Evolution of floral traits: Biogeography, pollination biology and phylogenetics in Macromeria viridiflora

Abstract

Macromeria viridiflora is an herbaceous perennial in which floral traits vary geographically. In my dissertation research, I analyzed geographic variation in plant morphology and pollinator assemblages. I conducted experiments to determine the breeding system of the plants, and used visitation rate and pollen deposition to compare effectiveness of floral visitors as pollinators. I analyzed aspects of pollinator attractants and rewards in the flowers and placed this into the context of pollinator syndromes. In addition, I used phylogenetic analysis of the genus to determine polarity of change in corolla size within the species. Analysis of morphometric data from eight sites across the range of the species revealed significant among-population variation in vegetative and floral traits. Flower size variation is particularly strong and follows a latitudinal cline. Hawkmoths and hummingbirds were the main floral visitors throughout the range. The large-bodied hummingbirds visiting plants in the southern regions are not present in the northern regions, where flowers are visited by hummingbirds with barely half the body size and much shorter bills. This difference in bill size of hummingbird pollinators mirrors the geographic variation in flower size in M. viridiflora, suggesting that pollinator-mediated selection may be acting upon the species. Flowers of M. viridiflora have several characteristics that fit both the hummingbird and hawkmoth pollinator syndromes, namely copious sucrose-rich nectar and long floral tubes. However, they also have characteristics that correspond with a single major pollinator. This plant therefore presents a compromise floral syndrome that attracts two classes of pollinators. Breeding system studies showed that whereas plants are self-compatible and occasionally produce seed autogamously, pollinators are important for reproductive success in the plants. Combining visitation rate and pollen deposition as measures of pollinator effectiveness, hummingbirds were found to be the most effective pollinators at both sites. Phylogenetic analysis produced a single most parsimonious tree that supports the monophyly of the genus. Mapping of corolla size onto the phylogeny indicates that floral size has changed many times within the genus, and that very large corolla size in southern populations of Macromeria viridiflora has been derived from a smaller-flowered ancestor.