Phenotypic variance for each of several bristle number characters (abdominal, sternopleural, second and third coxal) was partitioned using both hierarchal and dialled designs. Heritabilities and genetic correlations were estimated from parent-offspring regressions and correlations and half-sib correlations.A high proportion of the genetic variance for abdominal bristle number was due to epistatic and sex-linked gene action, but most of the genetic variance for the other characters was additive autosomal.The genetic correlations among sternopleural, and second and third coxal bristle numbers were all high, but that between abdominals and sternopleurals was low, while those between abdominals and either second or third coxals were virtually zero. An appreciable proportion of the covariance between abdominal and sternopleural bristle numbers was non-additive genetic.The diallel method gave more reliable estimates of genetic parameters when non-additive or sex-linked genetic variation was present.

SUMMARYThe effect of subdivision of a population on response to artificial directional selection for abdominal bristle number in Drosophila melanogaster was compared using large, replicated lines. Three different population structures were compared: (i) selection in an Undivided, large population with 50 pairs of parents (treatment U); (ii) selection in each of 10 sublines which were reconstituted every 6th generation by Crossing after Culling the 5 lowest sublines (treatment CC); and (iii) selection in each of 10 sublines which were reconstituted every 6th generation by Crossing after Retaining all 10 sublines (treatment CR). At the end of three cycles of selection and crossing, neither CR nor CC was superior to U; sublining did not increase response to selection. These results agree with the predictions arising from an entirely additive model and provide no evidence for the presence of epistasis.A comparison of 50-pair lines (U) with several 5-pair lines was made over 31 generations. For the 50-pair lines, there was close agreement between response predicted from the base population (using ih2σp) and observed response throughout all 31 generations of selection. Although the best of the 5-pair lines exceeded the 50-pair lines in the early generations, average response to directional selection in the 5-pair lines soon fell behind that predicted from ih2σp, and soon reached a plateau.

Reverse and relaxed selection were carried out in sublines which were derived from six replicate lines of Drosophila during 86-89 generations of selection for increased abdominal bristle number, and the reverse selection sublines were reciprocally crossed with selection lines of their origin.The results of serial relaxed selection initiated at different generations of selection confirm that the accelerated responses observed in the selection lines were largely due to deleterious genes, particularly lethals, with large effects on the selected character. The decline in mean bristle number under relaxed selection was not much different between crowded and uncrowded relaxed sublines.Reverse selection initiated at generation 57 was very effective, though it failed to bring the mean back to the base population level, and the genetic differences between replicate sublines (two from each of the six lines) indicate that low bristle number genes were probably rare in the selection lines. The genes which were still segregating after 57 generations of selection, on the average, did not show any directional dominance. The contribution of the X-chromosome to selection response was proportional to its chromosome length.

SummaryRegular cycles of intermittent mass selection have been compared with continuous mass selection at the same selection intensity for the quantitative trait of bristle number on one abdominal segment in a non-inbred strain of Drosophila melanogaster. All lines had 20 pairs of parents per generation. There were four replicates of each intermittent selection treatment, six of continuous selection at 20% selection intensity, and two at 10% selection intensity.In the treatment designations, the numbers of selected and non-selected generations per cycle are given first and second respectively, separated by a colon. The intermittent selection treatments, with number of generations of actual selection in parentheses, were 1:3 (5) and 1:1 (17) at 10% selection intensity, and 3:1 (24) and 6:2 (24) at 20% selection intensity. Rates of response of all treatments except 1:3 had declined by the time they were terminated, although none appeared plateaued. Sublines (continuous, 2:1, and 1:1 selection at an intensity of 20%) split off two replicates of treatment 1:1 at termination revealed little variation still available for utilization under selection in these two lines.All intermittent selection treatments gave similar amounts of response per generation of actual selection to those of the relevant continuous selection treatments.Measures of reproductive fitness, bristle number changes over five generations of relaxed selection under crowded mass culture, phenotypic variance for bristle number, and frequencies of lethal genes were obtained for almost all lines. As selection progressed, continuous selection lines were more prone to (1) exhibit increases in phenotypic variance for abdominal bristle number, and (2) have larger declines in abdominal bristle number on relaxation of selection.Although some replicates of almost all treatments showed little or no loss of reproductive fitness, fitness declined more under continuous than under intermittent selection.Failure to get increased response per generation of actual selection in the intermittent selection lines relative to their continuous selection controls may have been because responses obtained in both sets oflines were largely due to concentration of a few genes which were individually of large effect, and initially present at low frequencies. The greater decline in fitness under continuous selection suggests that linked complexes of genes deleterious to fitness and ones responsible for response in bristle number were important.

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