Comparative analysis of chloroplast genomes of Sanguisorba species and insights into phylogenetic implications and molecular dating

Sanguisorba is a small genus which consists of about 15 species widely distributed in North America and Eurasia. Previous studies on Sanguisorba are useful for understanding its phylogeny and chloroplast (cp) genome evolution, however, the cp genome resources of Sanguisorba are still limited, and the phylogeny and molecular dating for Sanguisorba and its relatives still need to be further explored. Here, we report four cp genomes of Sanguisorba and conduct comparative analysis of these four Sanguisorba cp genomes plus five previously published ones. The nine cp genomes have a typical quadripartite structure with a total length of 154 282-155 730 bp and their gene content, gene structure and gene order are relatively conserved. Analysis of single copy (SC)/inverted repeat (IR) boundaries shows only very slight boundary differences among the Sanguisorba cp genomes. Eight variation hotspots were screened as excellent candidate markers for future studies in Sanguisorba. Phylogenetic analysis indicated that Sanguisorba is monophyletic and a member of Agrimonieae subtribe Sanguisorbinae. Within Sanguisorba, S. filiformis is the sister group of the six other taxa in the present sampling. Estimation of divergence times indicates that the subtribes Agrimoniinae and Sanguisorbinae diverged at the transition between the Oligocene and the Miocene, and divergent times of Agrimonieae genera ranged from the late Miocene to the Middle Pleistocene. This study enriches the available cp genome resources of Sanguisorba, and it is of great significance to further study of the phylogeny and evolution of Sanguisorba and its relatives.