Sex begets sex

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The UH team: Rolf Lohaus, Dr. Ricardo Azevedo, Suraj Srinivasan. The study was done in collaboration with Dr. Christina Burch and Kristen Dang at University of North Carolina at Chapel Hill.


Scientists have long wondered why organisms bother with sexual reproduction. It makes a whole lot more sense to just have a bunch of individuals that can clone themselves. In the March 2 issue of the journal Nature, a team headed by Dr. Ricardo Azevedo from the Department of Biology and Biochemistry at the University of Houston proposed that sexual reproduction actually selects for conditions that favor its own maintenance - a case of evolution forging its own path.

One advantage of sex is that it can help rid the genome of harmful mutations. When, as a result of sexual reproduction, organisms shuffle their genes, harmful mutations can be brought together in the same genome, making them more susceptible to the cleansing action of natural selection. But for this to work, mutations must be more harmful when combined in the same genome than when separated - a phenomenon known as negative epistasis. If negative epistasis were true, it would provide a powerful explanation for why sex has managed to persist for so long despite its numerous costs. But the phenomenon has yet to be widely demonstrated in nature and scientists have yet to figure out how such a thing evolved in the first place.

An experiment run in an artificial gene network model shows that negative epistasis is a natural byproduct of sex itself. The researchers created digital organisms that reproduced through sex in the same manner as real organisms. And like a regular organism, the virtual ones developed a natural buffer to resist change by mutations. This ability, called "genetic robustness," is thought to be one of the main benefits of sex. By shuffling genes, sex allows a population to spread its mutations across many individuals within a group. The mutations become diluted and can be effectively dealt with by an individual's genetic repair system.

But the researchers found that the protection only works when the digital organisms were facing a few mutations at a time. When assaulted by many at once, their repair systems became overwhelmed and the organisms died. Dr. Azevedo thinks this happens in real life, too.