Purging of inbreeding depression and fitness decline in bottlenecked populations of Drosophila melanogaster

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Drastic reductions in population size, or bottlenecks, are thought to significantly erode genetic variability and reduce fitness. However, it has been suggested that a population can be purged of the genetic load responsible for reduced fitness when subjected to bottlenecks. To investigate this phenomenon, we put a number of Drosophila melanogaster isofemale lines known to differ in inbreeding depression through four ‘founder-flush’ bottleneck cycles with flush sizes of 5 or 100 pairs and assayed for relative fitness (single-pair productivity) after each cycle. Following the founder-flush phase, the isofemale lines, with a large flush size and a history of inbreeding depression, recovered most of the fitness lost from early inbreeding, consistent with purging. The same isofemale lines, with a small flush size, did not regain fitness, consistent with the greater effect of genetic drift in these isofemale lines. On the other hand, the isofemale lines that did not show initial inbreeding depression declined in fitness after repeated bottlenecks, independent of the flush size. These results suggest that the nature of genetic variation in fitness may greatly influence the way in which populations respond to bottlenecks and that stochastic processes play an important role. Consequently, an attempt intentionally to purge a population of detrimental variation through inbreeding appears to be a risky strategy, particularly in the genetic management of endangered species.

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