Levitan Don
Department of Biological Science, Florida State University, Tallahassee, Florida, 32306-1100.
Evolution. 1998 Aug;52(4):1043-1056. doi: 10.1111/j.1558-5646.1998.tb01832.x.
Evolutionary biologists generally invoke male competition and female choice as mechanisms driving sexual selection. However, in broadcast-spawning organisms sperm may be limiting and females may compete, in the Darwinian sense, for increased mating success. In this study, I investigate how species differences in egg and sperm traits result in different patterns of fertilization among three closely related sea urchins (Strongylocentrotus purpuratus, S. franciscanus, and S. droebachiensis). Field studies demonstrate that all three species achieve similar percentages of eggs fertilized when eggs and sperm are released simultaneously. However, when sperm must disperse before encountering eggs, differences arise among species such that those with the smaller eggs and faster but shorter-lived sperm achieve relatively fewer fertilizations than do species with larger eggs and slower but longer-lived sperm. A field hybridization experiment, field estimates of sperm dispersal, correlations of egg size to field rates of fertilization, laboratory studies of fertilization kinetics, and a simulation model all suggest that it is attributes of the egg (probably egg size) that are responsible for the differences. These patterns of fertilization match the species' patterns of dispersion; species that do well only when sperm and eggs are released in close proximity are more aggregated, species that do relatively well when sperm and eggs are released farther apart are more dispersed. These results are consistent with the notion that eggs of different species are adapted to maximize reproductive success under different degrees of sperm limitation and suggest that male competition and female choice may not be an appropriate dichotomy in broadcast-spawning organisms.
进化生物学家通常将雄性竞争和雌性选择作为驱动性选择的机制。然而，在体外受精生物中，精子可能是有限的，从达尔文主义的角度来看，雌性可能会为提高交配成功率而竞争。在本研究中，我调查了三种亲缘关系密切的海胆（紫海胆、加州紫海胆和大西洋球海胆）的卵子和精子特征差异如何导致不同的受精模式。野外研究表明，当卵子和精子同时释放时，这三种海胆的卵子受精百分比相似。然而，当精子在遇到卵子之前必须扩散时，不同物种之间就会出现差异，那些卵子较小、精子游动速度快但寿命短的物种，其受精率相对低于卵子较大、精子游动速度慢但寿命长的物种。一项野外杂交实验、精子扩散的野外估计、卵子大小与野外受精率的相关性、受精动力学的实验室研究以及一个模拟模型均表明，造成这些差异的原因是卵子的属性（可能是卵子大小）。这些受精模式与物种的分布模式相匹配；只有当精子和卵子在近距离释放时才能良好受精的物种分布更集中，而当精子和卵子在较远的距离释放时受精情况相对较好的物种分布更分散。这些结果与以下观点一致，即不同物种的卵子经过适应性进化，以便在不同程度的精子限制下实现繁殖成功率最大化，这表明在体外受精生物中，雄性竞争和雌性选择可能不是一个合适的二分法。
