GENETIC DISSECTION OF STRUCTURAL AND FUNCTIONAL COMPONENTS OF SYNAPTIC PLASTICITY .1. FASCICLIN-II CONTROLS SYNAPTIC STABILIZATION AND GROWTH

The glutamatergic neuromuscular synapse in Drosophila forms and differ entiates into distinct boutons in the embryo and grows by sprouting ne w boutons throughout larval life. We demonstrate that two axons form s imilar to 18 boutons on muscles 7 and 6 by hatching and grow to simila r to 180 boutons by third instar. We further show that, after synapse formation, the hemophilic cell adhesion molecule Fasciclin II (Fas II) is localized both pre- and postsynaptically where it controls synapse stabilization. In FasII null mutants, synapse formation is normal, bu t boutons then retract during larval development. Synapse elimination and resulting lethality are rescued by transgenes that drive Fas II ex pression both pre- and postsynaptically; driving Fas II expression on either side alone is insufficient. Fas II can also control synaptic gr owth; various FasII alleles lead to either an increase or decrease in sprouting, depending upon the level of Fas II.