A brown algal sex pheromone reverses the sign of phototaxis by cAMP/Ca2+-dependent signaling in the male gametes of Mutimo cylindricus (Cutleriaceae)

Abstract

Male gametes of the brown alga Mutimo cylindricus show positive phototaxis soon after spawning in seawater but gradually change the sign of phototaxis with time. This conversion appears to need the decrease of intracellular Ca²⁺ concentration. In this study, we revealed that the conversion of male gamete phototactic sign, positive to negative, was accelerated by mixing with female gametes. The supernatant after the centrifugation of female gamete suspension showed the same activity to change the phototactic sign, suggesting that a factor released from female gametes was responsible for the reaction. A known brown algal sex pheromone, ectocarpene, induced chemotaxis of male gametes of M. cylindricus. The addition of this compound induced the change of phototactic sign, clearly indicating that a sex pheromone is essential for the reversal. An inhibitor of phosphodiesterase, theophylline, inhibited the chemotaxis and phototactic sign reversion by a factor released from female gametes and ectocarpene. Measurements of cyclic nucleotides showed that the increase in intracellular concentration of cAMP, not cGMP, was parallel to the change of phototactic sign. The inhibition of phototactic sign by theophylline was not observed in low Ca²⁺ sea water. These results suggest that a signaling pathway mediated by cAMP and Ca²⁺ concentrations drives the interconversion between two important behaviors of male gametes, phototaxis and chemotaxis.

Keywords: Brown algae; Chemotaxis; Flagellar motility; Gametes; Pheromone; Phototaxis.