Genetic control of development of the mushroom bodies, the associative learning centers in the Drosophila brain, by the eyeless, twin of eyeless, and Dachshund genes.

Mushroom bodies (MBs) are the centers for olfactory associative learning and elementary cognitive functions in the Drosophila brain. By high-resolution neuroanatomy, we show that eyeless (ey), twin of eyeless, and dachshund (dac), which are implicated in eye development, also are expressed in the developing MBs. Mutations of ey completely disrupted the MB neuropils, and a null mutation of dac resulted in marked disruption and aberrant axonal projections. Genetic analyses demonstrated that, whereas ey and dac synergistically control the structural development of the MBs, the two genes are regulated independently in the course of MB development. These data argue for a distinct combinatorial code of regulatory genes for MBs as compared with eye development and suggest conserved roles of Pax6 homologs in the genetic programs of the olfactory learning centers of complex brains.