Supplementary figure legends:

Supplementary Figure 1 Two motor units with some branches that co-innervate the same muscle fibers show no differences in the distribution or number of their axonal branches a, b Histograms showing the distribution of branch numbers (see methods) at co-innervated neuromuscular junctions (a) and the branch numbers of all junctions within the two motor units described in the text(b). The distributions of branch numbers of the CFP and YFP axon at the co-innervated junctions were not significantly different from each other (P=0.35, K-S test) or from the rest of their respective motor units (P=0.41, K-S test). c A graph showing YFP branch number versus CFP branch number at each co-innervated neuromuscular junction. The twelve co-innervated junctions were contacted as often by a proximal branch of the CFP axon as the YFP axon, suggesting no systematic bias in their innervation.

Supplementary Figure 2 Pair wise competitions among three motor units suggest a hierarchy in competitive vigor. a-c, Three neuromuscular junctions from muscle III were co-innervated by the same CFP axon (blue) and brightly labeled YFP axon (brYFP, colorized orange). In each case the axon terminals are segregated into largely non-overlapping regions. The axon calibers also do not show any significant bias. These motor axons thus appear to be evenly matched. d, e Two neuromuscular junctions co-innervated by the same CFP axon shown in a-c except now facing a different competitor: a dimly expressing YFP axon (dYFP, colorized yellow). In both of these cases the dYFP axon occupies nearly all of the territory and the CFP axon co-occupies one edge (asterisks in insets). These images may reveal a stage in the process of competition where axonal invasion (in this case by the dYFP axon) precedes axon withdrawal (by the CFP axon). Confirmation of this sequence of events will require time lapse imaging of two spectrally non-overlapping labeled inputs (previous time lapse was done with only one spectrally distinct axon --see ref 17 for discussion). f, At the entrance region of another junction innervated by the dYFP axon is a retraction bulb from the brYFP axon. g, One junction was associated with all three labeled axons and shows the features seen at each of the pair wise interactions. Thus the dYFP occupies all the territory, the CFP axon overlaps on one edge and the brYFP axon is a retracting process as in (f). h, Schematic of the connectivity of three motor units shown in panels a-g. From these sets of interactions we infer a hierarchy of competitive vigor between the three axons: dYFP>CFP (panels d, e and g); CFP = brYFP (panels a-c); dYFP>brYFP (f,g).