Insights into the morphology and evolution of orbicules in the Spermacoce clade (Spermacoceae-Rubiaceae) and implications for systematics

Abstract

The orbicules are tiny structures of sporopollenin that occur on the interior wall of anthers in several groups of plants, and they are associated with pollen grains and tapetal cells. Although their function remains still unresolved, they have been widely used in systematics. Rubiaceae is one of the most studied families in this aspect. However, scarce information is available about the orbicules in the tribe Spermacoceae, especially in the Spermacoce clade. The main objectives of this work were to investigate the occurrence, general morphology, evolution, and systematic utility of the orbicular characters in the Spermacoce clade. We investigated the presence/absence, size, abundance, shape, and ornamentation of orbicules in 104 specimens from 84 species in the Spermacoce clade using scanning electron microscopy (SEM). On the basis of these features, the ancestral state reconstruction analyses were performed on a custom-made phylogeny by stochastic character mapping. The orbicules are present in 38 (44%) of 84 analysed species, corresponding to 13 genera (56%) of the Spermacoce clade. Seven genera have orbicules in all analysed species, eight genera lack orbicules in all analysed species, and six genera show both presence and absence of orbicules. The absence of orbicules was estimated to be the ancestral state for the Spermacoce clade and the presence of orbicules evolved several times independently within Clade B. The absence of orbicules in the common ancestor of the Spermacoce clade can be seen as one of the independent losses that occurred during the evolutionary history of the Rubiaceae.