Natural Hybrid Origin of the Controversial "Species" Clematis pinnata (Ranunculaceae) Based on Multidisciplinary Evidence

Hybridization has been proved to be one of the driving forces of plant evolution and speciation. In this study, we tested the homoploid hybrid origin of Clematis pinnata, a taxonomically controversial species from northern China, based on morphological statistics, flow cytometry, niche modelling, and phylogenomic analysis. The morphologic analysis showed intermediacy of C. pinnata between the putative parents, C. brevicaudata and C. heracleifolia or C. tubulosa. The homoploidy among all the tested species was confirmed by flow cytometry analysis. Niche modelling demonstrated that C. pinnata has not been adapted to novel ecological niche which is independent with its putative parents. Plastome phylogeny indicated that plants of C. pinnata in different populations originated by different hybridization events. Phylonet and HyDe analyses based on transcriptome data verified the hybrid origin of C. pinnata from C. brevicaudata with C. heracleifolia or C. tubulosa, and genetic contribution of the parents are almost same. Introgression between C. brevicaudata and C. heracleifolia or C. tubulosa was also detected in this study. All the evidence showed that C. pinnata is probably a F1 of its parental species. The present study also indicated that interspecific hybridization is a common mechanism to generate diversity and variation of Clematis, and may play an important role in evolution and diversification of this predominantly diploid genus.