Systematics of Cuscuta chinensis species complex (subgenus Grammica, Convolvulaceae): evidence for long-distance dispersal and one new species

Cuscuta chinensis species complex is a small clade from subgenus Grammica (Cuscuta, Convolvulaceae). Many species of this clade exhibit crest- or dome-like multicellular appendages with stomata on the midvein/carena of calyx lobes. Basic morphology, scanning electron microscopy, and DNA sequence data from the plastid trnL-F region and the nuclear internal transcribed spacer (ITS) were used to investigate the phylogenetic relationships within the group and test the species limits. Based on their morphological and molecular similarly, C. chinensis and C. applanata represent one single species, and the latter was retained as a variety of the former. While the clade is centered in the southern USA and Mexico, C. chinensis var. chinensis has a disjunct distribution in Australia and Asia, which is likely the result of relatively recent long-distance dispersal. Cuscuta alata from Mexico, previously considered synonymous to C. applanata, was found to be a distinct species based on both molecular and morphological evidence. Cuscuta potosina var. potosina and var. globifera segregate into two different subclades of the C. chinensis species complex. Because these two taxa are also different morphologically, var. globifera is described as a new species, C. azteca. Cuscuta campestris, an almost cosmopolitan weed species from another clade ("Clade B"; "C. pentagona complex"), is commonly mis-identified in Asia as C. chinensis, which raises questions about the identity of the Cuscuta plants used in widely commercialized medicinal herbal mixtures. A taxonomic treatment with an identification key, descriptions, and illustrations is provided.