THE EVOLUTION OF DISTYLY: POLLEN TRANSFER IN ARTIFICIAL FLOWERS

A recent model by Lloyd and Webb derives conditions necessary for the evolution of distyly based on pollen-transfer probabilities between ancestral morphs and invading mutants. We used bumblebees visiting artificial flowers to measure the parameters of the model. Our findings supported the first evolutionary step proposed by the model, establishment of a stigma-height polymorphism. Conditions for the subsequent establishment of an anther-height polymorphism were not satisfied by pollen-transfer patterns alone. Because conditions for the first step are considered more onerous, however, and because the second stage depends on inbreeding depression as well as pollen-transfer patterns, we interpret our results as supporting the plausibility of the Lloyd-Webb model. Video images of bees visiting glass-sided artificial flowers demonstrate a mechanism for disassortative pollination between the ancestral and mutant morph. In general, pollen-transfer probabilities were negatively correlated with the height difference between anthers of the donor and stigma of the recipient. Style length affects bee feeding posture in such a way that disassortative pollination could feasibly occur in the absence of an anther-height polymorphism.