New insights into infrageneric relationships of Lonicera (Caprifoliaceae) as revealed by nuclear ribosomal DNA cistron data and plastid phylogenomics

<p>The discontinuous geographic distribution pattern of plants in the north temperate zone has been a focus of biogeographic research, especially concerning the mechanisms behind the formation of such a pattern and the spatial and temporal evolution of this intermittent distribution pattern. Hypotheses of boreotropical origin, land bridge migration, and out-of-Tibet have been proposed to explain the formation of the discontinuous distribution pattern. The distribution of <em>Lonicera</em> shows a typical Europe-Asia-North America discontinuous distribution, which makes for a good case study to investigate the above three hypotheses. In this study, we inferred the phylogeny based on plastid genomes and a nuclear data set with broad taxon sampling, covering 83 species representing two subgenera and four sections. Both nuclear and plastid phylogenetic analyses found section <em>Isika</em> polyphyletic, while sections <em>Nintooa</em>, <em>Isoxylosteum</em>, and <em>Coelxylosteum</em> were monophyletic in subgenus <em>Chamaecerasus</em>. Based on the nuclear and chloroplast phylogeny, we suggest transferring L. <em>maximowiczii</em> and L. <em>tangutica</em> into section <em>Nintooa</em>. Reconstruction of ancestral areas suggests that <em>Lonicera</em> originated in the Qinghai-Tibetan Plateau (QTP) and/or Asia, and subsequently dispersed to other regions. The aridification of the Asian interior may have facilitated the rapid radiation of <em>Lonicera</em> in the region. At the same time, the uplifts of the Tibetan Plateau appear to have triggered the spread and recent rapid diversification of the genus on the QTP and adjacent areas. Overall, our results deepen the understanding of the evolutionary diversification history of <em>Lonicera</em>.</p>