionary biologist who studies the evolution of sex at the University of British Columbia, Canada. "This area is plagued with an imbalance between lots and lots of theory and very little experimental data to test these theories," she says. "I think that this probably sets a new gold standard for experimental tests of the evolution of sex."