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 α′ / β′ neurons is essential for the formation of adult α ′/ β ′ and α / β lobes. Of γ , α ′/ β ′ and α / β neurons, which are derived sequentially from the same four MB neuroblasts, only γ and α / β neurons expressed fasII . When fasII was misexpressed in developing MB neurons, defects resulted, including loss or misdirection of adult α ′/ β ′ lobes and concurrent misdirection of α / β lobes. Although no gross anatomical defects were apparent in the larval MB lobes, α ′/ β ′ lobes collapsed at the pupal stage when the larval lobe of γ neurons degenerated. In addition, α / β lobes, which developed at this time, were misdirected in close relationship with the collapse of α ′/ β ′ lobes. These defects did not occur when fasII was overexpressed in only γ and α / β neurons, indicating that ectopic expression of fasII in α ′/ β ′ neurons is required for the defects. Our findings also suggest that the α ′/ β ′ lobe play a role in guiding the pathfinding by α / β axons.