Molecular phylogeny of Galium L. of the tribe Rubieae (Rubiaceae) – Emphasis on Chinese species and recognition of a new genus Pseudogalium

Abstract

Galium L. is the largest genus in the tribe Rubieae, with about 667 species distributed worldwide. Previous researches mainly focused on species from the Americas and Europe. In the present paper, we greatly increased the number of samples examined from eastern Asia (especially China), representing the most comprehensive sampling of Galium to date. A total of 194 species and variations (subspecies) of Galium were sampled to determine phylogenetic relationships, using two nuclear and five chloroplast markers. Our data are largely consistent with all previous phylogenetic results and confirmed that Galium is non-monophyletic, as are most of its sections. Most members of Galium, including the Chinese taxa, fall into three large clades mixed with other genera from the Galium s.l. group; the exception being the distinct Galium paradoxum Maxim., the first diverged lineage in the Galium s.l. group, which was treated as a new genus (Pseudogalium L.-E. Yang, Z.-L. Nie & H. Sun, gen. nov.). The Galium s.s is a well-supported clade comprised entirely of Galium species, usually with six or more leaves per whorl, mostly from the Old World. Samples from G. maximowiczii (Kom.) Pobed, G. sect. Depauperata and sect. Aparinoides, together with a few from Asperula sect. Glabella and Microphysa (Schrenk ex Fisch. & C.A. Mey.) Pobed., form the second clade. The third clade comprises taxa purely from Galium that usually have four leaves per whorl, from both the New and Old World. Our results also indicated that the monotypic genus Microphysa should be retained and clarified phylogenetic relationships of some specific confused taxa from China. Unlike prior inferences, the combination of opposite leaves associated with two stipules is proposed as the ancestral characteristic of the Galium s.l. group and even the tribe. In addition, the shapes of different corolla and inflorescence types are important for distinguishing some taxa within Rubieae.