Morphological diversity and evolutionary changes of pollinaria in Hoya (Marsdenieae: Apocynaceae)

Abstract

The pollinarium morphologies of 85 species and four subspecies of Hoya were investigated in the present study. The longest pollinium was found in Hoya multiflora, the widest in H. archboldiana, and the smallest in H. bilobata. Pollinium shape showed the widest variation range of all palynological characters, and nine shapes were distinguished in this study. A pellucid margin was present in the pollinium of 84 taxa, and absent in five species. In total, 10 types of pollinarium were recognized based on the following characters: presence/absence of a pellucid margin and the caudicle wing, the ratio of corpusculum/pollinarium length, width/length of pollinium, and curving orientation of the caudicle wings. Types V and X were found in a greater number of taxa compared with the other eight types. Type V is a combination of obliquely elongate or oblong pollinium, presence of a pellucid margin and caudicle wing, and corpusculum length less than half of the pollinium. Type X combines pollinium with variable shapes, a pellucid margin, unwinged and twisted caudicle, and corpusculum with obvious lateral extensions. The characters of 46 taxa with a resolved position in the latest phylogeny of Hoya were mapped and ancestral reconstruction analysis was carried out, indicating that the presence of a pellucid margin in the pollinium, corpusculum length less than half of the pollinium, and rhomboid-shaped corpusculum with lower extensions represent the ancestral state of the genus. Clade II is characterized by the absence of a pellucid margin and twisted caudicles. Loss of the pellucid margin occurring in Clade II and sometimes in other clades is considered independent reversals to the plesiomorphic state. The presence of a pellucid margin and rhomboid corpusculum, and absence of caudicle wings characterizes Clade IV. Ancestral reconstruction analysis revealed that pollinarium type V was the ancestral state of the core lineage Clade V. The species in this clade share presence of both a pellucid margin and caudicle wing, straight caudicle, and pollinium width of 201–300 µm. Clade VI (the largest lineage) as well as Clades I and III have diverse palynological characters, but pollinarium type X and type V is probably the ancestral state of Clades VI and I, respectively.