To Lump or to Split? Revision of <i>Cuscuta</i> section <i>Indecorae</i> Using a Combined Morphometric, Phylogenetic, and Host Range Approach

Abstract— Cuscuta section Indecorae is an infrageneric clade of Cuscuta subgenus Grammica that originated in North America which includes three species: C. indecora , C. coryli , and C. warneri . Some forms of C. indecora are agricultural weeds, while C. warneri and C. coryli face conservation problems regionally or globally. Cuscuta indecora is a morphologically diverse species with three recognized varieties: C. indecora var. indecora , C. indecora var. longisepala , and C. indecora var. attenuata . Two recent broad-scale molecular phylogenetic studies raised questions about the circumscription of C. indecora and the validity of its infraspecific taxa. Therefore, we conducted an extensive morphometric study, and used DNA sequences from plastid and nuclear ribosomal non-coding regions to reconstruct evolutionary relationships among taxa. In addition, we studied the host range of all taxa using herbarium specimens, bipartite networks, and Venn diagrams. Results from all types of evidence confirmed the circumscription of the three species, but differed on the infraspecific structure of C. indecora . The molecular phylogeny inferred C. indecora as paraphyletic, consisting of two distinct clades, with C. coryli nested within one of them. The morphometric and host range studies uncovered extensive overlaps both among the traditional varieties of C. indecora as well as between the two molecular lineages of C. indecora . Previously, Iva annua had been considered the sole host of C. indecora var. attenuata ( C. attenuata ) and this presumed host specificity was the main criterion for accepting this taxon. We found that the type of C. indecora var. longisepala was also growing on Iva annua , which together with the morphological and genetical similarity, indicates that C. indecora var. attenuata is identical to C. indecora var. longisepala . Overall, considering the lack of morphological, host range, and geographical patterns distinguishing either the current three varieties of C. indecora or the two cryptic lineages, the most appropriate solution is to tentatively recognize C . indecora as variable species without recognizing any infraspecific taxa.