Simulation of gamete behaviors and the evolution of anisogamy: reproductive strategies of marine green algae

In marine green algae, isogamous or slightly anisogamous species are taxonomically widespread. They produce positively phototactic gametes 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. This was because they could search for potential mates on the 2-D water surface rather than randomly in three dimensions. Also, phototactic behavior clarified the difference between isogamy and slight anisogamy. Isogamous species produced more zygotes than slightly anisogamous ones only under the phototactic conditions. Our results suggested that ‘sperm limitation’ might be easily resolved particularly in the slightly anisogamous species. Some more markedly anisogamous species produce the smaller male gametes without any phototactic devices and the larger positively phototactic female gametes. In such species, female gametes attract their partners using a sexual pheromone. This pheromonal attraction system might have played a key role in the evolution of anisogamy, because it could enable markedly anisogamous species achieve 2-D search efficiencies on the water surface. The mating systems appear to be tightly tuned o the environmental conditions of their habitats.