Flower-size dimorphism avoids geitonogamous pollination in a nectarless monoecious plant Akebia quinata

Akebia quinata is a nectarless monoecious vine and has a flower-size dimorphism in which female flowers are twice as large as male flowers. In a field experiment, we examined the effects of resemblance between male and female flowers on pollinator behavior and the amount of geitonogamous pollination. When the same numbers of female and male flowers were presented in an artificial floral array, solitary bees tended to visit female flowers first and then visit male flowers in a foraging sequence. However, when sepals of male flowers were glued to female flowers from which the original sepals had been removed to produce floral monomorphism, solitary bees frequently moved from male to female flowers. Consequently, the amount of pollen deposited on stigmas per array visit was increased. The results show that floral monomorphism increases geitonogamous pollination, and flower-size dimorphism in A. quinata can avoid geitonogamous female 1 male pollination. We discuss the effect of pollinator selectivity on the evolution of flower-size dimorphism and intersexual floral mimicry.