SELECTION IMPOSED BY POLLINATION MODE MINIMALLY INFLUENCES EVOLUTION OF POLLEN MORPHOLOGY IN THALICTRUM (RANUNCULACEAE)

Premise of research. Pollen grain morphology varies widely among angiosperms; however, the selective pressures responsible for this diversification are not fully resolved, and both pollination and fertilization success vary with aperture number and pollen grain size. As such, pollen transmission conditions may exert specific selective pressures on pollen morphology. This study examines the hypothesis that evolutionary shifts in pollen grain size and aperture number correlate with transitions between insect and wind pollination systems. Methodology. Within the genus Thalictrum (Ranunculaceae), we use phylogenetic comparative methods to examine the correlated evolution between pollen morphology (grain size and aperture number) and the selective context of pollination mode. Furthermore, we implement Hansen's adaptation-inertia model, which envisions phenotypes evolving toward an adaptive optimum, based on strong stabilizing selection. Additionally, we calculate the phylogenetic heritabilities of pollen traits and test phylogenetic correlations between them. Pivotal results. Pollination mode explained only a small amount of the variation in pollen grain size and aperture number. Pollen grain size shows high phylogenetic heritability and marginally lower variation within wind-pollinated species relative to insect-pollinated species. The calculated optimum aperture number was greater for insect-pollinated species than it was for wind-pollinated species. Consistent with previous work, we find a high phylogenetic heritability for pollination mode in a pruned phylogeny. Conclusions. Differences in calculated optima for aperture number and coefficient of variation in size between pollination modes correspond with predictions related to pollen competition intensity and the constraints of pollen transmission. However, the effect of pollination mode on the evolution of aperture number and grain size within Thalictrum is small.