Down the tube: Pollinators, predators, and the evolution of flower shape in the alpine skypilot, Polemonium viscosum

We address how a conflict between pollinator attraction and avoidance of fl ower predation influences the evolution of flower shape in Polemonium visco sum. Flower shape in P. viscosum is the product of an isometric relationshi p between genetically correlated (r(A) = 0.70) corolla flare and length. Bu mblebee pollinators preferentially visit flowers that are more flared and h ave longer tubes, selecting for a funnel-shaped corolla. However, flower sh ape also influences nectar-foraging ants that sever the style at its point of attachment to the ovary. Surveys of ant damage show that plants having f lowers with flared, short corollas are most vulnerable to ant predation. Co nsistent with this result, the ratio of corolla length to flare is signific antly greater in a krummholz (high predation risk) population than in a tun dra (low predation risk) population. To explicitly test whether the evoluti on of a better defended flower would exact a cost in pollination, we create d tubular flowers by constricting the corolla during development. Performan ce of tubular flowers and natural controls was compared for defensive and a ttractive functions. In choice trials, ants entered control flowers signifi cantly more often than tubular ones, confirming that the evolution of tubul ar flowers would reduce the risk of predation. However, in a bumblebee-poll inated population, tubular flowers received significantly less pollen and s et fewer seeds than controls. A fitness model incorporating these data pred icts that in the absence of the genetic correlation between corolla length and flare, intermittent selection for defense could allow tubular flowers t o spread in the krummholz population. However, in the tundra, where bumbleb ees account for nearly all pollination, the model predicts that tubular flo wers should always confer a fitness disadvantage.