Stehlik Ivana, Barrett Spencer C H
Department of Botany, University of Toronto, 25 Willcocks Street, Toronto, Ontario M5S 3B2, Canada.
Evolution. 2006 Jun;60(6):1207-14.
Determining the mechanisms governing sex-ratio variation in dioecious organisms represents a central problem in evolutionary biology. It has been proposed that in plants with sex chromosomes competition between pollen tubes of female- versus male-determining microgametophytes (certation) causes female-biased primary sex ratios. Experimental support for this hypothesis is limited and recent workers have cast doubt on whether pollen-tube competition can modify sex ratios in dioecious plants. Here we investigate the influence of variation in pollination intensity on sex ratios in Rumex nivalis, a wind-pollinated alpine herb with strongly female-biased sex ratios. In a garden experiment, we experimentally manipulated pollination intensity using three concentric rings of female recipient plants at different distances from a central group of male pollen donors. This design enabled us to test the hypothesis that increasing pollen load size, by intensifying gametophyte competition, promotes female-biased sex ratios in R. nivalis. We detected a significant decline in pollen load at successive distance classes with concomitant reductions in seed set. Sex ratios of progeny were always female biased, but plants at the closest distance to male donors exhibited significantly greater female bias than more distant plants. The amount of female bias was positively correlated with the seed set of inflorescences. Hand pollination of stigmas resulted in approximately 100-fold higher stigmatic pollen loads than wind-pollinated stigmas and produced exceptionally female-biased progenies (female frequency = 0.96). Our results are the first to demonstrate a functional relation between stigmatic pollen capture, seed set, and sex ratio and suggest that certation can contribute towards female-biased sex ratios in dioecious plants.
确定雌雄异株生物性别比例变化的机制是进化生物学中的一个核心问题。有人提出，在具有性染色体的植物中，决定雌性与雄性的小配子体（花粉管竞争）之间的竞争会导致雌性偏向的初级性别比例。这一假设的实验支持有限，最近的研究人员对花粉管竞争是否能改变雌雄异株植物的性别比例表示怀疑。在这里，我们研究了授粉强度变化对雪兔耳草（一种风媒传粉的高山草本植物，性别比例强烈偏向雌性）性别比例的影响。在一项花园实验中，我们使用三组同心环的雌性受体植物，它们与中央一组雄性花粉供体的距离不同，以此来实验性地操纵授粉强度。这种设计使我们能够检验这样一个假设，即通过加强配子体竞争增加花粉负载量，会促进雪兔耳草中雌性偏向的性别比例。我们检测到在连续的距离类别中花粉负载量显著下降，同时结实率也随之降低。后代的性别比例总是偏向雌性，但离雄性供体最近的植物比距离较远的植物表现出明显更大的雌性偏向。雌性偏向的程度与花序的结实率呈正相关。对柱头进行人工授粉所产生的柱头花粉负载量比风媒传粉的柱头高出约100倍，并产生了异常偏向雌性的后代（雌性频率 = 0.96）。我们的结果首次证明了柱头花粉捕获、结实率和性别比例之间的功能关系，并表明花粉管竞争可能有助于雌雄异株植物中雌性偏向的性别比例。
