Congratulations to Lauric Reynes (EMBIO ) who defended his thesis in person and by videoconference on Tuesday, June 1, 2021 at 14:00 (Amphitheater OCEANOMED

On the following topic:

"Genetic connectivity and structure of Ericaria zosteroides (Fucales) and Laminaria rodriguezii (Laminariales) populations of the French coasts".
Thesis supervisor : Thierry Thibault, MCF AMU
Co-director: Didier Aurelle, MCF AMU

The forests of brown algae (Phaeophyceae); orders Laminariales (kelp) and Fucales (Cystoseira, Sargassum, Ericaria, Gongolaria, Fucus for the Mediterranean) are subject to combinations of pressures of anthropic origin. The response of marine forests is nevertheless highly variable depending on the species and populations considered, but also on the causes of their decline. The future of these ecosystems will depend on the ability of the populations of these species to evolve in this changing environment.

In my thesis, I studied the factors that may control the structure and genetic diversity of marine forests, focusing on two endemic species of the Mediterranean basin, the Fucale Ericaria zosteroides and the deep-sea kelp Laminaria rodriguezii. These species can form vast marine forests, of great ecological and socio-economic value. They are characterized by a sparse distribution and low dispersal potential, with potential consequences on their response to global change.

In a first part, I focused on population connectivity using a partial genome sequencing method (RAD-seq). The analysis of the genetic structure indicates a strong genetic differentiation at short distances and a reduced connectivity, both for E. zosteroides and L. rodriguezii populations. By coupling genetic structure and marine current modelling, I studied the role of propagule dispersal, using E. zosteroides populations in Provence as a case study. The results obtained show that genetic structure is predicted more by marine currents than by spatial isolation. This supports the hypothesis that the drift of fertile branches can be a vector of gene flow. In addition, RAD-seq detected outlier loci that may be involved in local adaptation to depth.

In a second part, I was interested in the mode of reproduction of the kelp L. rodriguezii, this species being one of the few in the genus Laminaria to reproduce by vegetative and sexual reproduction. Partial clonality, in interaction with other evolutionary forces, such as genetic drift and mutations control the evolution of genetic diversity. Here, I used RAD-seq to study genome-wide signatures of clonality. The results obtained confirm the impact of clonality on the genomic diversity of L. rodriguezii, with varying levels between populations. The comparison with a sexually reproduced congenic species (Laminaria digitata) shows that other factors, notably genetic drift, can have confounding effects.

In addition to the fundamental questions raised by these results, the existence of strong short-range genetic structure in sparse populations has important implications for the conservation of these species.