A gain-of-function mutation of wishful thinking (wit), wit d02492, was identified as an enhancer in our screen. To further investigate the interaction between wit and Smn at the NMJ, we used the neuron-specific driver, elavGAL4 to express WIT in neurons. (A–F) The morphology of the NMJ, as judged by bouton numbers, between muscles 6 and 7 in the A2 segment was observed in different genetic backgrounds using the pre-synaptic (Synaptotagmin) and post-synaptic (Discs large) markers, shown in green and red, respectively. The following genotypes were examined: (A) elavGAL4/+, (B) elavGAL4, Smn 73Ao/+, (C) elavGAL4, Smn f01109/+, (D) elavGAL4/UAS-wit2A, (E) elavGAL4, Smn 73Ao/UAS-wit2A, (F) elavGAL4, Smn f01109/UAS-wit2A, (G) Bouton counts for genotypes from (A–F and wild-type). Consistent with previous reports, neural induced expression of the UAS-wit2A transgene had no obvious effect on NMJ bouton number. A synergistic effect was observed upon the addition of a single Smn allele (Smn 73Ao or Smn f01109) to this background, leading to a reduction of NMJ bouton numbers. The phenotype was more severe in the Smn f01109 background. Smn f01109 showed an approximate 50% reduction in bouton numbers while Smn 73Ao reduced the bouton count by 20%. elavGAL4, Smn 73Ao/+ (B) and elavGAL4, Smn f01109/+ (C) individuals display no significant reduction in NMJ bouton numbers compared to wild-type (G). Bouton counts were determined as above. Error bars are s.e.m.; * P<0.02 was determined by the ANOVA multiple comparisons test to wild-type and all controls. n was 15–20 animals for each genotype. Bouton numbers for each genotype were normalized to the ratio of muscle areas. Scale bars represent 20 µm.