On the following subject:
"Influence of multiscale connectivity via larval dispersal on population structure and biodiversity patterns in the Mediterranean Sea".
Under the supervision of Anne Chenuil, Research Director and Vincent Rossi, Research Officer
Summary:
The living world can be described as a vast and complex network of connections: organisms, by moving themselves in space and time, are the links between populations or localities. Such movements are the result of dispersal, i.e. the movement of an individual from its place of birth to its place of reproduction. Dispersal mechanisms are extremely varied: they depend on the ability of each organism to move by itself or with the help of a third party. These are referred to as active and passive dispersal respectively. In the ocean, passive dispersal is facilitated by the intrinsic physical properties of the marine environment: many adult benthic to semi-sedentary species disperse during their early life stages by releasing cohorts of propagules (eggs, larvae, fruits, etc.) into the water column, which are then transported by marine currents. Connectivity between populations is achieved when the propagules survive such a pelagic phase and then settle in a favourable habitat to reproduce. Demographically, connectivity is a constituent of the spatial dynamics of populations: it is a key ecological process in the regulation and persistence of populations. Genetically, connectivity is a vector of gene flow, which, along with other evolutionary forces, modulates contemporary patterns of biodiversity. Connectivity via dispersal is thus a determining factor in the resilience of populations in the face of anthropogenic pressures: understanding it is essential for the proper management and protection of ecosystems, particularly for the design of marine protected areas (MPAs) or fisheries management. In this thesis, we defined a general framework for characterising demographic connectivity and locating spawning areas for any species with a pelagic phase, by combining the use of a Lagrangian model with data from otolith and biogeographic analyses. We showed that connectivity patterns are explained by the spatio-temporal variability of ocean circulation, and quantified the role of MPAs in supplying propagules to unprotected areas. We then defined analytically the probabilities of genetic connections resulting from successive and cumulative dispersal events, thus accumulating the different possibilities of gene flow between populations over several generations. For a fixed number of generations, filial connectivity, which quantifies the probability that one population is related to another, was distinguished from coalescent connectivity, which quantifies the underlying probability that two populations share common 'ancestor populations'. Our results show that hydrodynamic barriers, hitherto considered to be the cause of genetic structuring, are indeed permeable to coalescent connectivity. These models of filial and coalescent connectivity were used to estimate gene flow in 47 species with biphasic life cycles covering a wide phylogenetic range, compiled in a meta-analysis covering 58 population genetics studies in the Mediterranean Sea. The newly defined coalescent connectivity returns the best gene flow predictions for all taxa and explains about 50 % of the variability in genetic differentiations observed at the meta-analysis scale. Furthermore, our results suggest a close relationship between the temporal (i.e. number of dispersal events) and spatial (i.e. extent of genetic diversity patterns) scales of genetic connectivity, illustrating the eco-evolutionary interactions characteristic of such a process. Finally, we studied the influence of the precise consideration of the habitat obtained by niche models in the prediction of gene flow. In addition to establishing new methodological tools for a better understanding of demographic and genetic connectivity, this thesis identifies new avenues of research that will help to better assess the impact of gene flow and other evolutionary forces on marine biodiversity.