To lump or to split? Revision of Cuscuta section Indecorae using a combined morphometric, phylogenetic and host range approach

Cuscuta section Indecorae is an infrageneric clade of subgenus Grammica that originated in N 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: indecora, longisepala, and 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 var. longisepala was also growing on Iva annua, which together with the morphological and genetical similarity, indicates that var. attenuata is identical to 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.

Methods
1) Section_sect_Indecorae_MORPHOMETRIC-matrix. A total of 47 morphological characters were measured/scored using rehydrated plant material from 188 herbarium specimens. A total of 47 morphological characters were measured/scored (see article and Readme file). Empty cells in the Excel spreadsheet represent missing data for that particular trait and specimen. The extent of morphological variation was visualized with both principal coordinates analysis (PCoA or multidimensional scaling) and unweighted pair-group average (UPGMA), using the Gower’s coefficient of similarity.

2). Cuscuta-sect-indecorae-host-matrix. 964 herbarium specimens for which the host plants could be identified at species, genus, or family level were used to calculate a HFI index at these three taxonomic levels. The HFI shows the frequency (percentage) of a Cuscuta taxon to be encountered on a particular host (at family, genus, and species level), in relation to the total number of host occurrences. HFI data were then mapped into a directed bipartite network using Cytoscape to visualize host range and frequency of occurrence across section.