Morgan Martin T, Barrett Spencer C H
Department of Botany, University of Toronto, Toronto, ON, M5S 1A1, CANADA.
Evolution. 1988 May;42(3):496-504. doi: 10.1111/j.1558-5646.1988.tb04155.x.
Theoretical models of floral-morph frequencies in tristylous species predict a single equilibrium with all three morphs represented in equal proportions (isoplethy). North American populations of Pontederia cordata exhibit considerable heterogeneity of morph frequencies between populations, with the short-styled morph often in excess of isoplethic expectations and the long-styled morph commonly underrepresented. In a previous study, it was proposed that anisoplethic population structure in P. cordata is the result of differential male fertility, owing to genetic differences in pollen production among the morphs. In this study, the influence of historical factors on morph frequencies prior to equilibrium was investigated using a deterministic computer model. Nonequilibrium frequencies are strongly influenced by the genotypes of founding individuals, and, because tristyly is under the control of two diallelic loci, phenotypic equilibrium is approached asymptotically. The model indicates that in nonequilibrium populations the short-styled morph will be in excess and the long-styled morph will be underrepresented. This suggests that historical factors play an important role in determining population structure in P. cordata. Several features of the population ecology of the species lend support to this interpretation. Historical factors should be taken into account when interpreting data from population surveys of morph frequencies in tristylous species and of other genetic polymorphisms not under single-locus control.
三型花柱植物花形态频率的理论模型预测，三种形态以相等比例（等频率）呈现时会达到单一平衡状态。北美梭鱼草种群的形态频率在不同种群间存在显著差异，短花柱形态的频率往往超过等频率预期，而长花柱形态的频率通常低于预期。在之前的一项研究中，有人提出梭鱼草的不等频率种群结构是雄性育性差异的结果，这是由于不同形态在花粉产生上存在遗传差异。在本研究中，使用确定性计算机模型研究了平衡之前历史因素对形态频率的影响。非平衡频率受到奠基个体基因型的强烈影响，并且由于三型花柱受两个双等位基因位点控制，表型平衡是渐近达到的。该模型表明，在非平衡种群中，短花柱形态的频率会过高，而长花柱形态的频率会过低。这表明历史因素在决定梭鱼草的种群结构中起着重要作用。该物种种群生态学的几个特征支持了这一解释。在解释三型花柱植物形态频率以及其他非单基因座控制的遗传多态性的种群调查数据时，应考虑历史因素。
