POLYGENIC MUTATION IN DROSOPHILA-MELANOGASTER - THE CAUSAL RELATIONSHIP OF BRISTLE NUMBER TO FITNESS

The association between sternopleural and abdominal bristle number and fitness in Drosophila melanogaster was determined for sublines of an initially highly inbred strain that were maintained by divergent artif icial selection for 150 generations or by random mating for 180 genera tions. Replicate selection lines had more extreme bristle numbers than those that were maintained without artificial selection at the same c ensus size for approximately the same number of generations. The avera ge fitness, estimated by a single generation of competition against a compound autosome strain, was 0.17 for lines selected for high and low abdominal bristle numbers and 0.19 for lines selected for high and lo w sternopleural bristle number. The average fitness of unselected line s, 0.46, was significantly higher than that of the selection lines. Th e fitnesses and the relationships of bristle number to fitness in prog eny of all possible crosses of high x high (H x H), high x low (H x L) and low x low (L x L) selection lines were examined to determine whet her the observed intermediate optima were caused by direct stabilizing selection on bristle number or by apparent stabilizing selection medi ated through deleterious pleiotropic fitness effects of mutations affe cting bristle number. Although bristle number was nearly additive for progeny of H x H, H x L and L x L crosses among sternopleural bristle selection lines, their mean fitnesses were not significantly different from each other, or from the mean fitness of the unselected lines, su ggesting partly or completely recessive pleiotropic fitness effects ca use apparent stabilizing selection. The average fitness of the progeny of H X H abdominal bristle selection lines was not significantly diff erent from the fitness of unselected lines, but the mean fitness of th e progeny of L x L crosses was not significantly different from that o f the pure low lines. This is consistent with direct selection against low but not high abdominal bristle number, but the interpretation is confounded by variation in average degree of dominance for fitness (on average recessive in the high abdominal bristle selection lines and a dditive in the low abdominal bristle selection lines). Neither direct stabilizing selection nor pleiotropy, therefore, can account for all t he observations.