Early development of the Drosophila mushroom body: the roles of eyeless and dachshund

The mushroom body (MB) is a uniquely identifiable brain structure present i n most arthropods. Functional studies have established its role in learning and memory. Here we describe the early embryonic origin of the four neurob lasts that give rise to the mushroom body and follow its morphogenesis thro ugh later embryonic stages. In the late embryo, axons of MB neurons lay dow n a characteristic pattern of pathways, eyeless (ey) and dachshund (dac) ar e expressed in the progenitor cells and neurons of the MB in the embryo and larva. In the larval brains of the hypomorphic ey(R) strain, we find that beside an overall reduction of MB neurons, one MB pathway, the medial lobe, is malformed or missing. Overexpression of eyeless in MBs under the contro l of an MB-specific promoter results in a converse type of axon pathway abn ormality, i.e. malformation or loss of the dorsal lobe. In contrast, loss o f dachshund results in deformation of the dorsal lobe, whereas no lobe abno rmalities can be detected following dachshund overexpression. These results indicate that ey and dachshund may have a role in axon pathway selection d uring embryogenesis.