Phylogenetic conservation and shifts of pollination niche in generalist epiphytic cacti

Abstract

Plant-pollinator interactions are influenced, among other factors, by evolutionary history of the organisms. Thus, closely related species are expected to interact with similar pollinator assemblages. The aim of this study was to examine the evolution of pollination niches in neotropical epiphytic cacti. We recorded floral visitors and determined the pollination niche of 11 species of Rhipsalis and one species of Hatiora by using bipartite modularity, a metric of complex networks that is effective to identify pollination niches. Afterwards we explored how these niches evolved along the phylogeny of Rhipsalis. We recorded a total of 56 insect species belonging to nine functional groups pollinating those species of Rhipsalis. The most frequent floral visitors were bees. Pollination systems in Rhipsalis broadly combined a high phenotypic floral generalization with moderate richness of pollinator species and some functional pollinator group specialization. Four pollination niches were identified, mainly characterized by variations in the frequency of the functional groups of pollinators. The closer the species of Rhipsalis were, the more they interacted with the same functional groups of pollinators, as indicated by a positive phylogenetic signal for pollinating niches. The most likely ancestral pollination niche was associated with short-tongued bees. From the niche defined by small bees at least three shifts occurred towards the niches defined by extra-small bees, short-tongued medium bees, and flies. Considering the perspective of the evolution of each pollinator group, short-tongued bees were conserved along the branches of Rhipsalis phylogeny, with niche broadening or shifts associated with other groups such as flies, large bees, wasps, beetles and butterflies. Distinct generalist pollination niches have evolved in the genus. Shifts in pollination niches were related to differences in floral morphology. Moreover, the distribution of pollination niches only partially corresponded to the infrageneric classification of Rhipsalis. Our results support that generalist pollination systems encompass different pollinating niches, which may be consistently distributed across clades in angiosperm lineages.