Testing alternative methods for purging genetic load using the housefly (Musca domestica L.).

When a population faces long-term inbreeding, artificial selection, in principle, can enhance natural selection processes for purging the exposed genetic load. However, strong purge pressures might actually decrease fitness through the inadvertent fixation of deleterious alleles and allelic combinations. We tested lines of the housefly (Musca domestica L.) for the effectiveness of artificial selection to promote the adaptation to small population size. Specifically, replicate populations were held at average census sizes of 54 for nine generations or 30 for 14 generations while being subjected to artificial selection pressure for increased fitness in overall mating propensity (i.e., the proportion of virgin male-female pairs initiating copulation within 30 min), while also undergoing selection to create differences among lines in multivariate components of courtship performance. In the 14-generation experiment, a subset of the lines were derived from a founder-flush population (i.e., derived from three male-female pairs). In both experiments, we also maintained parallel non-selection lines to assess the potential for natural purging through serial inbreeding alone. Sub-populations derived from a stock newly derived from the wild responded to artificial selection for increased mating propensity, but only in the short-term, with eventual rebounds back to the original levels. Serial inbreeding in these lines simply reduced mating propensity. In sub-populations derived from the same base population, but 36 generations later, both artificial selection and serial inbreeding increased mating propensity, but mainly to restore the level found upon establishment in the laboratory. Founder-flush lines responded as well as the non-bottlenecked controls, so we base our major conclusions on the comparisons between fresh-caught and long-term laboratory stocks. We suggest that the effectiveness of the alternative purge protocols depended upon the amount of genetic load already exposed, such that prolonged periods of relaxed or altered selection pressures of the laboratory rendered a population more responsive to purging protocols.