Floral biology of the velvetseed Guettarda platypoda DC. (Rubiaceae): Atypical distyly or style dimorphism?

Distylous species have flowers with short styles and long stamens (thrums), and flowers with the inverse condition (pins) in different individuals. Studies suggested that style dimorphism (floral morphs that differ only in style height) is an intermediate stage in the evolution of distyly. We found populations of Guettarda platypoda DC. with apparent morphological variations of distyly and our objective was to assess stylar condition and other reproductive characteristics of G. platypoda. We analyzed the anthesis period, scent glands, nectar features, pollen number and viability, and measured lengths of corolla, anther and stigmatic lobes, heights of stigma and anther, and proportion of individuals of the floral morphs within the population. We characterized the species' reproductive system and investigated the behavior and frequency of its flower visitors. Two flower morphs were observed, one with a short style (thrum) and the other with a long style (pin); the anthers have similar height. Anthesis occurred between 1700 h and 1100 h. The volume of nectar, number of pollen grains and pollen viability did not differ between morphs, but the thrum showed more concentrated nectar. Both morphs had scent glands. The lengths of anthers and stigmas were similar among morphs, but the corolla was longer in the thrums. The influence of the corolla was the same on the different whorls in the pins, but stronger on stamen height than on stigma position in the thrums. The population is isoplethic and formed fruits in all treatments, with a partial incompatibility system for the thrums. Its visitors were three moth species, including two sphingids. The style dimorphism found in this sphingophilous species may indicate that G. platypoda represents an intermediate stage in the evolution towards distyly.