Morphogenetically Specific Mutability in DROSOPHILA ANANASSAE.

A stock exhibiting hypermutability with respect to visible mutants (Om) affecting optic morphology was subjected to genetic analysis. The production of Om mutants, independently recovered with a frequency of two per 10(4), is restricted to females and depends primarily on homozygosity of their X chromosomes; in heterozygotes, Om mutability is stimulated by the presence of either one of two extrachromosomally replicating elements previously identified in other stocks having cryptic mutability systems. The semidominant and nonpleiotropic Om mutants are not associated with gross rearrangements and they map to at least 15 loci. Most of the loci defined by mapping are represented by two or more Om mutants which, despite considerable interlocus mimicry, sometimes display locus-specific phenotypes. Om mutants are moderately unstable, and they are subject to dominant suppressors that arise spontaneously at either of two X-linked loci. An interpretation of these observations invokes an X-linked transposable element (tom) that specifically inserts into control sequences shared by a set of structural genes involved in eye morphogenesis.