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

Abstract

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. convergent evolution | evolutionarily stable strategy | fertilization kinetics

The evolution of sexes: A specific test of the disruptive selection theory

“…Togashi et al. () describe an individual‐based population genetic model of disruptive selection that also considers gamete collision kinetics. They use this model to explain the evolution of gamete dimorphism in marine green algae living in various environments.…”

“…The parameters of this model could, however, be set to reflect the different environmental conditions specified by Togashi et al. (). For example, conditions favoring a large zygote may be reflected in large β.…”

“…Or, conditions favoring large gametes may be represented by large δ and are expected to result in the “large isogamy” of Togashi et al. () if the optimal gamete size is less than the threshold for gamete viability ( m * < δ) (Bulmer & Parker, ).…”

Positive size–speed relationships in gametes and vegetative cells of Chlamydomonas reinhardtii ; implications for the evolution of sperm

“…; Togashi et al. ) and the results of these models may alter when underlying assumption is changed from negative to positive. Although their assumptions may have been based on previous datasets, these were often inconclusive, or obtained from populations displaying full oogamy (Cox and Sethian ).…”

What do isogamous organisms teach us about sex and the two sexes?

“…Presumably, this is possible due to the very small difference in gamete size, where female gametes have not yet become the sole resource provider. In some algae, the size difference between male and female gametes relates to the number of cell divisions of the mother cell, which may differ by only one division [46,47].…”