Molecular Structure and Phylogenetic Analyses of the Plastomes of Eight Sorbus Sensu Stricto Species

Sorbus L. is a genus of controversy on its taxonomic concept due to great variations in morphological characteristics. Sorbus sensu stricto species, being monophyletic, are characterized as pinnate leaves. However, phylogenetic relationships of these species are somewhat fluid based on morphological characteristics and genetic markers. In this study, the chloroplast (cp) genomes of eight Sorbus s. str. species were characterized and compared with those of twelve released species in this genus. Our results indicated that the plastomes of the twenty Sorbus species had a conserved quadripartite structure, and 129 annotated genes had the same order and showed a good collinearity. Additionally, numerous SSRs were observed in the cp genome of each Sorbus species; most of the sequence variations appeared in non-coding regions, and four intergenic regions were identified as mutation spots. By contrast, protein-coding genes showed low variations under purifying selection. The phylogenetic and molecular-dating analysis showed that Sorbus was resolved into two major clades, in which S. americana forms one clade originating at 51.78 Ma, and the rest of Sorbus formed another clade, splitting at 36.77 Ma into two sister groups with high support values. These results provide a basis for further studying the phylogenetic relationship and biogeography of Sorbus species.