Chromosomal analysis of D. melanogaster long-term selected lines

Crosses and chromosomal substitutions among five selected lines of Drosophila melanogaster were carried out. These lines came from the same natural population after long-term selection for increasing dorsocentral bristle number. Two of them (Ac-27S and Ac-27P) show a bristle number around 16, and the other three (S-27S, S-27P, and N-21) present very extreme phenotypes, between 35 and 40 bristles. Selected genes were located on the three major chromosomes. Recessivity and synergistic interactions among the selected chromosomes account for the existence of hidden genetic variability, which can be released by selection, for a trait that scarcely shows phenotypic variability in populations. Genes responsible for selection response in lines Ac-27S and Ac-27P are present in all the selected lines, while lines S-27S, S-27P, and N-21 have accumulated additional increasing alleles common to the three of them. Long-term response and the extreme phenotypes achieved by these three lines point to variation that arose de novo during the selection process. However, this idea disagrees with the low genetic variability found between them. All these facts are consistent with the idea that variability originated in the course of selection by a low-probability nonrandom mechanism such as the occurrence of rare recombination events.