Phylogenomics reveals patterns ancient hybridization and differential diversification that contribute to phylogenetic conflict in willows, poplars, and close relatives

<b>Abstract</b><br/><p>Despite the economic, ecological, and scientific importance of the genera <em>Salix</em> L. (willows) and <em>Populus</em> L. (poplars, cottonwoods, and aspens) Salicaceae, we know little about the sources of differences in species diversity between the genera and of the phylogenetic conflict that often confounds estimating phylogenetic trees. <em>Salix</em> subgenera and sections, in particular, have been difficult to classify, with one recent attempt termed a ‘spectacular failure’ due to a speculated radiation of the subgenera <em>Vetrix</em> and <em>Chamaetia</em>. Here we use targeted sequence capture to understand the evolutionary history of this portion of the Salicaceae plant family. Our phylogenetic hypothesis was based on 787 gene regions and identified extensive phylogenetic conflict among genes. Our analysis supported some previously described subgeneric relationships and confirmed polyphyly of others. Using an f<sub>branch</sub> analysis we identified several cases of hybridization in deep branches of the phylogeny, which likely contributed to discordance among gene trees. In addition, we identified a rapid increase in diversification rate near the origination of the <em>Vetrix</em>-<em>Chamaetia</em> clade in <em>Salix</em>. This region of the tree coincided with several nodes that lacked strong statistical support, indicating a possible increase in incomplete lineage sorting due to rapid diversification. The extraordinary level of both recent and ancient hybridization in both <em>Salix</em> and <em>Populus</em> have played important roles in the diversification and diversity in these two genera.</p>