Data from: Plant reproductive strategies vary under low and high pollinator densities

Long-term variation in the population density of honey bees (Apis mellifera) across landscapes has been shown to correlate with variation in the floral traits of plant populations in these landscapes, suggesting that variations in pollinator population density and foraging rates can drive floral trait evolution of their host plants. However, it remained to be determined whether this variation in plant traits is associated with adaptive variation in plant reproductive strategies under conditions of high and low pollinator densities. Here we conducted a reciprocal transplant experiment to examine how this variation in floral traits, under conditions of either high and low pollinator density, impacted seed production in the Tibetan lotus (Saussurea nigrescens). In 2014 and 2015, we recorded the floral traits, pollinator visitation rates, and seed production of S. nigrescens populations grown in both home sites and foreign sites, where sites varied in honey bee population density. Our results demonstrated that the floral traits reflected those of their original population, regardless of their current location. However, seed production varied with both population origin and transplant site. Seed number was positively correlated with flower abundance in the pollinator-rich sites, but with nectar production in the pollinator-poor sites. Pollinator visitation rate was also positively correlated with flower number at pollinator-rich sites, and with nectar volume at pollinator-poor sites. Overall, the local genotype had higher seed production than nonlocal genotypes in home sites. However, when pollen is hand-supplemented, plants from pollinator-rich populations had higher seed production than plants from pollinator-poor populations, regardless of whether they were transplanted to pollinator-rich or -poor sites. These results suggest the plant genotypic differences primarily drive variation in pollinator attraction, and this ultimately drives variation in seed: ovule ratio. Thus, our results suggest that flowering plant species use different reproductive strategies to respond to high or low pollinator densities.

已有研究表明，不同景观中西方蜜蜂（Apis mellifera）的种群密度长期变化，与该景观内植物种群的花部性状（floral traits）变异存在相关性，这提示传粉者种群密度和觅食速率（foraging rates）的差异，可驱动其寄主植物的花部性状演化。然而，在传粉者密度高低不同的条件下，植物性状的这种差异是否与其繁殖策略的适应性变异相关，仍有待探明。本研究通过开展互作移植实验（reciprocal transplant experiment），探究在传粉者密度高低不同的条件下，花部性状的差异如何影响西藏风毛菊（Saussurea nigrescens）的种子产量。2014年与2015年，我们对种植于原生境与外来境的显脉风毛菊（S. nigrescens）种群的花部性状、传粉者访花率（pollinator visitation rates）及种子产量（seed production）进行了记录，这些样地的西方蜜蜂种群密度存在差异。研究结果显示，无论植株当前所处的生境如何，其花部性状均与其源种群的性状一致。但种子产量则随种群来源与移植生境的不同而发生变化。在传粉者丰富的生境中，种子数量与花朵数量呈正相关；而在传粉者匮乏的生境中，种子数量则与花蜜产量呈正相关。传粉者访花率在传粉者丰富的生境中同样与花朵数量呈正相关，而在传粉者匮乏的生境中则与花蜜体积呈正相关。总体而言，在原生境中，本地基因型植株的种子产量高于非本地基因型植株。但当通过人工补充花粉后，来自传粉者丰富种群的植株的种子产量，高于来自传粉者匮乏种群的植株，无论其被移植到传粉者丰富还是匮乏的生境中。上述结果表明，植物的基因型差异主要驱动传粉者吸引能力的差异，而这最终会影响种子与胚珠之比的变异。综上，本研究结果显示，有花植物会采用不同的繁殖策略，以应对传粉者密度高低不同的环境。