Random Convergence of Olfactory Inputs in the Drosophila Mushroom Body

The mushroom body (MB) in the fruit fly Drosophila melanogaster is an associative brain centre that translates odour representations into learned behavioural responses(1). Kenyon cells (KCs), the intrinsic neurons of the MB, integrate input from olfactory glomeruli to encode odours as sparse distributed patterns of neural activity(2,3). We have developed anatomic tracing techniques to identify the glomerular origin of the inputs that converge onto 200 individual KCs. Here we show that each KC integrates input from a different and apparently random combination of glomeruli. The glomerular inputs to individual KCs exhibit no discernible organization with respect to their odour tuning, anatomic features, or developmental origins. Moreover, different classes of KCs do not appear to preferentially integrate inputs from specific combinations of glomeruli. This organization of glomerular connections to the MB could allow the fly to contextualize novel sensory experiences, a feature consistent with the role of this brain centre in mediating learned olfactory associations and behaviours.