Evolutionary history and taxonomy of the Cuscuta umbellata complex (Convolvulaceae): Evidence of extensive hybridization from discordant nuclear and plastid phylogenies
The Cuscuta umbellata complex is one of the 15 major clades recently circumscribed in C. subg.Grammica. Most of its members occur in North America and the Caribbean (C. desmouliniana, C. lacerata, C. leptantha, C. liliputana, C. odontolepis, C. polyanthemos, C. tuberculata, C. umbellata), but three species (C. acuta, C. membranacea, C. umbellata) grow in South America, and one (C. hyalina) is found as a native species in India, Pakistan and Eastern to South Africa. Basic morphology, scanning electron microscopy and sequence data from the nuclear internal transcribed spacer (ITS) and the plastid trnL-F region were used to reconstruct the phylogeny, gain a better understanding of the evolutionary history, and determine species boundaries. Our results show that in its currently accepted delimitation C. umbellata is polyphyletic. Discordances between phylogenies derived from plastid and nuclear data strongly suggest that at least four independent hybridization events have occurred in the evolution of this species group, rendering relationships among its members more complex than previously thought. One of these reticulation events involves C. umbellata var. reflexa, a taxon that has been considered synonymous to C. umbellata var. umbellata in the last decades. This hybrid is morphologically intermediate but distinct from its putative parents, C. odontolepis or C. acuta on the maternal side, and C. umbellata (var. umbellata) on the paternal side, which supports its treatment as a new species, C. legitima. Cuscuta umbellata is further redefined to exclude C. umbellata var. dubia, which is merged into C. desmouliniana. A new classification is provided, together with an identification key, descriptions, illustrations, and geographical distributions for the twelve species of the clade.
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