Cryptic adaptive radiation in tropical forest trees in New Caledonia.

The causes of the species richness of tropical trees are poorly understood, in particular the roles of ecological factors such as soil composition. The nickel(Ni)-hyperaccumulating tree genus Geissois (Cunoniaceae) from the South-west Pacific was chosen as a model of diversification on different substrates. Here, we investigated the leaf element compositions, spatial distributions and phylogeny of all species of Geissois occurring on New Caledonia. We found that New Caledonian Geissois descended from a single colonist and diversified relatively quickly into 13 species. Species on ultramafic and nonultramafic substrates showed contrasting patterns of leaf element composition and range overlap. Those on nonultramafic substrates were largely sympatric but had distinct leaf element compositions. By contrast, species on ultramafic substrates showed similar leaf element composition, but occurred in many cases exclusively in allopatry. Further, earlier work showed that at least three out of these seven species use different molecules to bind Ni. Geissois qualifies as a cryptic adaptive radiation, and may be the first such example in a lineage of tropical forest trees. Variation in biochemical strategies for coping with both typical and adverse soil conditions may help to explain the diversification and coexistence of tropical forest trees on similar soil types.

New Caledonian lineages of Psychotria (Rubiaceae) reveal different evolutionary histories and the largest documented plant radiation for the archipelago.

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.

Nickel resistance determinants in bradyrhizobium strains from nodules of the endemic New Caledonia legume Serianthes calycina.

Bradyrhizobium strains, isolated in New Caledonia from nodules of the endemic legume Serianthes calycina growing in nickel-rich soils, were able to grow in the presence of 15 mM NiCl2. The genomes of these strains harbored two Ni resistance determinants, the cnr and nre operons. By constructing a cnrA mutant, we demonstrated that the cnr operon determines the high nickel resistance in Bradyrhizobium strains.