The Mating System of Verbascum thapsus (Scrophulariaceae): The Effect of Plant Height

Several factors attract pollinators to individual plants. Among them, plant height is one of the least studied. Previous studies have demonstrated that taller plants often receive larger pollen loads, indicating that they may experience higher rates of outcrossing and have more diverse pollen donor arrays. In this study we sampled three populations consisting of at least 50 flowering individuals to estimate the mating system of the biennial weed Verbascum thapsus. Six tall and six short plants in each population were chosen for study. Mating system estimates were based on progeny arrays from three zones within the infructescence of each plant. Estimates of the mating system imply that plant height and flower position within the inflorescence affect outcrossing rates. Taller individuals in all three populations experienced higher rates of outcrossing than shorter individuals during both years of the study. The top infructescence section of short plants during the second year had significantly higher outcrossing than the middle or lower sections. However, in taller plants the middle section of the infructescence had higher outcrossing than the top and lower sections. The effective number of outcrossed pollen donors per maternal plant was also higher for taller plants. Our results indicate that tall plants receive larger and more genetically diverse pollen loads while shorter individuals may experience some pollen limitation leading to increased selfing.