Height differences in clonal stands of Tussilago farfara promote outcrossing by influencing pollinator behaviour

Plants with mixed mating systems balance the advantages of selfing and the costs of inbreeding. Previous studies have shown that plant species with the ability to self-pollinate and cross pollinate have strategies to promote outcrossing to increase genetic diversity. Various features of floral morphology are thought to be deliberate mechanisms to manipulate plant mating systems. I hypothesized that within-plant variation in flower stem height is a morphological trait that can reduce geitonogamy by increasing pollinator movement among plants. This hypothesis was tested using coltsfoot (Tussilago farfara); coltsfoot plants host several flowering stems that differ in height, with each stem having a single, terminal flowerhead. I used cut flowers to create populations of coltsfoot in which each plant had four inflorescences with equal or unequal flower stalk lengths and measured frequency of insect pollinators that would stay among flowers within a plant or move to other plants. All pollinators (bee flies, hover flies, solitary bees and wasps) showed a marked discrimination in favor of leaving plants with flowers of different heights and stayed when plants had flowers of the same height. This study shows that variation in flower heights is important for reducing geitonogamy in coltsfoot and suggests that the evolution of this morphological trait should be considered in studies of mating systems.