Marine Biosystems Research Center, Chiba University, Kamogawa 299-5502, Japan.
Proc Natl Acad Sci U S A. 2012 Aug 21;109(34):13692-7. doi: 10.1073/pnas.1203495109. Epub 2012 Aug 6.
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.
雌雄配子大小差异（即产生大小不同配子）的进化是性别二态性建立的第一步，也是性选择的基础现象。人们认为雌雄配子大小差异起源于大小均等配子的雌雄同体（isogamy），大多数理论家认为这是原始状态。尽管几乎所有的植物和动物物种都是雌雄配子大小差异的，但现存的海洋绿藻物种表现出多种交配系统，包括雌雄同体和雌雄配子大小差异。海洋绿藻的雌雄同体有两种形式：配子极小的雌雄同体和配子较大的雌雄同体。基于对受精成功和受精卵存活的分歧选择（帕克、贝克和史密斯理论），我们通过分析不同配子大小种群入侵模拟中的随机过程，探讨了环境变化如何有助于这种复杂交配系统的进化。我们发现，两种形式的雌雄同体都可以从其他雌雄同体祖先通过雌雄配子大小差异进化而来。这种无维分析解释了海洋绿藻中所有类型交配系统的进化稳定性，即使在相同的环境中也是如此。这些结果表明，进化轨迹以及配子/受精卵的最优性在配子大小的进化中起着重要作用。
