The role of gaseous metabolites in phototaxis byDictyostelium discoideumslugs

Three low molecular mass metabolites of Dictyostelium discoideum have been postulated to play roles in slug behavior as extracellular chemical messengers—cAMP, NH3 and slug turning factor (STF). Whereas the proposed function of cAMP in a slug behaviour is as a tip activator, STF and NH3 are putative nonvolatile and gaseous tip inhibitors respectively. During phototaxis, light is focused by the slug's translucent cylindrical tip onto its distal side, where it is hypothesized in both cases to stimulate secretion of tip inhibitor and thence a turn towards the light source. Under both hypotheses, accumulation at high cell density of high background concentrations of the tip inhibitor should impair slug phototaxis. I confirm here that this is so for nonvolatile metabolite(s) (STF), but show that it is not so for gaseous metabolite(s) (NH3). The steady state gradients of NH3 that can be achieved across a slug tip by lateral differences in the rates of secretion are shown here to be too shallow to elicit the chemotactic responses required to explain slug phototaxis. This conclusion is based on the rate of diffusion of NH3 in air and the previously measured rate of secretion of NH3 and chemotactic sensitivity of slugs to NH3 gradients. The active tip inhibitor controlling slug phototaxis is unlikely to be gaseous NH3 or any other volatile metabolite.