Molecular analyses of the disjunctly distributed Osmunda regalis and O. japonica (Osmundaceae), with particular reference to introgression and hybridization

Abstract

Osmunda regalis and O. japonica are geographically isolated independent species. Contrary to this morphologically based taxonomy, previous plastid DNA data have shown that O. regalis is paraphyletic, with the American O. regalis sister to O. japonica and O. lancea. It remains to be confirmed whether this paraphyletic topology results from hybridization, budding speciation, or classification issues. To clarify the evolutionary history of O. regalis and O. japonica, we performed multiple molecular phylogenetic analyses using 38 worldwide samples and sequences from 15 nuclear and six plastid DNA regions. The phylogenetic trees of the nuclear and plastid single-region analyses were incongruent. Phylogenetic networks were also discordant between the nuclear and plastid regions. Statistical analysis for detecting the introgressed sequences found hybridization signals between the American O. regalis and O. japonica lineage, particularly in plastid regions. Along with the paleobotanical data, the results suggest that the ancient introgression occurred at an estimated time of 12.8–4.1 Ma in East Asia, when and where both species coexisted. Post-hybridization geographical fluctuation led to the present amphi-Pacific distribution pattern. The phylogenetic network suggests that the New World O. regalis spread across continents in a relatively short time, while the Old World O. regalis experienced long-term geographical isolation between (sub)continents.