Evolutionary trajectories explain the diversified evolution of isogamy and anisogamy in marine green algae.

The evolution of anisogamy (the production of gametes of different size) is the first step in the establishment of sexual dimorphism, and it is a fundamental phenomenon underlying sexual selection. It is believed that anisogamy originated from isogamy (production of gametes of equal size), which is considered by most theorists to be the ancestral condition. Although nearly all plant and animal species are anisogamous, extant species of marine green algae exhibit a diversity of mating systems including both isogamy and anisogamy. Isogamy in marine green algae is of two forms: isogamy with extremely small gametes and isogamy with larger gametes. Based on disruptive selection for fertilization success and zygote survival (theory of Parker, Baker, and Smith), we explored how environmental changes can contribute to the evolution of such complex mating systems by analyzing the stochastic process in the invasion simulations of populations of differing gamete sizes. We find that both forms of isogamy can evolve from other isogamous ancestors through anisogamy. The resulting dimensionless analysis accounts for the evolutionary stability of all types of mating systems in marine green algae, even in the same environment. These results imply that evolutionary trajectories as well as the optimality of gametes/zygotes played an important role in the evolution of gamete size.

Underwater fertilization dynamics of marine green algae.

We studied the fertilization dynamics of marine green algae with both analytical methods and numerical simulations. In this study, we focused on a new factor, gametic investment per unit volume of the space in which gametes searched for their partners, and compared the numbers of zygotes formed at lower investments with those at higher investments. As a function of the gametic investment for various anisogamy ratios, we found there was generally a crossover region for each series where, for gametic investments larger than the crossover region, isogamy prevailed with the highest number of zygotes formed, while for gametic investments smaller than the crossover region, anisogamy dominated. These results may explain both the stable maintenance of isogamy in shallow water and the distribution of anisogamous species in deep water, since in shallow water the gametic investments typically exceed this crossover region and vice versa. Comparisons of field data from marine green algae are consistent with this hypothesis. Also, we showed that the cost of sex was approximately twofold in zygote formation when comparing isogamous species with mating types to those without mating types.