Darwin's cross-promotion hypothesis and the evolution of stylar polymorphism

There is growing appreciation that the ecological factors which impact on rates of pollen transfer can contribute significantly to reproductive trait evolution in plants. In heterostylous species, several studies support Darwin's claim that the reciprocal positions of stigmas and anthers enhance inter-morph mating in comparison to intra-morph mating and thus the maintenance of the polymorphism. In this study, we evaluate the relative importance of intra-morph and inter-morph pollen transfers in Narcissus assoanus, a species with dimorphic variation in style length but non-reciprocity of anther positions. This stigma-height dimorphism represents a transitional stage in theoretical models of the evolution of distyly. Seed set variation on recipient plants with donor plants of a single morph in experimental arrays in a natural population illustrate that inter-morph cross-pollination is more efficient that intra-morph cross-pollination as a result of high rates of pollen transfer from long-styled to short-styled plants. The observed rates of pollen transfer satisfy the theoretical conditions for the establishment of a stigma-height dimorphism in an ancestral monomorphic long-styled population in pollen-limited situations. These results provide experimental evidence for the Darwinian hypothesis that enhanced inter-morph cross-pollination facilitates not only the maintenance of heterostyly but also the establishment of transitional forms implicated in the evolution of this polymorphism.