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.

Phylogenies recurrently demonstrate that oceanic island systems have been home to rapid clade diversification and adaptive radiations. The existence of adaptive radiations posits a central role of natural selection causing ecological divergence and speciation, and some plant radiations have been highlighted as paradigmatic examples of such radiations. However, neutral processes may also drive speciation during clade radiations, with ecological divergence occurring following speciation. Here, we document an exceptionally rapid and unique radiation of Lamiaceae within the New Caledonian biodiversity hotspot. Specifically, we investigated various biological, ecological, and geographical drivers of species diversification within the genus Oxera . We found that Oxera underwent an initial process of rapid cladogenesis likely triggered by a dramatic period of aridity during the early Pliocene. This early diversification of Oxera was associated with an important phase of ecological diversification triggered by significant shifts of pollination syndromes, dispersal modes, and life forms. Finally, recent diversification of Oxera appears to have been further driven by the interplay of allopatry and habitat shifts likely related to climatic oscillations. This suggests that Oxera could be regarded as an adaptive radiation at an early evolutionary stage that has been obscured by more recent joint habitat diversification and neutral geographical processes. Diversification within Oxera has perhaps been triggered by varied ecological and biological drivers acting in a leapfrog pattern, but geographic processes may have been an equally important driver. We suspect that strictly adaptive radiations may be rare in plants and that most events of rapid clade diversification may have involved a mixture of geographical and ecological divergence.