![]() ![]() ![]() The lack of a strong phenotype in single-gene mutants of genes that belong to large families is hardly surprising. Beat protein is a secreted member of the immunoglobulin superfamily (IgSF) ( Bazan and Goodman, 1997). Reducing the levels of the homophilic axonal cell adhesion molecule (CAM) Fasciclin II (Fas II) on motor axons partially suppresses the beat phenotype and allows specific motor axons to exit at the appropriate choice point and contact their target muscles ( Fambrough and Goodman, 1996). ![]() In beat loss-of-function mutants, motor axons extending along the major motor nerves in the periphery are unable to overcome axon-axon adhesion to other motor axons, and fail to defasciculate at their appropriate branch points, instead continuing to extend along the motor nerves and growing past their target muscles. The Beat protein appears to function as an anti-adhesive factor secreted by motoneuron growth cones at defasciculation choice points. A genetic screen for mutations that perturb the pattern of motor axon projections in the fruit fly Drosophila melanogaster led to the discovery of the beaten path ( beat) gene ( VanVactor et al., 1993). We present evidence on the expansion of the family of beat-like genes in Drosophila. ![]()
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