Purging of deleterious mutations through sexual selection: negative evidence. Jing Zhu, James Fry. Biology Dept, University of Rochester, Rochester, NY.

   Simple population-genetic models, when parameterized with available estimates of mutation rates, predict that recurrent deleterious mutations should severely depress the reproductive capacity of populations of higher eukaryotes, including Drosophila. One way that the mutational genetic load could be mitigated is through sexual selection: if deleterious mutations are eliminated primarily because of their effects on male mating or fertilization success, then little reduction of population mean fitness need occur. The sexual selection hypothesis requires that there be substantial overlap between mutations that reduce female fitness and those that reduce male mating and fertilization success, and that selection against deleterious mutations is on average considerably stronger in males than in females. We tested these assumptions by allowing spontaneous mutations to accumulate in an outbred population in the near-absence of selection, using the Middle Class Neighborhood (MCN) design. After more than 70 generations of mutation accumulation, we created lines from this population whose genomes were derived primarily from either males that were consistently successful at obtaining matings in competitive mating trials ("stud" lines), or males that were consistently unsuccessful ("dud" lines). Males from stud lines (descendents of the original males) had substantially higher mating success than males from the dud lines, but females from the stud and dud lines did not differ in reproductive output, giving no evidence that mutations that depressed mating success also depressed female fitness. Moreover, relative to stud and dud lines from a control population in which selection had been allowed to operate, the stud and dud lines from the MCN population showed similar declines in male mating success and female reproductive output, giving no evidence for generally stronger selection in males.