Congratulations to Terence Legrand (OPLC) who defended his thesis on Monday March 21, 2022

On the following topic:

"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 Fellow

Abstract:

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 result from dispersal, that is the movement of an individual from its birth site to its reproduction site. Dispersal mechanisms are extremely varied: they result from the capacity of each organism to move by itself or thanks to a third party. We speak respectively of active or passive dispersal. In the ocean, passive dispersal is facilitated by the intrinsic physical properties of the marine environment: many benthic to semi-sedentary adult 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 propagules survive such a pelagic phase and then settle on a favorable habitat to reproduce. Demographically, connectivity is constitutive 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 critical to the resilience of populations to anthropogenic pressures: understanding it is essential for proper ecosystem management and conservation, including the design of marine protected areas (MPAs) and fisheries management. In this thesis, we have defined a general framework to characterize the demographic connectivity and to locate the spawning areas for any species with a pelagic phase, by combining the use of a Lagrangian model with otolith and biogeographic analysis data. We showed that connectivity patterns are explained by the spatio-temporal variability of ocean circulation, and quantified the role of MPAs in the supply of propagules to unprotected areas. We then analytically defined 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, so far considered 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 biphasic life history species spanning a wide phylogenetic range, compiled in a meta-analysis covering 58 population genetic studies in the Mediterranean Sea. The newly defined coalescent connectivity returns the best gene flow predictions across taxa and explains approximately 50% of the variability in genetic differentiations observed across the meta-analysis. 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. This thesis, beyond establishing new methodological tools for a better understanding of demographic and genetic connectivity, identifies new avenues of research that will contribute to better assess the impact of gene flow and other evolutionary forces on marine biodiversity.