The effects of disruptive and stabilizing selection on body size inDrosophila melanogaster. III. Genetic analysis of two lines with different reactions to disruptive selection with mating of opposite extremes

A genetic analysis was made of two lines which when subjected to disruptive selection with compulsary mating of opposite extremes (D^?) showed a different response viz. one, D^?-1, showing predominantly an increase of environmental variance and possibly interaction variance, the other, D^?-2, showing an increase of genetic variance. Crosses between extreme flies within lines revealed that D^?-1 genomes from large flies are dominant to genomes from small individuals. In D^?-2 the genetic variation is predominantly additive variance. Tests for dominant chromosome effect in crosses with an inbred stock with recessive markers showed clear third chromosome differences in D^?-2 and not in D^?-1. Chromosome exchange between extreme flies corroborated the importance of genetic differences in D^?-2. A factor or complex of factors with large effect decreasing body size is located on third chromosomes from small flies in D^?-2. Interaction between chromosomes has a similar magnitude in the two lines. Crowding and temperature experiments did not reveal an increased general sensitivity to environmental factors in D^?-1, which was suggested by the enlarged environmental variance of this line.