Abstract
The genetic basis of a sexually dimorphic quantitative character in Drosophila melanogaster was investigated by means of two-way directional selection for increased and decreased differences between male and female wing length. The sex dimorphism (SD), defined as the mean wing length difference between the sexes, within families, provided the criterion for selection.-The two lines (High SD, Low SD) diverged rapidly during the 15 generations of selection, indicating the presence of extensive genetic variability for the genotype-sex interaction underlying the observed sexual dimorphism. There was evidence that genetic variability persisted in both lines when selection was relaxed. Most of the divergence between the two lines remained after 10 generations of relaxed selection.-The change in the level of sex dimorphism in the High line was due primarily to a decrease in male wing length; in the Low line most of the change in SD was the result of a decrease in female wing length. An overall reduction in wing length in both sexes in both lines is interpreted as an effect of inbreeding.-The distribution and nature of the genetic control underlying the SD characteristic of the two selection lines was investigated by chromosome substitution between selection lines using a marked inversion technique. The two lines differed by factors located on each of the three major chromosome pairs. Chromosome III had the greatest effect on the difference in SD level between lines, and showed an overall additive effect when present in homozygous versus heterozygous combination. Chromosome II had the least effect, with a significant dominance effect of the High II being evident when heterozygotes were compared with homozygotes. The effect of the X chromosome was intermediate. There was some evidence of interaction between non-homologous chromosomes.