Supermatrix analyses reveal the importance of outgroup, gene and taxon sampling in Onosma (Boraginaceae) phylogenetics

Tribe Lithospermeae (Boraginaceae) consists of ca. 26 genera and 470 species, in which Onosma constitutes approximately one third of the species (~150). Although the tribe is strongly supported as monophyletic, both generic and species boundaries remain ambiguous. Among them, not only the phylogenetic position of Eastern Asian Onosma species, but also the taxonomic limits of the genus remain unclear. Whether Eastern Asian Onosma is monophyletic, or the genus should be widened to include Maharanga, and maybe Cystostemon, are still open questions. For these reasons, I performed 16 phylogenetic analyses with different taxon coverages, alignments, gene regions and outgroups, with up to 746 taxa of tribe Lithospermeae and with five DNA regions, using data from GenBank. The results, with the widest taxon coverage to date, show that while genus Onosma is not monophyletic in any of the analyses, the phylogenetic relationships among Onosma s.s., Eastern Asian Onosma, Maharanga and Cystostemon differ among analyses. However, the approximately unbiased (AU) test showed that the topology (((Eastern Asian Onosma+Maharanga) Cystostemon) Onosma s.s.) is overwhelmingly supported. Therefore, the current study highlights the importance of taxon, gene and outgroup sampling in Onosma phylogenetics.