Complex patterns of reticulate evolution in opportunistic weeds (Potentilla L., Rosaceae), as revealed by low-copy nuclear markers

BACKGROUND: Most cinquefoils (Potentilla L., Rosaceae) are polyploids, ranging from tetraploid (4x) to dodecaploid (12x), diploids being a rare exception. Previous studies based on ribosomal and chloroplast data indicated that Norwegian cinquefoil (P. norvegica L.) has genetic material from two separate clades within Potentilla; the Argentea and the Ivesioid clades – and thus a possible history of hybridization and polyploidization (allopolyploidy). In order to trace the putative allopolyploid origin of the species, sequence data from low-copy, biparentally inherited, nuclear markers were used. Specimens covering the circumpolar distribution of P. norvegica and its two subspecies were included, along with the morphologically similar P. intermedia. Potentilla species of low ploidy level known to belong to other relevant clades were also included. RESULTS: Gene trees based on three low-copy nuclear markers, obtained by Bayesian Inference and Maximum Likelihood analyses, showed slightly different topologies. This is likely due to genomic reorganizations following genome duplication, but the gene trees were not in conflict with a species tree of presumably diploid taxa obtained by Multispecies Coalescent analysis. The results show that both P. norvegica and P. intermedia are allopolyploids with a shared evolutionary history involving at least four parental lineages, three from the Argentea clade and one from the Ivesioid clade. CONCLUSIONS: This is the first time that reticulate evolution has been proven in the genus Potentilla, and shows the importance of continuing working with low-copy markers in order to properly resolve its evolutionary history. Several hybridization events between the Argentea and Ivesioid clades may have given rise to the species of Wolf’s grex Rivales. To better estimate when and where these hybridizations occurred, other Argentea, Ivesioid and Rivales species should be included in future studies.