Phylogenetic affinities of Myrioneuron and Cyanoneuron, generic limits of the tribe Argostemmateae and description of a new Asian tribe, Cyanoneuroneae (Rubiaceae)

Argostemmateae (Rubiaceae, subfam. Rubioideae) are a mostly tropical Asian group of about 200 species currently classified in four morphologically distinct genera (Argostemma, Mouretia, Mycetia, Neohymenopogon). The monophyly of the tribe and Mycetia is strongly supported by molecular data, however, the tropical Asian genus Myrioneuron, traditionally associated with Mycetia based on its berry fruits, has not previously been investigated. The Bornean and Sulawesian genus Cyanoneuron, described based on the species of Myrioneuron with drupaceous fruits, had not been sequenced. Therefore, the phylogenetic positions of Cyanoneuron and Myrioneuron within Rubiaceae and their generic status have yet to be assessed with molecular data. These genera have tentatively been placed in tribe Spermacoceae (Rubioideae). We reconstructed a robust phylogeny of Rubioideae with sequence data from five plastid regions of 176 accessions and using the Bayesian Markov chain Monte Carlo and parsimony methods. Once the positions of Cyanoneuron and Myrioneuron were revealed, a robust phylogeny of the Spermacoceae alliance was reconstructed with the combined plastid and nuclear data (nrETS, nrITS) from 61 accessions to reassess its tribal limits. Mycetia and Myrioneuron are non-monophyletic and intermixed, and formed a well-supported clade diagnosed by berry fruits. We formally transfer Myrioneuron to Mycetia (older name), and present nine new combinations in the latter genus. Cyanoneuron was resolved with high support as monophyletic, and appears to be closely related to the Chinese monogeneric tribes Foonchewieae and Dunnieae. A new tribe Cyanoneuroneae is described to accommodate Cyanoneuron. This tribe is morphologically distinct from related tribes by its stipules apically divided into multiple linear segments, condensed-cymose inflorescences and drupe-like fruits with numerous small seeds. A new key to the genera of Argostemmateae is provided.