Phototaxis of Dictyostelium discoideum Slugs
b'During the slug stage of cellular slime mold Dictyostelium discoideum, up\\nto 105 cells coordinate their movement and migrate as a single organism.\\nSlugs have a cylindrical shape with tip and tail; their morphological polarity\\ncorresponds to the polarity of migration. A large body of results suggest that\\ncyclic AMP-mediated cell-cell signaling is the mechanism coordinating\\nmulticellular movement. Waves of cyclic AMP generated at the anterior tip\\npropagate towards the tail and induce the chemotactic movement of cells\\ntoward the tip. Slugs exhibit highly sensitive environmental reactions:\\nphototaxis, chemotaxis and thermotaxis. Although many studies have\\ninvestigated how Dictyostelium slugs move toward a light source, the\\nmechanism of phototaxis is still unclear. It has been known that slugs turn\\ntowards the light at the anterior end. In addition, previous research identified\\nmutations and drug treatments that interfere with phototaxis but the strategy\\nfor analyzing phototaxis has been limited to low resolution both temporarily\\nand spatially.\\nIn this thesis methods have been developed to analyze phototactic behavior\\non two different scales, the slug level and cellular level. The analyses\\nrevealed dynamic features of slug behavior during phototaxis which have\\nnot been previously described. Following light irradiation slugs moved with\\napproximately 50% higher speed; they showed prominent serpentine\\nmovement of their tip as if they were scanning and correcting migration\\ndirection; they elongated and decreased the diameter of their body; and their\\ntip remained lifted off the substrate for long periods. The analysis of cell\\nmovement during phototactic turning showed that the cell movement pattern\\nwas unlike any predicted from earlier hypotheses. Some cells in the anterior\\nzone moved away from the light source across the slug, thus increasing\\nthe volume on the \\u201cdark\\u201d side (\\u201casymmetric cell accumulation\\u201d) and bending\\nthe anterior zone like a lever-arm toward the light source. Furthermore, it was discovered that light irradiation enhances secretion of cyclic AMP from\\nthe slug and that light interferes with cyclic AMP cell-cell signaling during\\nother multicellular stages as well. A model for phototaxis has been proposed\\nbased on these results. Laterally irradiated light is focused on the distal side\\nof the slug by a lens effect and locally induces cyclic AMP release. Some\\ncells accumulate chemotactically on the side away from the light source and\\ncause a bending of the anterior zone towards the light source. Since cell\\nmovement within the slug is organized by cyclic AMP waves, light induced\\ncyclic AMP release interferes with the endogenous signaling pattern. The\\nconsequence is an overall change in the shape and the behavior of slug.\\nThe mechanism by which light induces the release of cyclic AMP from slug\\ncells may involve a histidine kinase phosphorelay pathway, since such a\\npathway is known to be functional in Dictyostelium and is used for\\nenvironmental responses in many other organisms.'