Comparative floral structure and systematics in the sarracenioid clade (Actinidiaceae, Roridulaceae and Sarraceniaceae) of Ericales

Abstract

In molecular phylogenetic studies, Actinidiaceae, Roridulaceae and Sarraceniaceae form a strongly supported clade, which is sister to the ericoid clade (Clethraceae, Cyrillaceae and Ericaceae). In pre-molecular classifications, the sarracenioid families were often not affiliated with other ericalean taxa or considered to be closely related with each other, as they differ conspicuously in their habit, mode of nutrient uptake and/or superficial floral structure. In order to interpret the findings of molecular phylogenetic analyses from a morphological point of view, a detailed comparative study of floral morphology, anatomy and histology of these three families is presented. In addition, earlier literature is reviewed. The three families share a series of general and, at the level of Ericales, most likely plesiomorphic floral features, including pentamery, actinomorphy and hypogyny. Other, more specialized features, such as polystemony, choripetaly and integument number, turn out to be homoplasious in the sarracenioid clade. A floral feature shared by the three families is late anther inversion, which, in Ericales, is restricted to the sarracenioids and ericoids. Potential synapomorphies for the sarracenioids include vesicles that appear to contain condensed tannins in floral tissue, proximally thick petals, ovules with a nucellar hypostase and the presence of iridoid compounds. For the subclade of Actinidiaceae and Roridulaceae, potential synapomorphies include the presence of raphides and mucilage cells in floral tissue, a secretory inner surface in the gynoecium and the absence of synlateral vasculature in the ovary. Floral features in the clade are discussed and compared with the other families of Ericales. Further structural studies in other clades of Ericales and a well-resolved molecular phylogeny of the order are needed to test the systematic value of these features further. Some features may turn out to be true synapomorphies, whereas others may turn out to be widespread in Ericales and therefore plesiomorphic for the order.