Potential variance affecting homeotic Ultrabithorax and Antennapedia phenotypes in Drosophila melanogaster

Introgression of homeotic mutations into wild-type genetic backgrounds resu lts in a wide variety of phenotypes and implies that major effect modifiers of extreme phenotypes are not uncommon in natural populations of Drosophil a. A composite interval mapping procedure was used to demonstrate that one major effect locus accounts for three-quarters of the variance for haltere to wing margin transformation in Ultrabithorax flies, yet has no obvious ef fect on wild-type development. Several other genetic backgrounds result in enlargement of the haltere-significantly beyond the normal range of haploin sufficient phenotypes, suggesting genetic variation in cofactors that media te homeotic protein function. Introgression of Antennapedia produces lines with heritable phenotypes ranging from almost complete suppression to perfe ct antennal leg formation, as well as transformations that are restricted t o either the distal or proximal portion of the appendage. It is argued that the existence of "potential" variance, which is genetic variation whose ef fects are not observable in wild-type individuals, is a prerequisite for th e uncoupling of genetic from phenotypic divergence.