Variation in the degree of reciprocal herkogamy affects the degree of legitimate pollination in a distylous species

Distyly is a widespread floral polymorphism characterized by the flowers within a population showing reciprocal placement of the anthers and stigma. Darwin hypothesizes that distyly evolves to promote precise pollen transfer between morphs. Primula chungensis exhibits two types of anther heights, and these two types of anthers show pollen of two different size classes. To understand whether the stigma could capture more pollen grains from the anthers of the pollen donor as the separation between the stigma of pollen receiver and the anther of pollen donor decreased, the present research assessed the source of the pollen load in a series of open-pollinated flowers with continuous variation of style lengths. Individuals with continuous variation of style length were tagged, and the selected flowers in the tagged plants were emasculated the day before dehiscence. The stigma of the emasculated flowers was fixed in fuchsin gel at the end of blooming. We assessed the pollen sources on each stigma by taking photos under a microscope and measured the diameter of each conspecific pollen grain with ImageJ. We found that a shorter distance from the stigmas to the anthers of a pollen donor gave the flower a higher capacity to receive pollen from those anthers. Our result provides a new evidence that distyly could promote the pollen transfer between morphs, which is consistent with Darwin’s hypothesis of disassortative pollination. An alternative hypothesis for the evolution of distyly (e.g. selfing avoidance) might also be true, but less likely, because self-incompatibility would greatly avoid self-fertilization for many distylous species.
Distyly, a well-known floral syndrome that was first identified by Charles Darwin, would be an ideal model to determine how a key floral syndrome is shaped by nature. Primula chungensis, a distylous species with simultaneous homo-, short- and long-styled morphs, exhibits some variations in the length of style. Using this species, we found that the stigma captured more compatible pollen as the separation between the stigma of the pollen receiver and the anther of the pollen donor decreased. Our results provide clear and novel evidence for the disassortative pollination hypothesis proposed by Charles Darwin, which will be helpful for our future understanding of the evolution of distyly.