GENETIC-ANALYSIS OF THE MECHANISMS CONTROLLING TARGET SELECTION - TARGET-DERIVED FASCICLIN-II REGULATES THE PATTERN OF SYNAPSE FORMATION

In Drosophila, motoneuron growth cones initially probe many potential muscle targets but later withdraw most of these contacts to form stere otypic synapses with only one or a few muscles. Prior to synapse forma tion, Fasciclin II (Fas II) is expressed at low levels on muscle. Duri ng synapse formation, Fas II concentrates at the synapse and disappear s from the rest of the muscle. We previously showed that Fas II is req uired both pre- and postsynaptically for synaptic stabilization. Here, we show that the differential expression of target derived Pas II has a profound influence on the patterning of synapse formation. A transi ent increase in muscle Fas II stabilizes growth cone contacts and lead s to novel synapses that are functional and stable; targets that norma lly receive two inputs can now receive up to six inputs. Changing the relative levels of Fas II on neighboring muscles leads to dramatic shi fts in target selection.