Parasites and mutational load: an experimental test of a pluralistic theory for the evolution of sex

Ecological and mutational explanations for the evolution of sexual reproduction have usually been considered independently. Although many of these explanations have yielded promising theoretical results, experimental support for their ability to overcome a twofold cost of sex has been limited. For this reason, it has recently been argued that a pluralistic approach, combining effects from multiple models, may be necessary to explain the apparent advantage of sex. One such pluralistic model proposes that parasite load and synergistic epistasis between deleterious mutations might interact to create an advantage for recombination. Here, we test this proposal by comparing the fitness functions of parasitized and parasite-free genotypes of Escherichia coli bearing known numbers of transposon-insertion mutations. In both classes, we failed to detect any evidence for synergistic epistasis. However, the average effect of deleterious mutations was greater in parasitized than parasite-free genotypes. This effect might broaden the conditions under which another proposed model combining parasite-host coevolutionary dynamics and mutation accumulation can explain the maintenance of sex. These results suggest that, on average, deleterious mutations act multiplicatively with each other but in synergy with infection in determining fitness.