aRESEARCH ARTICLE Drosophila semaphorin2b is Required for the Axon Guidance of a Subset of Embryonic Neurons

Background: The process of axon guidance is important in establishing functional neural circuits. The dif-ferential expression of cell-autonomous axon guidance factors is crucial for allowing axons of different neurons to take unique trajectories in response to spatially and temporally restricted cell non-autono-mous axon guidance factors. A key motivation in the field is to provide adequate explanations for axon behavior with respect to the differential expression of these factors. Results: We report the characteriza-tion of a predicted secreted semaphorin family member, semaphorin2b (Sema-2b) in Drosophila embryonic axon guidance. Misexpression of Sema-2b in neurons causes highly penetrant axon guidance phenotypes in specific longitudinal and motoneuron pathways; however, expression of Sema-2b in muscles traversed by these motoneurons has no effect on axon guidance. In Sema-2b loss-of-function embryos, specific moto-neuron and interneuron axon pathways display guidance defects. Specific visualization of the neurons that normally express Sema-2b reveals that this neuronal cohort is strongly affected by Sema-2b loss-of-function alleles. Conclusions: While secreted semaphorins have been implicated as cell non-autonomous chemorepellants in a variety of contexts, here we report previously undescribed Sema-2b loss-of-function and misexpression phenotypes that are consistent with a cell-autonomous role for Sema-2b. Developmen