Cuscuta californica complex (sensu lato, s.l.) is a western North American group of species in which the infrastaminal scales are reduced, making the morphological delimitation of species particularly challenging. A revision of this group was prompted by the discovery of an apparent new species from central California based primarily on molecular means. Prompted by this finding, the morphological limits of C. californica s.l. species were comprehensively re-evaluated through a morphometric study. DNA sequences from plastid (trnL–F region and rbcL), nuclear ribosomal ITS and 26S rDNA, as well as a low-copy nuclear pentatricopeptide repeat (PPR) gene were used to reconstruct evolutionary relationships among taxa. Last but not least, the host range of relevant taxa was determined using herbarium specimens. Molecular results strongly supported the new species, C. difficilis, which was found to be morphologically separated from C. brachycalyx only by subtle calyx lobe and corolla tube shape differences. Despite sharing some of the hosts, all the members of C. californica s.l. exhibited a differentiation of their host ranges. An identification key and description of the new species were provided together with a discussion on the systematics and host range of C. californica s.l.. Hybridization, accompanied by plastid capture, was suggested as a possible mechanism of speciation for C. brachycalyx.

Please see "materials and methods" within the paper.