Pollen aperture heteromorphism. Variation in pollen-type proportions alongaltitudinal transects in Viola calcarata

Some species produce pollen grains with different aperture numbers within a single individual (pollen aperture heteromorphism). In the pansy Viola div ersifolia, aperture number is positively correlated with pollen germination speed, and negatively correlated with viability. In V calcarata, young fiv e-aperturate pollen grains germinate faster than four-aperturate ones. Hete romorphism could thus be favoured when pollination is unpredictable, as pla nts produce both very competitive and long-lived pollen grains. Depending o n the efficiency of the pollinators, different proportions of pollen types will be optimal. In insect-pollinated species, such as V:calcarata, pollina tion efficiency generally decreases as elevation increases. We therefore ex pect a decrease in mean aperture number as altitude increases. This was fou nd in four transects tout of six). Pollinator activity therefore has a pote ntial impact on pollen morphology. (C) Academie des Sciences/ Elsevier, Par is.