Comparative genomics of figworts (Scrophularia, Scrophulariaceae), with implications for the evolution of Scrophularia and Lamiales

The figwort genus Scrophularia L. (Scrophulariaceae) comprises 200-300 species and is widespread throughout the temperate Northern Hemisphere. Due to reticulate evolution resulting from hybridization and polyploidization, the taxonomy and phylogeny of Scrophularia is notoriously challenging. Here we report the complete chloroplast (cp) genome sequences of S. henryi Hemsl. and S. dentata Royle ex Benth. and compare them with those of S. takesimensis Nakai and S. buergeriana Miq. The Scrophularia cp genomes ranged from 152 425 to 153 631 bp in length. Each cp genome contained 113 unigenes, consisting of 78 protein-coding genes, 31 transfer RNA genes, and 4 ribosomal RNA genes. Gene order, gene content, AT content and IR/SC boundary structure were nearly identical among them. Nine cpDNA markers (trnH-psbA, rps15, rps18-rpl20, rpl32-trnL, trnS-trnG, ycf15-trnL, rps4-trnT, ndhF-rpl32, and rps16-trnQ) with more than 2% variable sites were identified. Our phylogenetic analyses including 55 genera from Lamiales strongly supported a sister relationship between ((Bignoniaceae + Verbenaceae) + Pedaliaceae) and (Acanthaceae + Lentibulariaceae). Within Scrophulariaceae, a topology of (S. dentata + (S. takesimensis + (S. buergeriana + S. henryi))) was strongly supported. The crown age of Lamiales was estimated to be 85.1 Ma (95% highest posterior density, 70.6-99.8 Ma). The higher core Lamiales originated at 65.6 Ma (95% highest posterior density, 51.4-79.4 Ma), with a subsequent radiation that occurred in the Paleocene (between 55.4 and 62.3 Ma) and gave birth to the diversified families. Our study provides a robust phylogeny and a temporal framework for further investigation of the evolution of Lamiales.