Pollen release mechanisms and androecium structure in Solanum (Solanaceae): Does anther morphology predict pollination strategy?

Abstract

Most species of Solanum L. (Solanaceae) exhibit a floral morphology typical of the genus: Yellow poricidal anthers with rigid walls contrasting in color with the corolla. However, some species of Solanum sect. Cyphomandropsis differ from most of the other species of Solanum by having flowers without contrasting colors and large anthers with flexible walls. These features resemble those of some closely related species belonging to Solanum sect. Pachyphylla that exhibit a bellows pollination mechanism whereby male euglossine bees cause the compression of thin anther walls and trigger pneumatic pollen release without vibration. Herein we studied the reproductive and pollination biology of a population of Solanum luridifuscescens (sect. Cyphomandropsis), a species with purple corolla and anthers, expecting to find a bellows mechanism of pollination. Both artificial mechanical stimuli applied with forceps and vibrations transmitted with an electric toothbrush resulted in the release of pollen from the anthers. Females of six species of bees visited the flowers and vibrated the anthers to collect pollen. Only one male euglossine bee visited the flowers, without causing pollen release. Solanum luridifuscescens exhibits pollen flower characteristics that are common among species of Solanum, such as the absence of nectar and the presence of poricidal anthers that release dry pollen by vibration. However, it also exhibits features that resemble mature anthers of perfume flowers typical of Solanum sect. Pachyphylla, such as a dorsally developed connective covered with purple papillae that do not contrast with the corolla plus flexible anther walls that are two or three cell layers thick on the lateral and ventral surfaces. Despite this distinct anther morphology, pollen of the flowers of S. luridifuscescens was removed exclusively by female bees using a typical vibratory mechanism. Morphological traits alone, thus, are not sufficient to predict the pollination mechanism of species of Solanum.