Barrett Spencer C H, Morgan Martin T, Husband Brian C
Department of Botany, University of Toronto, Toronto, ON, M5S 3B2, CANADA.
Evolution. 1989 Nov;43(7):1398-1416. doi: 10.1111/j.1558-5646.1989.tb02591.x.
Eichhornia paniculata (Pontederiaceae) displays a wide range of outcrossing levels as a result of the dissolution of the tristylous genetic polymorphism and the evolution of semihomostyly. Population surveys, comparison of fitness components of the style morphs, and computer simulations were used to investigate the breakdown of tristyly and the selective mechanisms responsible for the evolution of self-fertilization. Of 110 populations surveyed in northeast Brazil and Jamaica, 53% were trimorphic, 25% were dimorphic, and 22% were monomorphic for style morph. The short (S) morph was underrepresented in trimorphic populations and absent from nontrimorphic populations. The mid (M) morph predominated in dimorphic populations and was the only morph in monomorphic populations. Stamen modifications promoting selfing, associated with semihomostyle evolution, were largely confined to the M morph. They were rare in trimorphic populations, common in dimorphic populations, and often fixed in monomorphic populations. Stochastic simulations and comparisons of fruit set in natural populations indicate that founder events, population bottlenecks, and lowered fertility of the S morph due to an absence of long-tongued pollinators can each account for loss of the S morph from trimorphic populations. A reduced level of disassortative mating can accentuate the rate at which the S morph is lost by both random and deterministic processes. Nontrimorphic populations occur at the geographical margins of the region surveyed and tend to be smaller and less dense than trimorphic populations. These observations and the higher fruit set of the M morph relative to the L morph in dimorphic populations suggest that reproductive assurance, favoring selfing variants of the M morph under conditions of low pollinator service, has been of primary importance in the origin of most monomorphic populations. Where pollinator service is reliable, however, automatic selection of selfing genes, aided by mating asymmetries between the morphs, can cause the M morph to spread to fixation in dimorphic populations.
由于三型花柱遗传多态性的瓦解和半同型花柱的进化，圆锥凤眼莲（雨久花科）表现出广泛的异交水平。通过种群调查、花柱形态适合度成分的比较以及计算机模拟，来研究三型花柱的崩溃以及导致自花受精进化的选择机制。在巴西东北部和牙买加调查的110个种群中，53%为三型花柱，25%为二型花柱，22%为单型花柱。短花柱（S）形态在三型花柱种群中占比不足，在非三型花柱种群中不存在。中型花柱（M）形态在二型花柱种群中占主导地位，是单型花柱种群中的唯一形态。与半同型花柱进化相关的促进自交的雄蕊修饰主要局限于M形态。它们在三型花柱种群中很少见，在二型花柱种群中很常见，并且在单型花柱种群中常常固定下来。随机模拟和自然种群中坐果情况的比较表明，奠基者事件、种群瓶颈以及由于缺乏长舌传粉者导致S形态的育性降低，都可能导致三型花柱种群中S形态的丧失。异交交配水平的降低会加速S形态通过随机和确定性过程丧失的速率。非三型花柱种群出现在调查区域的地理边缘，往往比三型花柱种群更小、密度更低。这些观察结果以及二型花柱种群中M形态相对于L形态更高的坐果率表明，在传粉者服务水平较低的情况下，有利于M形态自交变体的繁殖保障在大多数单型花柱种群的起源中起了主要作用。然而，在传粉者服务可靠的地方，形态之间交配不对称辅助下的自交基因自动选择会导致M形态在二型花柱种群中扩散至固定状态。
