Reproductive Strategies of Marine Green Algae: the Evolution of Slight Anisogamy and the Environmental Conditions of Habitats

In marine green algae, isogamous or slightly anisogamous species are taxonomically widespread. They produce positively phototactic gametes with phototactic devices including an eye-spot in both sexes. We developed a new numerical simulator of gamete behavior using C++ and pseudo-parallelization methods to elucidate potential advantages of phototaxis. Input parameters were set based on experimental data. Each gamete swimming in a virtual rectangular test tank was tracked and the distances between the centers of nearby male and female were measured at each step to detect collisions. Our results shed light on the roles of gamete behavior and the mechanisms of the evolution of anisogamy and more derived forms of sexual dimorphism. We demonstrated that not only gametes with positive phototaxis were favored over those without particularly in shallow water because they could search for potential mates on the two-dimensional water surface rather than randomly in three dimensions, but also phototactic behavior clarified the difference between isogamy and slight anisogamy. Isogamous species produced significantly more zygotes than slightly anisogamous ones only under the phototactic conditions. Our results suggested that “sperm limitation” might be resolved in the slightly anisogamous species. In marine green algae, some more markedly anisogamous species produce the smaller male gametes that have no eye-spot and swim randomly. In contrast, the larger female gametes have an eye-spot and show positive phototaxis. As a result of careful experimental observations, we discovered the first pheromonal attraction system in marine green algae. This pheromonal attraction system might have played a key role in the evolution of anisogamy in marine green algae, because it may enable markedly anisogamous species achieve 2D search efficiencies on the water surface. The mating systems appear to be tightly tuned o the environmental conditions of their habitats.