Selection for canalization at two extra dorsocentral bristles in Drosophila melanogaster.

From 10 isofemale lines of D. melanogaster, the D2 line was established with the aim of obtaining an invariant phenotype at two extra dorsocentral bristles. Line D2 was also subdivided into two other lines, SA and ASD, based on their different bristle patterns. The SA line was selected for two symmetrical anterior extra bristles, and the ASD line was selected for two asymmetrical extra bristles, one anterior and one posterior. Only the SA line showed any canalizing response (estimated by the width of the probit transformation) at the two-extra-bristle class. Nevertheless, the results from the different lines were more consistent with the independent ones of both the anterior and posterior regions of the extra dorsocentral bristles. This analysis showed some independent genetic systems for each region, developmental canalization being at two extra symmetrical bristles per region in all the selected lines (D2, ASD, and SA). Therefore, this canalization did not depend directly on the extra-bristle positional pattern used in the selection. The wild-type canalizing system is suggested to explain the fast canalizing response in a phenotype that had not been previously canalized by natural selection.