Structural brain mutants: mushroom body defect (mud): a case study.

Single-gene mutants of Drosophila have not only increased our understanding of the biochemical processes underlying learning and memory processes, but also established structure-function relationships. The first relevant mutants were identified by Martin Heisenberg nearly 30 years ago in a screen for altered adult brain structure and were used to link the mushroom bodies in the central brain with olfactory learning and memory processes. Because the observed structural defects in the adult are the consequence of deregulated developmental processes, the characterization of these mutants can also provide insight into the genetic programs underlying the establishment, maintenance, and remodeling of functional neuronal circuits. As an example for the value of this approach, we trace the history of mushroom body defect (mud), from the original anatomical description of the mutation to most recent insights of the function of the protein as a regulator of neuronal progenitor cell division.