Campbell Diane R, Waser Nickolas M, Price Mary V
Department of Ecology and Evolutionary Biology, University of California, Irvine, California, 92717.
Rocky Mountain Biological Laboratory, Crested Butte, Colorado, 81224.
Evolution. 1994 Feb;48(1):55-68. doi: 10.1111/j.1558-5646.1994.tb01294.x.
Experimental manipulation of a trait can be used to distinguish direct selection from selection of correlated traits and to identify mechanisms of selection. Here we use experiments to investigate phenotypic selection of stigma position in angiosperm flowers. In natural populations of the subalpine herb Ipomopsis aggregata, plants with more strongly exserted stigmas receive more pollen per flower, indicating selection favoring stigma exsertion during the pollination stage of the life cycle. We pose four hypotheses for this association, two involving direct selection on stigma position and two involving indirect selection of a correlated floral trait. The first three hypotheses were tested using hand pollinations that mimicked natural hummingbird visitation, and by presenting captive hummingbirds with a series of flowers that differed in stigma and anther positions, sex ratio, and presence of anthers. In these experiments, pollen deposition either was independent of stigma exsertion or was highest on inserted stigmas, suggesting direct selection against exserted stigmas. In natural populations, however, stigma exsertion is highly correlated with time spent by the protandrous flowers in the pistillate phase. When we manipulated the latter trait in the field, pollen deposition increased with duration of exposure to hummingbirds, indicating indirect selection for stigma exsertion. Stigma exsertion and time spent in the pistillate phase are genetically and phenotypically correlated, as shown by a quantitative genetic experiment conducted in the field with paternal half sibships. Our results suggest that the evolution of stigma position can be driven by selection of a genetically correlated trait.
对某一性状进行实验性操控，可用于区分对直接性状的选择与对相关性状的选择，并确定选择机制。在此，我们通过实验来研究被子植物花朵柱头位置的表型选择。在亚高山草本植物聚花漏斗菜的自然种群中，柱头伸出程度更强的植株每朵花接收的花粉更多，这表明在生命周期的授粉阶段存在有利于柱头伸出的选择。我们针对这种关联提出了四个假说，其中两个涉及对柱头位置的直接选择，另外两个涉及对相关花部性状的间接选择。前三个假说通过模拟自然蜂鸟访花的人工授粉实验进行检验，实验中向圈养的蜂鸟展示了一系列柱头和花药位置、性别比例以及花药有无不同的花朵。在这些实验中，花粉沉积要么与柱头伸出无关，要么在柱头内缩的花朵上最高，这表明存在对伸出柱头的直接选择。然而，在自然种群中，柱头伸出与雄性先熟花处于雌蕊期的时长高度相关。当我们在田间操控后一性状时，花粉沉积随着暴露于蜂鸟的时长增加，这表明存在对柱头伸出的间接选择。如在田间对父本半同胞家系进行的数量遗传学实验所示，柱头伸出与处于雌蕊期的时长在遗传和表型上均相关。我们的结果表明，柱头位置的进化可能由对一个遗传相关性状的选择所驱动。
