Evolutionary dynamics of emblematic Araucaria species (Araucariaceae) in New Caledonia: nuclear and chloroplast markers suggest recent diversification, introgression, and a tight link between genetics and geography within species.

Myriam Gaudeul,Philip Thomas,Martin F Gardner,Richard A Ennos,Pete M Hollingsworth

doi:10.1186/s12862-014-0171-6

Abstract

BackgroundNew Caledonia harbours a highly diverse and endemic flora, and 13 (out of the 19 worldwide) species of Araucaria are endemic to this territory. Their phylogenetic relationships remain largely unresolved. Using nuclear microsatellites and chloroplast DNA sequencing, we focused on five closely related Araucaria species to investigate among-species relationships and the distribution of within-species genetic diversity across New Caledonia.ResultsThe species could be clearly distinguished here, except A. montana and A. laubenfelsii that were not differentiated and, at most, form a genetic cline. Given their apparent morphological and ecological similarity, we suggested that these two species may be considered as a single evolutionary unit. We observed cases of nuclear admixture and incongruence between nuclear and chloroplast data, probably explained by introgression and shared ancestral polymorphism. Ancient hybridization was evidenced between A. biramulata and A. laubenfelsii in Mt Do, and is strongly suspected between A. biramulata and A. rulei in Mt Tonta. In both cases, extensive asymmetrical backcrossing eliminated the influence of one parent in the nuclear DNA composition. Shared ancestral polymorphism was also observed for cpDNA, suggesting that species diverged recently, have large effective sizes and/or that cpDNA experienced slow rates of molecular evolution. Within-species genetic structure was pronounced, probably because of low gene flow and significant inbreeding, and appeared clearly influenced by geography. This may be due to survival in distinct refugia during Quaternary climatic oscillations.ConclusionsThe study species probably diverged recently and/or are characterized by a slow rate of cpDNA sequence evolution, and introgression is strongly suspected. Within-species genetic structure is tightly linked with geography. We underline the conservation implications of our results, and highlight several perspectives.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0171-6) contains supplementary material, which is available to authorized users.

Highlights

New Caledonia harbours a highly diverse and endemic flora, and 13 species of Araucaria are endemic to this territory
We examined the genetic structure of four species that are largely distributed in New Caledonia: A. montana and A. laubenfelsii, A. rulei, and A. scopulorum
The major clusters were: 1) the three A. scopulorum populations from the northwestern coast; 2) the five A. scopulorum populations from the eastern coast; 3) all A. rulei populations; 4) the two northeastern A. montana populations (Mt Panie and TonNon); 5) the Koniambo A. montana population; 6) the remaining six A. montana populations, three A. laubenfelsii populations and the A. biramulata Mt Do population; 7) the three A. biramulata populations Pandop, Kaala Gomen and Mt Tonta and two samples identified as A. montana in Pandop

Summary

Introduction

New Caledonia harbours a highly diverse and endemic flora, and 13 (out of the 19 worldwide) species of Araucaria are endemic to this territory. It is characterized by high endemism (77% for angiosperms and 100% for conifers; [7]) It is of continental Gondwanan origin and geographically isolated (1500 km east of Australia), recent studies have shown that the modern New Caledonian biodiversity largely originates from dispersal events and in situ species radiations [8,9]. 80 Ma; [11]) and congruent with the submersion of the island, most radiations would be relatively recent (< 15Ma; reviewed in [12,13]) In this context, studies at the interface between population genetics and phylogenetics seem best suited to examine how the biodiversity of New Caledonia emerged. Given the complex topology of the island, with many physical barriers, genetic drift could have a strong influence on the genetic structure relative to current gene flow, and cause significant differentiation among populations

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