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Abstract

mushroom-body-miniature (mbm) is one of ten genes identified in mass histology screens for mushroom body (MB) structural mutants (Heisenberg and Boehl, 1979.2. Narurforsch. 34C. 143). Although mbm is an autoso- ma1 gene (2-0.0 21B8-Cb). mutants exhibit a striking sexual dimorphism: MBs of females are nearly, if not completely absent, while those of males are reduced by about 30 percent. Reductions in associative odor learn- ing have been shown previously to be correlated with the severity of MB derangement or reduction, in both ge- netic dissection and tissue-specific ablation studies (Heisenberg et al., 1985. J. Neurogener. 2, l; de Belle and Heisenberg, 1993. J. Neurogenef. 8,225; de Belle and Heisenberg, 1994, Science 263,692). The sexual dimor- phism of mbrn closely follows this pattern. Here we report on a series of experiments in which we compare the anatomy and behaviour of mbm mutants in their original genetic background and in controlled genetic back- grounds derived from the wild type Cunron-S (CS). We find almost completely wild type MBs (at the level of the light microscope) in both mbm males and fe- males in CS genetic backgrounds (mbm (CS) and mbm; CS). Substitution of the X-chromosome from the origi- nal mbm stock revives the sexually dimorphic mutant structural phenotype. However. all mbrn mutants are impared in associative odor learning and memory. Only mbm (CSJ performs normally in sensory acuity control tests. We are currently carrying out experiments to characterize the heritable factors influencing mbm pheno- types, examining mbm anatomy in the genetic backgrounds of GAL4 MB-specific markers, and studying mbm behavior in additional learning and locomotor paradigms. We suggest that mbm is part of a system of genes that functions both during MB development and in the processing of sensory information during the establishment of memory.