Phylogenetic relationships, possible ancient hybridization, and biogeographic history of Abies (Pinaceae) based on data from nuclear, plastid, and mitochondrial genomes

Abies, the second largest genus of Pinaceae, consists of approximately 48 species occurring in the north temperate region. Previous molecular phylogenetic studies improved our understanding of relationships within the genus, but were limited by relying on only DNA sequence data from single genome and low taxonomic sampling. Here we use DNA data from three genomes (sequences of internal transcribed spacer of nrITS, three chloroplast DNA intergenic spacers, and two mitochondrial intergenic spacers) from 42 species to elucidate species relationships and construct the biogeographic history of Abies. We further estimated the divergence times of intercontinental disjunction using a relaxed molecular clock calibrated with three macro-fossils. Our phylogenetic analyses recovered six robust clades largely consistent with previous classifications of sections. A sister relationship between the eastern Asian and Europe-Mediterranean clades was highly supported. The monophyly of section Balsamea, disjunct in Far East and western North America, is supported by the nrITS data but not by the cpDNA data. Discordance on placement of section Balsamea between the paternally inherited cpDNA and maternally inherited mtDNA trees was also observed. The data suggested that ancient hybridization was likely involved in the origin of sect. Balsamea. Results from biogeographic analyses and divergence time estimation suggested an origin and early diversification of Abies in an area of high latitude around the Pacific during the Eocene. The present disjunction in eastern Asia and Europe-Mediterranean area of Abies was likely the result of southward migration and isolation by the Turgai Strait in the Late Eocene. An 'out-of-America' migration, for the origin of an eastern Asian and western North American disjunct species pairs in section Amabilis was supported. The results suggested a western North American origin of the section with subsequent dispersal across the Bering Land Bridge (BLB) to Japan during the Middle Miocene.