Phylogenomic insights into the reticulate evolution of Camellia sect. Paracamellia Sealy (Theaceae)

Polyploids are common in Camellia sect. Paracamellia, which contain many important oil crop species. However, their complex evolutionary history is largely unclear. In this study, 22 transcriptomes and 19 plastomes of related species of Camellia were sequenced and assembled, providing the most completed taxa sampling of Camellia sect. Oleifera and C. sect. Paracamellia. Phylogenetic trees were reconstructed with predicted single-copy nuclear genes and plastomes. Phylogenetic trees with nuclear genes demonstrated that C. sect. Oleifera should be merged into C. sect. Paracamellia. Cytonuclear discordance and network analyses suggested hybridizations among polyploid species and relatives. The divergence of major clades in C. sect. Paracamellia was dated to be during the middle to late Miocene from the ancestral Lingnan region, and a rapid diversification during the Quaternary was found, probably through hybridization and polyploidization. The tetraploid Camellia meiocarpa Hu may have originated from hybridization between closely related diploid species. The hexaploid Camellia oleifera C. Abel probably originated from hybridization between closely related diploid and tetraploid (e.g., C. meiocarpa) species. The octoploid Camellia vietnamensis T. C. Huang ex Hu could have originated from hybridization between hexaploid C. oleifera and the closely related diploid species. Hybridization and polyploidization played an important role in generating the rich variation of important fruit traits, especially increased fruit size in polyploid species.