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.

跨不同景观的西方蜜蜂（Apis mellifera）种群密度的长期变化，已被证实与这些景观中植物种群的花部性状（floral traits）变异相关，这表明传粉者（pollinator）种群密度和觅食速率的差异，能够驱动其宿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，植物性状的这种变异是否与植物繁殖策略的适应性变异相关，仍有待探明。本研究开展了交叉移植实验（reciprocal transplant experiment），以探究在传粉者密度高低两种情境下，花部性状的这种变异如何影响西藏风毛菊（Saussurea nigrescens）的种子产量。2014年与2015年，我们记录了种植于原生境与外来境的该植物种群的花部性状、传粉者访花率（pollinator visitation rates）以及种子产量，各样地的西方蜜蜂种群密度存在差异。研究结果显示，植株的花部性状与其原种群的性状保持一致，而非取决于当前的生长位置。不过，种子产量同时受种群来源与移植样地的双重影响。在传粉者丰富的样地中，种子数量与花的丰度呈正相关；而在传粉者匮乏的样地中，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样地中同样与花的数量呈正相关，而在传粉者匮乏的样地中则与花蜜体积呈正相关。总体而言，本地基因型在原生境中的种子产量高于非本地基因型。但当进行人工补充授粉（hand-supplemented pollen）时，来自传粉者丰富种群的植株，其种子产量高于来自传粉者匮乏种群的植株，无论它们被移植到传粉者丰富还是匮乏的样地中。这些结果表明，植物的基因型差异主要驱动传粉者吸引能力的变异，而这最终会影响种子与胚珠比（seed: ovule ratio）。综上，本研究结果显示，显花植物会采用不同的繁殖策略以响应传粉者密度的高低变化。