Ancient hybridization and combinatorial mechanism: drivers of adaptive evolution in Clematis sect. Fruticella (Ranunculaceae).

Hybridization can drive rapid evolution via a 'combinatorial mechanism' - the recombination of existing parental variation - but plant case studies are scarce. The drought-adapted shrubby Clematis sect. Fruticella provides an ideal system to test this model. We integrated phylogenomic data (> 6000 nuclear genes, complete plastomes) with morphological and ecological data for > 60 samples. We developed a robust workflow, including a novel visualization tool and rigorous false-positive filtering, to detect ancient hybridization and trace parental contributions. Our analyses reveal that sect. Fruticella and the related sect. Meclatis are of ancient hybrid origin, from admixture between the arid-adapted Clematis songorica and the mesic Otophora group ancestor. The sections combine key parental adaptive traits, and their distinct ecological adaptations strongly correlate with their differing proportions of this inherited ancestry. Our study provides strong evidence that ancient hybridization, acting through a combinatorial mechanism, was a pivotal driver in the adaptive evolution of sect. Fruticella. This process facilitated the colonization of novel ecological niches and continued to shape the diversification of these hybrid lineages.