Taxonomy, comparative genomics of Mullein (Verbascum, Scrophulariaceae), with implications for the evolution of Verbascum and Lamiales

Abstract

Background: The genus Verbascum L. (Scrophulariaceae) is distributed in Africa, Europe, and parts of Asia, with the Mediterranean having the most species variety. Several researchers have already worked on the phylogenetic and taxonomic analysis of Verbascum by using ITS data and chloroplast genome fragments and have produced different conclusions. The taxonomy and phylogenetic relationships of this genus are unclear. Results: The complete plastomes (cp) lengths for V. chaixii, V. songaricum, V. phoeniceum, V. blattaria, V. sinaiticum, V. thapsus, and V. brevipedicellatum ranged from 153,014 to 153,481 bp. The cp coded 114 unique genes comprising of 80 protein-coding genes, four ribosomal RNA (rRNA), and 30 tRNA genes. We detected variations in the repeat structures, gene expansion on the inverted repeat, and single copy (IR/SC) boundary regions. The substitution rate analysis indicated that some genes were under purifying selection pressure. Phylogenetic analysis supported the sister relationship of (Lentibulariaceae + Acanthaceae + Bignoniaceae + Verbenaceae + Pedaliaceae) and (Lamiaceae + Phyrymaceae + Orobanchaceae + Paulowniaceae + Mazaceae) in Lamiales. Within Scrophulariaceae, Verbascum was sister to Scrophularia, while Buddleja formed a monophyletic clade from (Scrophularia + Verbascum) with high bootstrap support values. The relationship of the nine species within Verbascum was highly supported. Conclusion: Based on the phylogenetic results, we proposed to reinstate the species status of V. brevipedicellatum (Engl.) Hub.-Mor. Additionally, three genera (Mazus, Lancea, and Dodartia) placed in the Phyrymaceae family formed a separate clade within Lamiaceae. The classification of the three genera was supported by previous studies. Thus, the current study also suggests the circumscription of these genera as documented previously to be reinstated. The divergence time of Lamiales was approximated to be 86.28 million years ago (Ma) (95% highest posterior density (HPD), 85.12–89.91 Ma). The complete plastomes sequence data of the Verbascum species will be important for understanding the Verbascum phylogenetic relationships and evolution in order Lamiales.