Canalization at two extra scutellar bristles: artificial selection, chromosome analysis, and effect of temperature.

Two isofemale lines (P1 and P2) from a wild-type population of Drosophila melanogaster were selected for an invariant phenotype, two symmetrical and anterior scutellar extra bristles. Both P1 and P2 lines responded quickly to direct selection for two extra bristles, but although this phenotype was canalized in the P2 line, it was not in the P1 line. This lack of canalization at two extra bristles, measured by probit width, is the first reported in the literature. Analysis of chromosomal contribution showed the P1 decanalized phenotype to be due to chromosome 2. Synthetic chromosomal combinations were tested at four development temperatures (17 degrees, 21 degrees, 24 degrees, and 30 degrees C), and no correlation was observed between canalization at two extra bristles measured by probit width and minimum phenotypic change due to different temperatures. In this test, development at the highest temperatures was accompanied by an increased tendency to return to the wild phenotype in the canalized phenotypes only, suggesting that this drastic change could be accounted for by the action of the wild-type canalizing system. Decanalized genotypes, or those canalizing at phenotypes other than the wild type, could be explained by mutations which, to a greater or lesser degree, alter the normal genetic system of canalization.