Draining the Swamp Dodders: Systematics of Cuscuta gronovii s.l. (Subgenus Grammica, Sect. Oxycarpae)

Abstract Cuscuta gronovii species complex (sensu lato, s.l.) is a North American group of species that belong to C. sect. Oxycarpae of subgenus Grammica. Members of C. gronovii s.l. are often associated with riparian or wet habitats, and their delimitation has been problematic historically. DNA sequences from multiple plastid and nuclear ribosomal non-coding regions were used to reconstruct evolutionary relationships among taxa. Molecular phylogeny supported the resurrection of C. saururi, a taxon that has been treated either as C. gronovii var. latiflora or a nomenclatural synonym of C. gronovii var. gronovii. Cuscuta saururi was inferred to be sister to C. cephalanthi, but the relationships among the rest of taxa, although forming a clade with moderate support, remained largely unresolved. Thus, the species limits of C. gronovii s.l. aggregate were also comprehensively investigated through a morphometric study. Plants with 4-merous flowers from New Brunswick and Nova Scotia, Canada, which were previously thought to be C. cephalanthi, were described as a new species, C. acadiana. Cuscuta acadiana, although fertile and producing abundant seeds, is unusual in that the majority of its pollen grains intergrade from zonocolpate, syncolpate, or pantocolpate, with variously shaped or twisted ectoapertures. Cuscuta gronovii var. calyptrata, a widely accepted but virtually unknown taxon, was typified and included in all the analyses. Cuscuta umbrosa was treated as a variety of C. gronovii (var. curta), and C. rostrata was maintained as a species. An identification key, detailed descriptions, illustrations, and a summary of geographical distribution, ecology, and conservation status were provided for all the members of C. gronovii s.l. The only taxon of this species complex found to be weedy, especially in cranberries, was C. gronovii var. gronovii.