Tripartite mushroom body architecture revealed by antigenic markers

We have explored the organization of the axonal lobes in Drosophila mushroo m bodies by using a panel of immunohistochemical markers. These markers con sist of antibodies to eight proteins expressed preferentially in the mushro om bodies: DAME, DCO, DRK, FASII, LEG, OAMB, PKA RII, and RUT. Previous to this work, four axonal lobes, two projecting dorsally (alpha and alpha') an d two medially (beta and gamma), had been described in Drosophila mushroom bodies. However, our analysis of immunohistochemically stained frontal and sagittal sections of the brain revealed three medially projecting lobes. Th e newly distinguished lobe, which we term beta', lies along the dorsal surf ace of beta, just posterior to gamma. In addition to resolving a fifth lobe , our studies revealed that there are specific lobe sets defined by equival ent marker expression levels. These sets are (1) the alpha and beta lobes, (2) the alpha' and beta' lobes, and (3) the gamma 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. Previou s 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 bo dy cell axons: (1) one branch in the alpha and one in the beta lobe, (2) on e branch in the alpha' and one in the beta' lobe, and (3) one unbranched ax on projecting to the heel and the gamma lobe. The fact that these neuron ty pes exhibit differential expression levels of a number of mushroom body gen es suggests that they may have corresponding functional differences. These functions may be conserved in the larvae, as several of these genes were ex pressed in larval and embryonic mushroom bodies as well. The basic mushroom body structure, including the denritic calyx, peduncle, and lobes, was alr eady visible by the late stages of embryogenesis. With new insights Into mu shroom body organization, and the characterization of markers for developin g mushroom bodies, we are beginning to understand how these structures form and function.