mab-20 encodes Semaphorin-2a and is required to prevent ectopic cell contacts during epidermal morphogenesis in Caenorhabditis elegans

The Semaphorins are a family of transmembrane proteins known to elicit grow th cone repulsion and collapse. We made and characterized a putative null m utant of the C, elegans gene semaphorin-2a (Ce-sema-2a), This mutant failed to complement mutants of mab-20 (Baird, S. E,, Fitch, D. H., Kassem, I. A, A. and Emmons, S. W. (1991) Development 113, 515-526), In addition to low- frequency axon guidance errors, mab-20 mutants have unexpected defects in e pidermal morphogenesis. Errant epidermal cell migrations affect secreted an d epidermal enclosure of the embryo, body shape and sensory rays of the mal e tail. These phenotypic traits are explained by the formation of inappropr iate contacts between cells of similar type and suggest that Ce-Sema-2a may normally prevent formation or stabilization of ectopic adhesive contacts b etween these cells.