Towards a monophyletic Cynoglossum: a dated molecular phylogeny and historical biogeography of Cynoglossinae (Boraginaceae)

Cynoglossoideae is the largest subfamily of Boraginaceae, and Cynoglosseae is its largest tribe, with the eponymous subtribe Cynoglossinae (ca. 200 spp.) being the most taxonomically challenging group. Cynoglossum remains problematic, especially regarding its relationships to a range of satellite genera (e.g. Paracaryum, Lindelofia, Mattiastrum, Rindera). We aim to elucidate the evolutionary relationships among Cynoglossinae and their historical biogeography by analyzing ca. 80 species of the subtribe. We employed PacBio sequencing of four plastid markers (trnL–trnF, rps16, rpl16, trnK–psbA) and one nuclear marker (nrITS), and reconstructed molecular phylogenies utilizing maximum parsimony, maximum likelihood, and Bayesian inference. Our analyses demonstrated a more highly resolved tree for the major clades of Cynoglossinae, strongly supporting a broader taxonomic circumscription of Cynoglossum. Our biogeographic reconstruction suggests that Cynoglossinae diverged from its sister group, Bothriosperminae, during the late Oligocene in East Asia and likely originated during the middle Miocene in an extensive area encompassing East Asia and the Irano-Turanian region. The lineage rapidly diversified, expanding its range towards Europe, including the Mediterranean region, and Africa, as well as southward to Australia from the middle Miocene to Pleistocene. The data indicate that the epichorous fruits of Cynoglossum have significantly contributed to multiple long-distance dispersal events and subsequent diversification.