Pollen-size comparisons among animal-pollinated angiosperms with different pollination characteristics

Abstract

Pollen volume varies among angiosperm species over five orders of magnitude, presumably because the functional advantages of pollen size depend on each species» particular pollination and fertilization conditions. This paper reports two studies that assess whether animal-pollinated species with different pollination systems differ correspondingly in pollen size, as would be expected if pollen size affected pollen transport. Analysis of ninePedicularisspecies detected significant interspecific variation in pollen size; however, the overall mean pollen volume of species pollinated primarily by nectar-collecting bees did not differ significantly from that of species pollinated by pollen-collecting bees. Similarly, comparison of bee- and bird-pollinated congeners in 16 genera (nine families) from Australia and North America found considerable variation in pollen diameter within and between genera, but this variation was not associated consistently with differences in primary pollinator type (bee>bird for three genera, bee≈bird for five genera, bee<bird for eight genera). These results imply that pollen-transport conditions likely play a minor role in the evolution of pollen size, at least for bee- and bird-pollinated species. Hence, interspecific pollen-size variation more probably reflects differences in conditions for pollen germination, pollen-tube growth and ovule fertilization.