Entangled evolutionary history of Cuscuta pentagona clade: A story involving hybridization and Darwin in the Galapagos

Abstract

AbstractThe distribution of Cuscuta subg. Grammica sect. Cleistogrammica (Cuscuta pentagona clade) is centered in North America (C. campestris, C. glabrior, C. harperi, C. pentagona, C. obtusiflora, C. plattensis, C. polygonorum, C. runyonii); however, long ‐distance dispersal was documented to Hawaii (C. sandwichiana), South America (C. gymnocarpa, C. stenolepis, and in part C. obtusiflora), Africa (C. bifurcata, C. schlechteri), Eurasia, and Australia (C. australis). Hybrid speciation has already been documented for some members of sect. Cleistogrammica (C. sandwichiana, C. bifurcata) but previous studies strongly suggested that the extent of reticulate evolution is underestimated in Cuscuta generally, and in this section in particular. Sequence data from the nuclear internal transcribed spacer (ITS) and the plastid trnL‐F region were used to reconstruct the phylogeny and gain a better understanding of the evolutionary history within the clade. Additionally, a morphometric analysis was conducted to test the phenetic distinctiveness of a select number of species with taxonomic problems: C. campestris, C. glabrior, C. gymnocarpa, and C. pentagona. Discordances between phylogenies derived from plastid and nuclear data showed that C. campestris is a hybrid, likely involving the C. runyonii / glabrior lineage as a maternal progenitor and an undiscovered species as a paternal progenitor. This latter species, an extinct or unsampled lineage, was itself inferred to be a hybrid between C. pentagona / harperi and C. australis / obtusiflora / polygonorum lineages. Both the evolutionary and morphometric results clearly showed that C. campestris is a distinct species and the negative consequences of its amalgamation with C. pentagona during the last decades are discussed. Cuscuta gymnocarpa, an enigmatic species described from specimens collected by Darwin from the Galapagos, was inferred as conspecific with C. campestris and proposed as a variety of the latter. Because C. gymnocarpa is only a form of C. campestris, the possible means of dispersal of the latter species to the Galapagos are discussed. Cuscuta modesta, a new species discovered while studying the systematics of the clade, is described and illustrated.