School of Life Sciences, University of KwaZulu-Natal, P. Bag X01, Scottsville, 3209, Pietermaritzburg, South Africa.
Am J Bot. 2012 Jun;99(6):1104-11. doi: 10.3732/ajb.1100535. Epub 2012 Jun 5.
A useful, but seldom applied, measure of the effectiveness of different pollinators is their contribution to the rate of outcrossing. This measure is particularly useful in facultatively autogamous plants for which seed set cannot be used as a direct measure of pollinator effectiveness. We used selective exclusion experiments to assess the importance of insects for outcrossing in Protea caffra, a facultatively autogamous shrub with scented flowers that are visited frequently by both birds and insects (mainly beetles).
Pollen loads on stigmas, pollen tube growth, seed set, seed mass, germination, and early seedling survivorship were similar for vertebrate-excluded and open-pollinated inflorescences. Pollen-supplementation mostly did not increase seed set, revealing resource limitation. Mean multilocus outcrossing rates, estimated using eight polymorphic allozyme loci, were similar for progeny from inflorescences excluded from bird visitors (0.65) and for those visited by both birds and insects (0.59). Wright's fixation indices indicated that the adult population is near Hardy-Weinberg equilibrium but differed markedly for maternal plants (F(IS) = -0.187 ± 0.065) and their early stage progeny (F(IS) = 0.258 ± 0.002). Since seed from self and cross hand-pollinations were equally viable in terms of germination, this discrepancy in F(IS) could be explained by inbreeding depression that occurs between germination and reproductive maturity.
Since outcrossing rates were not reduced when birds were excluded, we infer that insects are effective agents of cross pollination in P. caffra. This helps to explain the evolution of traits associated with insect pollination, such as fruity floral scent, in this species.
衡量不同传粉者效率的一个有用但很少应用的方法是它们对异交率的贡献。对于自交亲和植物，这种方法特别有用，因为不能将结实率直接作为传粉者效率的衡量标准。我们使用选择性排除实验来评估昆虫在花色斑斓的山龙眼（Protea caffra）中的传粉作用，这种植物是一种兼性自交的灌木，其有香味的花朵经常被鸟类和昆虫（主要是甲虫）访问。
排除脊椎动物后，柱头花粉负载量、花粉管生长、结实率、种子质量、萌发率和早期幼苗存活率与开放授粉的花序相似。花粉补充并没有显著增加结实率，这表明资源受到限制。使用 8 个多态性等位酶基因座估算的平均多位点异交率，对于排除鸟类访问的花序的后代（0.65）和同时被鸟类和昆虫访问的花序的后代（0.59）相似。Wright 的固定指数表明，成年种群接近 Hardy-Weinberg 平衡，但母本植物（F(IS) = -0.187 ± 0.065）和它们早期阶段的后代（F(IS) = 0.258 ± 0.002）差异显著。由于自交和异交授粉的种子在萌发方面具有同等活力，因此 F(IS) 的这种差异可以用萌发与生殖成熟之间发生的近交衰退来解释。
由于排除鸟类后异交率没有降低，我们推断昆虫是花色斑斓的山龙眼的有效异交传粉者。这有助于解释该物种与昆虫传粉相关的特征的进化，例如水果般的花香。
