Multilocus phylogenetic reconstruction informing polyploid relationships of Aconitum subgenus Lycoctonum (Ranunculaceae) in China

Polyploidization has long been recognized as one of the most important driving forces of plant evolution. Aconitum subgenus Lycoctonum (Ranunculaceae) has a wide distribution range and well-known background of polyploidy, thereby providing a potentially valuable model to explore polyploid origin and evolutionary history. However, the phylogeny of subg. Lycoctonum has not yet been completely resolved. In the current study, 29 species including diploid, tetraploid and hexaploid species were sampled in subg. Lycoctonum. Using four cpDNA regions (ndhF-trnL, psbA-trnH, psbD-trnT and trnT-L) and two nrDNA regions (internal transcribed spacer, ITS, and external transcribed spacer, ETS), phylogenetic relationship was first reconstructed for the polyploid species within subg. Lycoctonum. In combination with nuclear diversification rate estimation, cpDNA haplotype network, ancestral area reconstruction as well as morphological and karyotypic evidence, potential origin and divergence time were further assessed among the polyploid species. Hybridization was inferred for A. angustius and A. fine-tianum was suggested to be the potential maternal progenitor, due to their close phylogenetic relationship, highly similar morphologies and overlapping distribution range. Local origin was inferred for tetraploids in the Hengduan Mountains (HDM) with eight groups of chromosomes of four homeologous, which diverged approximately 3.00 Ma in the same period of the orogeny of the HDM. The hexaploid A. apetalum was inferred to suffer from geographical isolation due to the formation of the Qinghai-Tibetan Plateau (QTP) and the HDM. Hybridization and heterogeneous habitats in the HDM were suggested to play an important role in the polyploidization in subg. Lycoctonum.