Wax plants (Hoya, Apocynaceae) evolution: Epiphytism drives successful radiation

Abstract

AbstractHoya, a species‐rich, mostly epiphytic genus within tribe Marsdenieae (Apocynaceae), occurs in tropical and subtropical forests of the Indo‐Australian Archipelago (IAA). Previous molecular phylogenetic studies based on a restricted taxon sample have provided a first phylogenetic framework for the genus. Questions remain, however, about the evolution and relationships among the over 300 species of the genus. Here, we (1) present the first well‐resolved phylogenetic tree based on sequences of the plastid spacer regions trnH‐psbA, trnT‐trnL, and of the nuclear‐encoded ITS and 5′‐ETS regions, for 154 species covering all proposed sections and (2) we discuss the evolution of epiphytism and its possible role in the successful radiation of Hoya in the tropical forests of the IAA in light of the new phylogenetic results. According to our results, species of Hoya group into six major lineages (Clades I–VI) with widespread geographic distribution. The earliest‐diverging epiphytic lineages of Clade I are restricted to subtropical continental Asia or tropical to subtropical Australasia (Clade IV), whereas their non‐epiphytic relatives are limited to the tropical Indomalayan (Clade II/III) and Australasian (Clade II) parts of the IAA. Clade V (tropical Indomalaya and Australasia) and Clade VI, with outposts in the Himalayas, Japan, and throughout Australasia, represent the second phase of diversification within Hoya. We hypothesise that Hoya is of tropical to subtropical Indo‐Burma/Himalayan origin and that the common trait epiphytism is connected with the onset of monsoon climates during the Himalayan uplift. The subsequent dispersal of Hoya into the many subareas of the IAA region and eventually Australia and the Southwest Pacific, and its success‐ ful radiation there, are proposed to be linked to its epiphytic life style, a trait that is today only found in 10% of angiosperms.