Pollination syndromes are defined as suites of floral traits, which have evolved repeatedly across angiosperms in adaptation to distinct functional pollinator groups. Several studies show strong support for the concept of pollination syndromes while others raised concerns about their reliability.
Melastomataceae offer an ideal system to test the concept of pollination syndromes, since many different pollinators and specialized floral morphologies have been reported for the family. The large, pantropically distributed family is dominated by buzz-bee pollination (95.5% of species), where pollen is released from tubular, poricidal anthers through vibrations. In eight (of 23) tribes, shifts to different pollinators have occurred. These shifts seem to associate with changes in floral functional traits such as reward type, pollen expulsion mechanism and corolla shape. Shifts among major functional pollinator groups are generally regarded as a source of increased floral disparity.
We recorded 44 pollination-relevant functional floral traits across 411 species, spanning the whole family. We used machine learning algorithms to classify pollination syndromes for species with documented pollinators and employed these trained models to predict pollinators for species with unobserved pollinators. We ran ancestral character state estimation to determine directionality in pollinator shifts, and tested whether pollinator shifts mirror shifts in selection regimes using Ornstein-Uhlenbeck models. Furthermore, we contrasted floral disparity among the different syndromes, biogeography, and tribes to evaluate the relative role of pollinator shifts in generating floral diversity.
Our results indicate strong support for four well differentiated pollination syndromes within Melastomataceae: “buzz-bee” syndrome, “nectar-foraging vertebrate” syndrome (pollinated by mixed assemblages of vertebrates), “food-body-foraging vertebrates” syndrome (pollinated by birds), and “generalist” syndrome (pollinated by mixed insect assemblage). Pollination syndromes in Melastomataceae can be discriminated by six system specific floral traits of which reward type and pollen release mechanism are the two most important traits to distinguish the different syndromes. Also, we found that the “buzz-bee” and “nectar-foraging vertebrate” syndromes are highly diverse and pollinator shifts contribute to floral disparity. Finally, we reconstructed the “buzz-bee” syndrome as ancestral from which all other syndromes evolved repeatedly.