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 an cestral morphs and invading mutants. We used bumblebees visiting artif icial flowers to measure the parameters of the model. Our findings sup ported the first evolutionary step proposed by the model, establishmen t of a stigma-height polymorphism. Conditions for the subsequent estab lishment of an anther-height polymorphism were not satisfied by pollen -transfer patterns alone. Because conditions for the first step are co nsidered more onerous, however, and because the second stage depends o n inbreeding depression as well as pollen-transfer patterns, we interp ret our results as supporting the plausibility of the Lloyd-Webb model . Video images of bees visiting glass-sided artificial flowers demonst rate a mechanism for disassortative pollination between the ancestral and mutant morph. In general, pollen-transfer probabilities were negat ively correlated with the height difference between anthers of the don or and stigma of the recipient. Style length affects bee feeding postu re in such a way that disassortative pollination could feasibly occur in the absence of an anther-height polymorphism.