Phylogeny and biogeography of Pachygoneae (Menispermaceae), with consideration of the boreotropical flora hypothesis and resurrection of the genera Cebatha and Nephroia.

The tribe Pachygoneae consists of four genera with about 40 species, primarily distributed in tropical and subtropical Asia and America, also in Australasia and Africa. This tribe presents an ideal model to investigate the origin of the tropical and subtropical amphi-Pacific disjunction pattern. More specifically, it allows us to test whether the tropical lineages diverged earlier than the subtropical ones during the fragmentation of the boreotropical flora. In this study, we reconstructed the phylogeny of Pachygoneae using five plastid (rbcL, atpB, matK, ndhF, trnL-F) and one nuclear (26S rDNA) DNA regions. Our results indicate that Pachygoneae is not monophyletic unless Cocculus pendulus and Cocculus balfourii are excluded. We resurrected the genus Cebatha to include these two species and established a new tribe for this genus. Within Pachygoneae, the species of Cocculus are distributed in three different clades, among which two are recognized as two distinct genera, Cocculus s.str. and Nephroia resurrected, and one species is transferred into Pachygone. Our molecular dating and ancestral area reconstruction analyses suggest that Pachygoneae began to diversify in tropical Asia around the early-middle Eocene boundary (c. 48 Ma) and expanded into the New World by c. 44 Ma. In the New World, tropical Hyperbaena originated in the late Eocene (c. 40 Ma), whereas the subtropical Cocculus carolinus and Cocculus diversifolius originated later, in the early Oligocene (c. 32  Ma). These two timings correspond with the two climatic cooling intervals, which suggests that the formation and breakup of the boreotropical floral may have been responsible for the amphi-Pacific disjunct distribution within Pachygoneae. One overland migration event from Asia into Australasia appears to have occurred in the early to late Miocene.