Abstract
The Rubiaceae tribe Rubieae has a world-wide distribution with up to 1,000 species. These collectively exhibit an enormous ecological and morphological diversity, making Rubieae an excellent group for macro- and microevolutionary studies. Previous molecular phylogenetic analyses used only a limited sampling within the tribe or missed lineages crucial for understanding character evolution in this group. Here, we analyze sequences from two plastid spacer regions as well as morphological and biogeographic data from an extensive and evenly distributed sampling to establish a sound phylogenetic framework. This framework serves as a basis for our investigation of the evolution of important morphological characters and the biogeographic history of the Rubieae. The tribe includes three major clades, the Kelloggiinae Clade (Kelloggia), the Rubiinae Clade (Didymaea, Rubia) and the most species-rich Galiinae Clade (Asperula, Callipeltis, Crucianella, Cruciata, Galium, Mericarpaea, Phuopsis, Sherardia, Valantia). Within the Galiinae Clade, the largest genera Galium and Asperula are para- and polyphyletic, respectively. Smaller clades, however, usually correspond to currently recognized taxa (small genera or sections within genera), which may be used as starting points for a refined classification in this clade. Life-form (perennial versus annual), flower shape (long versus short corolla tube) and fruit characters (dry versus fleshy, with or without uncinate hairs) are highly homoplasious and have changed multiple times independently. Inference on the evolution of leaf whorls, a characteristic feature of the tribe, is sensitive to model choice. Multi-parted leaf whorls appear to have originated from opposite leaves with two small interpetiolar stipules that are subsequently enlarged and increased in number. Early diversification of Rubieae probably started during the Miocene in western Eurasia. Disjunctions between the Old and the New World possibly are due to connections via a North Atlantic land bridge. Diversification of the Galiineae Clade started later in the Miocene, probably in the Mediterranean, from where lineages reached, often multiple times, Africa, eastern Asia and further on the Americas and Australia.
Highlights
Rubiaceae is the fourth-largest family of angiosperms and includes about 12000, mostly woody and tropical species [1]
Phylogenetic relationships inferred from maximum parsimony and Bayesian methods are largely congruent and without any strongly supported incongruences (Figs 1 and 2 and S1)
Likelihoods estimated via stepping stone or path sampling support a relaxed clock model over a strict clock model, both in the data set including DNA sequences only as well as in the combined data set including morphological traits and geographic data
Summary
Rubiaceae is the fourth-largest family of angiosperms and includes about 12000, mostly woody and tropical species [1] Within this family, the predominantly herbaceous tribe Rubieae, comprising up to 1,000 species in about twelve genera [2], has achieved a world-wide distribution and an enormous ecological diversity with species in tropical habitats, Mediterranean bushlands, temperate deciduous forests or arctic and alpine tundra. The predominantly herbaceous tribe Rubieae, comprising up to 1,000 species in about twelve genera [2], has achieved a world-wide distribution and an enormous ecological diversity with species in tropical habitats, Mediterranean bushlands, temperate deciduous forests or arctic and alpine tundra This is paralleled by a considerable morphological diversity, e.g., with respect to life form (perennials, annuals), growth form (shrubs, vines, herbs), leaves and stipules, inflorescences, flower and fruit morphology and corresponding changes in pollination (e.g., tubular or rotate corollas for different visitors) and dispersal ecology (e.g., dry mericarps with uncinate hairs as means for epizoochory or fleshy fruits as means for endozoochory). Yang et al [13] addressed relationships within Rubieae using a broad sampling with an expanded focus on species occurring in China
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