Intraspecific variation in morphology of spiny pollen grains along an altitudinal gradient in an insect-pollinated shrub.

Intraspecific variations in pollen morphological traits are poorly studied. Interspecific variations are often associated with pollination systems and pollinator types. Altitudinal environmental changes, which can influence local pollinator assemblages, provide opportunities to explore differentiation in pollen traits of a single species over short distances. The aim of this study is to examine intraspecific variations in pollen traits of an insect-pollinated shrub, Weigela hortensis (Caprifoliaceae), along an altitudinal gradient. Pollen spine phenotypes (length, number, and density), pollen diameter, lipid mass (pollenkitt) around pollen grains, pollen production per flower, and pollinator assemblages were compared at four sites at different altitudes. Spine length and the spine length/diameter ratio of pollen grains were greater at higher altitudes but were not correlated with flower or plant size. Spine number and density increased as flower size increased, and pollen lipid mass decreased as plant size increased. Bees were the predominant pollinators at low-altitude sites whereas flies, specifically Oligoneura spp. (Acroceridae), increased in relative abundance with increasing altitude. The results of this study suggest that the increase in spine length with altitude was the result of selection favoring longer spines at higher-altitude sites and/or shorter spines at lower-altitude sites. The altitudinal variation in selection pressure on spine length could reflect changes in local pollinator assemblages with altitude.