Abstract
Mutations in 12 genes regulating Drosophila melanogaster mushroom body (MB) development were each studied in two genetic backgrounds. In
all cases, brain structure was qualitatively or quantitatively different after replacement of the "original" genetic background with that of the Canton Special
wild-type strain. The mushroom body miniature gene (mbm) was investigated in detail. mbm supports the maintenance of MB Kenyon cell fibers in third
instar larvae and their regrowth during metamorphosis. Adult mbm(1) mutant females are lacking many or most Kenyon cell fibers and are impaired in
MB-mediated associative odor learning. We show here that structural defects in mbm(1) are apparent only in combination with an X-linked,
dosage-dependent modifier (or modifiers). In the Canton Special genetic background, the mbm(1) anatomical phenotype is suppressed, and MBs develop to
a normal size. However, the olfactory learning phenotype is not fully restored, suggesting that submicroscopic defects persist in the MBs. Mutant mbm(1) flies
with full-sized MBs have normal retention but show a specific acquisition deficit that cannot be attributed to reductions in odor avoidance, shock reactivity, or
locomotor behavior. We propose that polymorphic gene interactions (in addition to ontogenetic factors) determine MB size and, concomitantly, the ability to
recognize and learn odors.
