Transposon-dependent mutant phenotypes at the Notch locus of Drosophila

Many mutations at complex genetic loci in the fruitfly Drosophila melanogaster are associated with insertions of transposable elements. At the Notch locus, members of one class of insertion-associated mutations, termed glossy-like, produce a recessive viable, smooth-eye phenotype with mottled pigmentation. Members of a second class, facet, produce a recessive viable, rough-eye phenotype with homogeneous pigmentation. Both classes of mutations fail to complement Notch lethal mutations, so they behave as Notch alleles. Here we report that each glossy-like mutation is associated with an insertion of the same transposable element (flea). Each flea insertion occurs in the same orientation, but at different locations within intervening sequences of the Notch locus. In contrast, each facet mutation is associated with insertion of a unique, non-flea, transposable element. Insertions producing a facet phenotype and insertions causing a glossy-like phenotype can break Notch intervening sequences at precisely the same location. This suggests that the type of insertion element rather than its position within an affected gene is the primary determinant of the phenotype observed.