With his low-fitness viruses, Chao collaborated with Thutrang Tran and Crystal Matthews in designing experiments to test whether viral sex leads to an increase in fitness. The scientists grew different pairs of the debilitated viruses together with their host bacteria, and they controlled how much sex was going on by altering the ratio of bacteria to viruses. When they put in far fewer bacteria than viruses, they ensured that multiple viruses would be infecting the same host bacterium. As they increased the likelihood of producing viral progeny that had genetic material from two parent viruses, they found that they were more likely to find progeny with increased fitness.
Here's why: An asexually reproducing virus will pass on all its harmful mutations to each of its descendants, but if the descendants have genetic material from two parents, they can end up with the best parts of both. Imagine that one virus has a deleterious mutation on segment 1, and the other one has a deleterious mutation on segment 2. If the viruses are reproducing in the same bacterium, out can pop a virus with neither bad segment.
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