The Last Stand of Angraecum: Biogeographic and Evolutionary History of Darwin's Muses
The Orchidaceae are notable for their remarkable diversity and are regarded as one of the most charismatic families of flowering plants. Notwithstanding the biological significance of Orchidaceae, numerous systematic challenges persist. Inter- and intrageneric relationships in several tribes and subtribes remain poorly understood, particularly within subtribe Angraecinae (tribe Vandeae), which impedes the exploration of its biogeographical and evolutionary history. While the evolutionary history of subtribe Angraecinae has recently been addressed by numerous authors, its biogeographical history has only been studied for a limited number of areas within its full geographic distribution and/or for only a few of its taxa. Herein, phylogenetic, divergence-time estimation, and historical-biogeographical analyses were conducted using a comprehensive molecular dataset of six loci for Angraecinae with the objective of reconstructing ancestral areas of distribution of the subtribe. The results of the biogeographical study indicate that Angraecinae originated during the Miocene and that Madagascar represents its ancestral area. The group has undergone several independent dispersal events to neighboring islands in the Indian Ocean and to continental Africa. However, a single major dispersal event towards continental Africa is responsible for all taxa of the Afro-Neotropical clade. The resulting phylogenies also corroborated the polyphyly of Angraecum, the subtribe’s most diverse genus. Furthermore, while sectional relationships were poorly resolved, the majority of the sections were also found to be polyphyletic. The absence of a robust phylogenetic classification of Angraecum continues to hinder the formulation of hypotheses regarding its evolutionary history. Phylogenetic analyses based on a limited number of markers derived from Sanger sequencing have proved to be insufficient for informing the construction of such a classification. Using Angiosperms353, a target sequence capture probe set, hundreds of low-copy nuclear genes were sequenced from a broad sampling of Angraecum species across most previously defined sections, along with representatives from associated genera. Phylogenetic analyses conducted in both concatenated and coalescence-based frameworks yielded a strongly supported phylogenetic hypothesis, coherent with the morphological characters of Angraecum species. It provided the basis for constructing a classification of Angraecum that better reflects evolutionary relationships and to redefine the sectional limits of the genus. Based on these results, 188 taxa (179 species, 3 subspecies and 5 varieties) of Angraecum are recognized, distributed among 11 sections. Thirteen species were excluded from the genus. By contrast, five species previously placed in Oeoniella Schltr. and Sobennikoffia Schltr. have herein been transferred to Angraecum, in two new sections. Finally, we confirmed that several species of Angraecum should also be excluded from the genus based on their phylogenetic position. This clade of five Angraecum species spread across four sections has consistently been found to be only distantly related to the rest of the genus and instead is closely related to Cryptopus Lindl., Oeonia Lindl. and allied genera. This clade was also recovered in the phylogenies resulting from two earlier studies, both using a small set of loci derived from Sanger sequencing and the phylogenomic dataset derived from high-throughput DNA sequencing. Based on the results reported here, synapomorphies could be identified for this clade of ex-Angraecum species and compared to the description and from material of other species representing eight sections of Angraecum (sensu stricto). In total, ten species share the morphology of the ex-Angraecum clade, including one taxonomic novelty. This clade is therefor described as a new genus of Angraecinae, Vavavoloina, or “hairy mouth” in Malagasy, referring to the hairs on the labellum at the spur entrance, which is one of its most striking features. Nine species are given new combinations and the taxonomic novelty is descrived as a new species, Vavavoloina punctata.
Biodiversity, Biology, Botany, Evolution, Orchidaceae, Angraecum, Systematics, Angiosperms353, Biogeography
City University of New York - Graduate Center
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