Abstract
The main objective of this thesis is to explore the phylogeographic differentiation patterns of several species that inhabit deep hydrothermal vents in the back-arc basins of the western Pacific. These vents constitute a discontinuous habitat as they are interspersed with volcanic arcs, subduction trenches and abyssal plains. The ecosystems associated with these sources are composed of an abundant fauna of species entirely dependent on bacterial chemosynthesis. In this highly fragmented context, the question arises as to the degree of connectivity of the populations of the different hydrothermal species as well as their phylogeographic history. Little studied until now, we approach these questions here with the methodologies of population genomics which allow us to get rid of the main limitations linked to the use of a small number of markers and bring a better resolution of the demo-genetic processes.The first chapter focuses on a gastropod emblematic of these ecosystems, Ifremeria nautilei (Provannidae) sampled throughout its range. By obtaining 10,500 independent genomic markers (ddRad), we identified a structure in two metapopulations separated by the Solomon Islands and Vanuatu and genetically almost homogenous on both sides. Demo-genetic reconstructions indicate that these metapopulations are currently connected by a weak, bi-directional and slightly asymmetric gene flow and that they would have diverged about 70,000 generations ago. However, a detailed analysis of loci that do not conform to the expected neutral distribution ("outliers") shows a limit of demographic connectivity at the basin scale within each metapopulation. This may have implications for site recolonization in the event of mass destruction.Chapter 2 compares the phylogeographic patterns of four species (I. nautilei, Alviniconcha kojimai, Shinkailepas tollmanni, and Eochionelasmus ohtai) using the same methodology as Chapter 1. As with I. nautilei, the other three species show a two-metapopulation structure with weak bi-directional gene flow slightly asymmetrically to the west. This bi-directionality might suggest that larval dispersal strategies are not unimodal and thus more complex than those proposed so far. Demo-genetic reconstructions show that most species would have diverged under a secondary contact pattern with a separation between metapopulations estimated at 70 000 - 100 000 generations with the exception of the gastropod A. kojimai whose divergence is very recent. However, the timing of secondary contact and gene flow levels vary from species to species, differences that are probably attributable to the interaction between the geological history of the region and their life history traits, which themselves condition their biogeographic distribution.