Tripartite Mushroom Body Architecture Revealed by Antigenic Markers

Abstract

We have explored the organization of the axonal lobes in Drosophila mushroom bodies by using a panel of immunohistochemical markers. These markers consist of antibodies to eight proteins expressed preferentially in the mushroom bodies: DAMB, DCO, DRK, FASII, LEO, OAMB, PKA RII, and RUT. Previous to this work, four axonal lobes, two projecting dorsally (α and α′) and two medially (β and γ), had been described inDrosophila mushroom bodies. However, our analysis of immunohistochemically stained frontal and sagittal sections of the brain revealed three medially projecting lobes. The newly distinguished lobe, which we term β′, lies along the dorsal surface of β, just posterior to γ. In addition to resolving a fifth lobe, our studies revealed that there are specific lobe sets defined by equivalent marker expression levels. These sets are (1) the α and β lobes, (2) the α′ and β′ lobes, and (3) the γ lobe and heel (a lateral projection formed by a hairpin turn of some of the peduncle fibers). All of the markers we have examined are consistent with these three sets. Previous Golgi studies demonstrate that each mushroom body cell projects one axon that branches into a dorsal lobe and a medial lobe, or one unbranched axon that projects medially. Taken together with the lobe sets listed above, we propose that there are three major projection configurations of mushroom body cell axons: (1) one branch in the α and one in the β lobe, (2) one branch in the α′ and one in the β′ lobe, and (3) one unbranched axon projecting to the heel and the γ lobe. The fact that these neuron types exhibit differential expression levels of a number of mushroom body genes suggests that they may have corresponding functional differences. These functions may be conserved in the larvae, as several of these genes were expressed in larval and embryonic mushroom bodies as well. The basic mushroom body structure, including the denritic calyx, peduncle, and lobes, was already visible by the late stages of embryogenesis. With new insights into mushroom body organization, and the characterization of markers for developing mushroom bodies, we are beginning to understand how these structures form and function.