What is the role of stamen appendages in the buzz-pollinated Huberia insignis (Melastomataceae)?

Abstract

In most species of Melastomataceae pollen is the only reward for pollinators. Pollen is locked within poricidal anthers and is only legitimately removed by bees that specifically buzz the anthers. Stamen appendages in many species of plants are morphological adaptations of the androecium for pollination, and are evident in several species of Melastomataceae. In an experimental study, anthers with large, coiled appendages released more pollen during artificial buzzing in flowers of the genus Huberia than anthers without appendages. Other aspects of reproductive biology and pollination ecology in Huberia are poorly known, including the relationship between size and shape of widely varying stamen appendages with pollen release. In the Brazilian Atlantic Forest, we studied the breeding system and pollinator assemblage of the endemic Huberia insignis, and we experimentally tested how stamen appendages are associated with pollination. With hand-pollination experiments, morphoanatomical analysis, observations of the pollinator assemblage, using flowers with and without stamen appendages, we tested pollinator preferences and the effect of appendage presence on pollen removal rates. Huberia insignis is self-compatible and requires pollinators, and bees that buzz-pollinate are the only pollinators, typically two species of Bombus (ca. 80% of visits). The presence of stamen appendages of H. insignis had no effects on attracting pollinators and pollen removal. Anthers and stamen appendages have neither secretory structures nor specialized biomechanical tissues. We conclude that stamen appendages play no role in the pollination of H. insignis. We suggest that these typically small and inconspicuous stamen appendages are vestigial.