Plastome phylogeny and lineage diversification of Salicaceae with focus on poplars and willows

Phylogenetic relationships and lineage diversification of the family Salicaceae sensu lato (s.l.) remain poorly understood. In this study, we examined phylogenetic relationships between 42 species from six genera based on the complete plastomes. Phylogenetic analyses of 77 protein coding genes of the plastomes produced good resolution of the interrelationships among most sampled species and the recovered clades. Of the sampled genera from the family, Flacourtia was identified as the most basal and the successive clades comprised both Itoa and Poliothyrsis, Idesia, two genera of the Salicaceae sensu stricto (s.s.) (Populus and Salix). Five major subclades were recovered within the Populus clade. These subclades and their interrelationships are largely inconsistent with morphological classifications and molecular phylogeny based on nuclear internal transcribed spacer sequence variations. Two major subclades were identified for the Salix clade. Molecular dating suggested that species diversification of the major subclades in the Populus and Salix clades occurred mainly within the recent Pliocene. In addition, we found that the rpl32 gene was lost and the rps7 gene evolved into a pseudogene multiple times in the sampled genera of the Salicaceae s.l. Compared with previous studies, our results provide a well-resolved phylogeny from the perspective of the plastomes.