Towards a Monophyletic Hoya (Marsdenieae, Apocynaceae): Inferences from the Chloroplast TrnL Region and the rbcL-atpB Spacer

The delimitation of the genus Hoya, with at least 200 species distributed from India and China to Australia, from its closest relatives in the Marsdenieae has long been problematic, precluding an understanding of the evolution and biogeography of the genus. Traditional circumscriptions of genera in the Hoya alliance have relied on features of the flower, but these overlap extensively between clades and may be evolutionarily labile. We obtained chloroplast DNA sequences to infer the phylogenetic relationships among a sample of 35 taxa of Hoya and 11 other genera in the tribe Marsdenieae, namely Absolmsia, Cionura, Dischidia, Dregea, Gongronema, Gunnessia, Madangia, Marsdenia, Micholitzia, Rhyssolobium, and Telosma. Trees were rooted with representatives of Asclepiadeae, Ceropegieae, Fockeeae, Periplocoideae, and Secamonoideae. Hoya and Dischidia form a monophyletic group, but the phylogenetic signal in the chloroplast data analyzed here was insufficient to statistically support the mutual monophyly of the two genera. A monophyletic Hoya, however, must include the monotypic Absolmsia, Madangia, and Micholitzia, a result congruent with their flower morphology. The data also identified several well-supported groups within Hoya. The morphologically unusual Gunnessia belongs firmly in the Marsdenieae, but it is not close to Hoya and Dischidia.