Recircumscription of Oxera (Lamiaceae: Ajugoideae) to include Faradaya based on molecular and anatomical data

Abstract

Oxera is a genus of c. 20 species from New Caledonia and Vanuatu. Its phylogenetic placement in Lamiaceae (Ajugoideae) remained uncertain until studies showed close affinities with the large pantropical genus Clerodendrum, the Indo-Pacific Faradaya (three species) and the monotypic Hosea from Borneo. We reassessed the relationships of Oxera with these genera by building a phylogenetic framework based on six plastid and six nuclear loci and on a sampling including 30 ingroup species. The results show that Clerodendrum and its closely related genera are sister to the alliance encompassing Faradaya, Hosea and Oxera. Hosea is sister to all species of Oxera and Faradaya. Faradaya is recovered as polyphyletic as it is partly nested within Oxera. Extensive morphological variability is observed in Oxera. However, Oxera also shares strong similarities with the three Faradaya species. Polyphyletic Faradaya is therefore placed in synonymy with Oxera and we propose three new combinations: O.amicorum, O.lehuntei and O.splendida. A unique synapomorphy supports this new circumscription of Oxera: the petiole vascularization consisting of several small vascular bundles arranged in an interrupted full circle (vs. a large bundle in a half circle in other ajugoids). Oxera thus comprises c. 37 species, with a diversity centre in New Caledonia. Biogeographical processes, plastid paralogy and vascularization shift are discussed.