Department of Plant, Soil and Insect Science, University of Massachusetts, Amherst, MA 01003, USA.
Oecologia. 2012 Apr;168(4):1033-41. doi: 10.1007/s00442-011-2153-3. Epub 2011 Oct 20.
The evolution of floral traits may be shaped by a community of floral visitors that affect plant fitness, including pollinators and floral antagonists. The role of nectar in attracting pollinators has been extensively studied, but its effects on floral antagonists are less understood. Furthermore, the composition of non-sugar nectar components, such as secondary compounds, may affect plant reproduction via changes in both pollinator and floral antagonist behavior. We manipulated the nectar alkaloid gelsemine in wild plants of the native perennial vine Gelsemium sempervirens. We crossed nectar gelsemine manipulations with a hand-pollination treatment, allowing us to determine the effect of both the trait and the interaction on plant female reproduction. We measured pollen deposition, pollen removal, and nectar robbing to assess whether gelsemine altered the behavior of mutualists and antagonists. High nectar gelsemine reduced conspecific pollen receipt by nearly half and also reduced the proportion of conspecific pollen grains received, but had no effect on nectar robbing. Although high nectar gelsemine reduced pollen removal, an estimate of male reproduction, by one-third, this effect was not statistically significant. Fruit set was limited by pollen receipt. However, this effect varied across sites such that the sites that were most pollen-limited were also the sites where nectar alkaloids had the least effect on pollen receipt, resulting in no significant effect of nectar alkaloids on fruit set. Finally, high nectar gelsemine significantly reduced seed weight; however, this effect was mediated by a mechanism other than pollen limitation. Taken together, our work suggests that nectar alkaloids are more costly than beneficial in our system, and that relatively small-scale spatial variation in trait effects and interactions could determine the selective impacts of traits such as nectar composition.
花部特征的进化可能受到影响植物适合度的花部访客群的塑造，包括传粉者和花部拮抗者。花蜜吸引传粉者的作用已经得到了广泛的研究，但花蜜对花部拮抗者的影响却知之甚少。此外，非糖花蜜成分的组成，如次生化合物，可能通过改变传粉者和花部拮抗者的行为来影响植物繁殖。我们在原产多年生藤本植物 Gelsemium sempervirens 的野生植物中操纵了花蜜生物碱钩吻素甲。我们将花蜜钩吻素甲的操纵与人工授粉处理相结合，从而可以确定该特征及其相互作用对植物雌性繁殖的影响。我们测量了花粉沉积、花粉去除和花蜜盗食，以评估钩吻素甲是否改变了共生者和拮抗者的行为。高浓度的花蜜钩吻素甲使同种花粉的接收量减少近一半，也减少了同种花粉粒的接收比例，但对花蜜盗食没有影响。虽然高浓度的花蜜钩吻素甲使花粉去除量，即雄性繁殖的一个估计值，减少了三分之一，但这种效果在统计学上并不显著。结实率受花粉接收量的限制。然而，这种效应在不同地点之间存在差异，花粉限制最严重的地点也是花蜜生物碱对花粉接收影响最小的地点，因此花蜜生物碱对结实率没有显著影响。最后，高浓度的花蜜钩吻素甲显著降低了种子重量；然而，这种效果是通过花粉限制以外的机制介导的。总之，我们的研究表明，在我们的系统中，花蜜生物碱的成本高于收益，而特征效应和相互作用的相对小规模空间变异性可能决定了如花蜜组成等特征的选择影响。
