GENETIC-ANALYSIS OF THE MECHANISMS CONTROLLING TARGET SELECTION - COMPLEMENTARY AND COMBINATORIAL FUNCTIONS OF NETRINS, SEMAPHORINS, AND IGCAMS

The molecular mechanisms controlling the ability of motor axons to rec ognize their appropriate muscle targets were dissected using Drosophil a genetics to add or subtract Netrin A, Netrin B, Semaphorin II, and F asciclin II alone or in combination, Fas II and Sema II are expressed by all muscles where they promote (Fas II) or inhibit (Sema II) promis cuous synaptogenesis, NetB is expressed by a subset of muscles where i t attracts some axons and repels others, However, growth cones in this system apparently do not rely solely on single molecular labels on in dividual targets, Rather, these growth cones assess the relative balan ce of attractive and repulsive forces and select their targets based o n the combinatorial and simultaneous input of multiple cues.