Rhopalobrachium is an endemic New Caledonian genus of Rubiaceae with controversial tribal position, because its protologue was apparently based on two distantly related species, Rhopalobrachium fragrans and Rhopalobrachium congestum. Despite its new circumscription as a monotypic genus (Rhopalobrachium fragrans) the tribal placement of Rhopalobrachium remained unknown. We investigated, for Rhopalobrachium fragrans, all morphological characters currently used for circumscribing tribes in the subfamily Ixoroideae sensu lato. The species has axillary inflorescences, with valvate flower aestivation, drupaceous fruits with a single pendulous ovule per locule, and 3-pororate pollen grains. We also performed Bayesian and parsimony analyses based on two chloroplast markers, rps16 intron and trnT-F region, to assess the phylogenetic position of Rhopalobrachium within Ixoroideae sensu lato. These analyses produced well-resolved and strongly supported topologies. The results of both the morphological survey and phylogenetic analyses indicated a position of Rhopalobrachium fragrans in the tribe Vanguerieae and a close relationship with the Pacific genus Cyclophyllum.
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