Phylogenomic conflict analyses in the apple genus Malus s.l. reveal widespread hybridization and allopolyploidy driving diversification, with insights into the complex biogeographic history in the Northern Hemisphere

Phylogenomic evidence from an increasing number of studies has demonstrated that different data sets and analytical approaches often reconstruct strongly supported but conflicting relationships. In this study, 785 single-copy nuclear genes and 75 complete plastomes were used to infer the phylogenetic relationships and estimate the historical biogeography of the apple genus Malus sensu lato, an economically important lineage disjunctly distributed in the Northern Hemisphere and involved in known and suspected hybridization and allopolyploidy events. The nuclear phylogeny recovered the monophyly of Malus s.l. (including Docynia); however, the genus was supported to be biphyletic in the plastid phylogeny. An ancient chloroplast capture event in the Eocene in western North America best explains the cytonuclear discordance. Our conflict analysis demonstrated that ILS, hybridization, and allopolyploidy could explain the widespread nuclear gene tree discordance. One deep hybridization event (Malus doumeri) and one recent event (Malus coronaria) were detected in Malus s.l. Furthermore, our historical biogeographic analysis integrating living and fossil data supported a widespread East Asian-western North American origin of Malus s.l. in the Eocene, followed by several extinction and dispersal events in the Northern Hemisphere. We also propose a general workflow for assessing phylogenomic discordance and biogeographic analysis using deep genome skimming data sets.