Pleistocene and Holocene sea level drops and rises have resulted in the extirpation of most inner reef obligate marine species by drying out the lagoons or over-flooding them, leaving genetic evidence of demographic perturbations in various inner reef taxa. Here, we investigated the effect of the last glaciations sea level fluctuations on the regional distribution and demography of the black lipped pearl oyster, Pinctada margaritifera across French Polynesia, by combining mitochondrial and nuclear markers.
We sequenced cytochrome oxidase subunit I (COI), exon and intron Pearlin nuclear genes from 406 and 372 P. margaritifera individuals respectively, collected in the Marquesas, the Tuamotu, the Society and the Gambier archipelagos of French Polynesia and in Papua New Guinea. We estimated substitution rates of each marker based on Bayesian methods and used coalescent based methods to infer molecular diversity, phylogeographical patterns and demographic history by constructing haplotype networks and Bayesian skyline plots.
P. margaritifera populations of Central Polynesia have undergone recent sudden expansion events resulting from a wide founder effect and/or bottleneck, between 2500 and 1000 years BP, when the sea level finally reached the modern level after subsequently decreasing of about 85 m and rising above the modern level by about 0.8m. The recent expansion dates highlights the fact that P. margaritifera needed the lagoons to recover their enclosed sheltering habitat, in order to expand. The nuclear data suggests that these populations seem to be gradually reaching equilibrium again thanks to high connectivity levels between populations. On the opposite, the Marquesas populations seem to have remained quite stable in time thanks to the absence of barrier reefs, allowing the populations to shift down slope with the sea level drop. We suggest that the Marquesas and Papua New Guinea populations could have represented refuge areas during the last glaciations event and that they probably contribute to the recolonization of Central Polynesia.
Keywords : marine biogeography, Pinctada margaritifera, Cytochrome oxydase I, Pearlin, Pacific Ocean, Phylogeography
