Whole Genome based Insights into the Phylogeny and Evolution of the Juglandaceae
Abstract Background: The walnut family (Juglandaceae) contains commercially important woody trees commonly called walnut, wingnut, pecan and hickory. Phylogenetic relationships in the Juglandaceae are problematic, and their historical diversification has not been clarified, in part because of low phylogenetic resolution and/or insufficient marker variability. Results: We reconstructed the backbone phylogenetic relationships of Juglandaceae using organelle and nuclear genome data from 27 species. The divergence time of Juglandaceae was estimated to be 78.7 Mya. The major lineages diversified in warm and dry habitats during the mid-Paleocene and early Eocene. The plastid, mitochondrial, and nuclear phylogenetic analyses all revealed three subfamilies, i.e., Juglandoideae, Engelhardioideae, Rhoipteleoideae. Five genera of Juglandoideae were strongly supported. Juglandaceae were estimated to have originated during the late Cretaceous, while Juglandoideae were estimated to have originated during the Paleocene, with evidence for rapid diversification events during several glacial and geological periods. The phylogenetic analyses of organelle sequences and nuclear genome yielded highly supported incongruence positions for J. cinerea, J. hopeiensis, and Platycarya strobilacea. Winged fruit were the ancestral condition in the Juglandoideae, but adaptation to novel regeneration regimes after the Creaceous-Paleogene boundary led to the independent evolution of zoochory among several genera of the Juglandaceae. Conclusions: A fully resolved, strongly supported, time-calibrated phylogenetic tree of Juglandaceae can provide an important framework for studying classification, diversification, biogeography, and comparative genomics of plant lineages.