Floral Variation and Distyly in Guettarda scabra (Rubiaceae)

Abstract

Controversy exists as to whether the tropical shrub Guettarda scabra (Rubiaceae) is distylous. Variations in stigma and anther position and floral morphology of G. scabra were studied in a population in south Florida. Stigma and anther height have unimodal distributions, but stigma-anther separation is bimodally distributed and can be used to identify a long-styled and a short-styled morph. Stigma width varies between morphs, but anther length, pollen diameter, and stigma papillae length do not. The morphs occur in a 1:1 ratio in the two populations studied. G. scabra is self-compatible and can pollinate itself. Styles of the two morphs have similar relative growth rates in early development. Stylar growth is inhibited in the short-styled morph when buds are approximately 12 mm long. Anther height differs between morphs because of different relative growth rates and because the long-styled morph corolla tube, where the anthers are attached, stops growth before the tube of the short-styled morph. Reciprocity between morphs for average stigma and anther height falls within the range of reciprocity found in other distylous Rubiaceae. Thus G. scabra is morphologically distylous but unusual among distylous species in the variation within morphs and overlap between morphs in stigma and anther heights.