Precise control of fasciclin II expression is required for adult mushroom body development in Drosophila

Fasciclin II (FASII) is a cell adhesion molecule that participates in axonal pathfinding, fasciculation and divergence in the Drosophila nervous system. Here, we examined spatio-temporal control of fasII expression during the development of adult mushroom body (MB) and found that suppression of fasII in alpha'/beta' neurons is essential for the formation of adult alpha'/beta' and alpha/beta lobes. Of gamma, alpha'/beta' and alpha/beta neurons, which are derived sequentially from the same four MB neuroblasts, only gamma and alpha/beta neurons expressed fasII. When fasII was misexpressed in developing MB neurons, defects resulted, including loss or misdirection of adult alpha'/beta' lobes and concurrent misdirection of alpha/beta lobes. Although no gross anatomical defects were apparent in the larval MB lobes, alpha'/beta' lobes collapsed at the pupal stage when the larval lobe of gamma neurons degenerated. In addition, alpha/beta lobes, which developed at this time, were misdirected in close relationship with the collapse of alpha'/beta' lobes. These defects did not occur when fasII was overexpressed in only gamma and alpha/beta neurons, indicating that ectopic expression of fasII in alpha'/beta' neurons is required for the defects. Our findings also suggest that the alpha'/beta' lobe play a role in guiding the pathfinding by alpha/beta axons.