Abstract

New Caledonia is a remote archipelago of the South-West Pacific, whose flora is rich, distinctive, and disharmonic. The interest of botanists has long been attracted by the spatio-temporal origin of this flora, but little attention has been paid to the modes of colonization and the diversification processes that have led to the archipelago’s modern flora. To date, no explosive plant radiation has yet been highlighted for New Caledonia. A dated phylogenetic framework on the second richest New Caledonian genus – Psychotria s.l. and its allied genera (tribes Psychotrieae and Palicoureeae, Rubiaceae; ca. 85 species) – is provided in this study to explore its patterns of colonization and diversification in the archipelago. This study is based on a comprehensive species sampling, two nuclear and four plastid loci. Results show that New Caledonia was colonized four times by Psychotria and its allied genera during the Neogene long after its mid-Eocene re-emergence from the sea. The Pacific clade of Psychotrieae, one of the largest plant diversifications in the Pacific islands and the Indo-Pacific region, is absent from New Caledonia, possibly due to niche competition. Although the four lineages colonized New Caledonia relatively simultaneously during the Neogene, they express different evolutionary histories, as revealed by unevenness in species richness and net diversification rates. The genus Geophila has not diversified on New Caledonia, as a non-endemic single species has been documented in the archipelago. The genus Margaritopsis had a moderate level of diversification (four species) similar to that on other Pacific islands. The Psychotria clade NC1 appears to be a relictual lineage, which probably underwent a drastic extinction, with a narrow ecological habitat and dispersal limitations. The Psychotria clade NC2 is the largest and youngest New Caledonian plant radiation, and has undergone the fastest recorded diversification of any endemic lineage in the archipelago, and could be the result of a ‘non-adaptive radiation’, originating from Australian rainforests.

In the past, the circumscription of the large genus Psychotria (Rubiaceae) was difficult, until molecular phylogenetic studies revealed its considerable paraphyly, enabling the delimitation of its major lineages and the grouping of related genera, and most notably the separation of Psychotria and its relatives (former Psychotrieae) into two tribes: Psychotrieae and Palicoureeae. The genus Margaritopsis, which is included in Palicoureeae, encompasses 27 Neotropical species, and in previous studies these have been shown to be close relatives of a group of eight Psychotria species that occur over a large region extending from South‐East Asia to tropical South Pacific through Malesia (= the AMP region, defined as including South‐East Asia, Malesia, tropical Australia, Melanesia, Micronesia, New Caledonia and Polynesia). A molecular phylogenetic study, using one nuclear DNA region (ITS) and four plastid DNA regions (ndhF, rps16, trnH‐psbA, trnT‐F), is undertaken in order to test the placement of 17 AMP Psychotria species within Palicoureeae. The phylogenetic results show that they form a monophyletic clade (= clade G), which also includes the monotypic Fijian genus Readea and Hodgkinsonia frutescens from Australia. Clade G is embedded in a well‐supported grade with five Neotropical representatives of Margaritopsis. A morphological survey based on twenty characters, with the potential to circumscribe generic entities, shows that the monophyly of clade G is supported by a character combination that is similar to species of Neotropical Margaritopsis, confirming their inclusion in this genus. Taxonomic and nomenclatural work on these species is required to formalize nomenclatural implications. Based on the same set of morphological characters, but in absence of molecular data, 28 other Psychotria species from the AMP region were detected as likely candidates for inclusion in Margaritopsis, allowing the estimation of species richness of clade G to be a minimum of 47 species. Within clade G, five well‐supported subclades and a Readea lineage are delimited and each of these is generally supported by unique morphological features. Phylogenetic topologies reveal biogeographical patterns, including a main route of dispersal from western to eastern parts of the AMP region, with subsequent dispersals between archipelagos in the region.