Dictyostelium RasD is required for normal phototaxis, but not differentiation

RasD, a Dictyostelium homolog of mammalian Pas, is maximally expressed duri ng the multicellular stage of development. Normal Dictyostelium aggregates are phototactic and thermotactic, moving towards sources of light and heat with great sensitivity. We show that disruption of the gene for rasD causes a near-total loss of phototaxis and thermotaxis in mutant aggregates, with out obvious effects on undirected movement. Previous experiments had sugges ted important roles for RasD in development and cell-type determination. Su rprisingly, rasD(-) cells show no obvious changes in these processes. These cells represent a novel class of phototaxis mutant, and indicate a role fo r a Pas pathway in the connections between stimuli and coordinated cell mov ement.