The Phylogenetic Significance of Fruit and Trichome Structures in Apiaceae Subfamily Mackinlayoideae

Fruit and trichome structures of Apiaceae subfamily Mackinlayoideae were studied in detail using light microscopy to identify morphological features that might be useful in delimiting this subfamily and to identify structural characters that can be used to define subclades identified by molecular phylogenetic studies (e.g. the Centella and Xanthosia clades). Three types of trichomes are present in all genera except Mackinlaya and Schoenolaena: equisetiform (Actinotus, Centella, and Chlaenosciadium), digitiform (Apiopetalum, Micropleura, and Pentapeltis), and dendritic (Xanthosia). Fruits usually have laterally compressed mericarps. Dorsal bundles and rib ducts are usually branching (or branching and anastomosing) in most taxa studied. Branching and anastomosing vittae only occur in Apiopetalum, which is also characterized by sclereids in the mesocarp. Two types of crystals were found, rhomboidal and druse. Carpophores are entirely absent from the subfamily, but homologous features are present in the form of ventral bundles. Within Mackinlayoideae, the Actinotus-Apiopetalum and Mackinlaya clades can be distinguished from the Centella clade (Centella, Micropleura, Pentapeltis, and Schoenolaena) and Xanthosia clade (Xanthosia and Chlaenosciadium) in having fleshy fruits, specialized trichomes, and sclereids in the fruit mesocarp. Despite its phylogenetic position, Mackinlayoideae are more similar to Araliaceae than to Azorelloideae or the rest of Apiaceae, suggesting the retention of plesiomorphic character states, such as digitiform trichomes, laterally compressed mericarps, and sclereids. The equisetiform and dendritic trichomes are synapomorphies of Centella and Xanthosia clades and the Xanthosia clade differs from the Centella clade in lacking digitiform trichomes. Overall, trichome and fruit characters provide useful structural features in defining Mackinlayoideae and in differentiating its subclades.