Barrett Spencer C H, Harder Lawrence D, Cole W W
Department of Botany, University of Toronto, Toronto, Ontario M5S 3B2, Canada.
Evolution. 2004 May;58(5):964-75. doi: 10.1111/j.0014-3820.2004.tb00431.x.
In sexually polymorphic species, reproductive morphology governs mating patterns and the character of negative frequency-dependent selection. If local environmental conditions cause sexual morphs to differ between populations, then frequency-dependent selection should create corresponding geographic variation in morph frequencies. We investigate this relation with a model of morph-ratio evolution and analysis of geographic variation in the heterostylous plant Narcissus triandrus. Unlike other tristylous species, N. triandrus possesses both imperfect reciprocity among morphs in sex-organ position and a self-incompatibility system that permits outcrossing within and between morphs. We sampled 137 populations throughout the Iberian Peninsula for floral-morph ratios, and measured floral morphology in 31 populations. Morph ratios exhibited three atypical features: (1) predominance of the long-styled (L) morph; (2) absence of the mid-styled (M) morph from 17.5% of populations; and (3) a negative relation between the frequencies of the L and M morphs among populations. Morph ratios varied geographically, with decreasing frequency of the M morph from the southeast to the northwest of the species' range. Much of this variation accompanied allometric change in the positions of sex organs, especially the mid-level organs, with the M morph declining in frequency and ultimately being lost in large-flowered populations. Using multivariate multiple regression, we demonstrate that variation in floral morphology among populations predicts this geographic variation in morph frequencies. Our theoretical analysis illustrates that patterns of pollen transfer governed by imperfect sex-organ reciprocity can select for unequal equilibrium morph ratios like those observed for N. triandrus. We interpret the L-biased morph ratios and the unusual morphology of N. triandrus as a consequence of its atypical intramorph compatibility system.
在具有性多态性的物种中，繁殖形态决定交配模式以及负频率依赖选择的特征。如果当地环境条件导致不同种群间的性形态存在差异，那么频率依赖选择应该会在形态频率上产生相应的地理变异。我们通过一个形态比例进化模型以及对异型花柱植物水仙（Narcissus triandrus）的地理变异分析来研究这种关系。与其他三型花柱物种不同，水仙在性器官位置上形态间不存在完美的相互对应关系，并且具有一个自交不亲和系统，该系统允许形态内和形态间进行异交。我们在伊比利亚半岛各地采样了137个种群以获取花形态比例，并在31个种群中测量了花形态。形态比例呈现出三个非典型特征：（1）长花柱（L）形态占主导；（2）17.5%的种群中不存在中型花柱（M）形态；（3）种群中L和M形态的频率之间呈负相关。形态比例在地理上存在差异，在该物种分布范围内，从东南部到西北部，M形态的频率逐渐降低。这种变异大多伴随着性器官位置的异速生长变化，尤其是中层器官，随着M形态频率的下降，最终在大花种群中消失。通过多变量多元回归分析，我们证明了种群间花形态的变异能够预测形态频率的这种地理变异。我们的理论分析表明，由不完美的性器官相互对应关系所支配的花粉传递模式能够选择出像在水仙中观察到的不均衡平衡形态比例。我们将水仙中偏向L型的形态比例以及不寻常的形态解释为其非典型的形态内亲和系统的结果。
