Divergent selection on the biomechanical properties of stamens under wind and insect pollination

Wind pollination has evolved from insect pollination in numerous angiosperm lineages and is associated with a characteristic syndrome of morphological traits. The traits initiating transitions to wind pollination and the ecological drivers involved are poorly understood. Here, we examine this problem in Thalictrum pubescens, an ambophilous (insect and wind pollination) species that probably represents a transitional state in the evolution of wind pollination. We investigated wind-induced pollen release by forced harmonic motion by measuring stamen natural frequency ( f_n), a key vibration parameter, and its variability among nine populations. We assessed the repeatability of f_n over consecutive growing seasons, the effect of this parameter on pollen release in a wind tunnel, and male reproductive success in the field using experimental manipulation of the presence or absence of pollinators. We found significant differences among populations and high repeatability within genotypes in f_n. The wind tunnel assay revealed a strong negative correlation between f_n and pollen release. Siring success was greatest for plants with lower f_n when pollinators were absent, but this advantage diminished when pollinators were present. Our biomechanical analysis of the wind-flower interface has identified f_n as a key trait for understanding early stages in the transition from insect to wind pollination.