On the following subject:
"Genetic connectivity and structure of Ericaria zosteroides (Fucales) and Laminaria rodriguezii (Laminariales) populations on the French coast".
Thesis supervisor: Thierry Thibault, MCF AMU
Co-director: Didier Aurelle, MCF AMU
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 anthropogenic origin. However, the response of marine forests is highly variable, depending on the species and populations under consideration, 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 likely to control the structure and genetic diversity of marine forests, focusing on two species endemic to the Mediterranean basin, the rockweed Ericaria zosteroides and the deepwater kelp Laminaria rodriguezii. These species can form vast marine forests of great ecological and socio-economic value. They are characterised by a sparse distribution and low dispersal potential, with potential consequences for their response to global change.
In the first part, I looked at population connectivity using a partial genome sequencing method (RAD-seq). Analysis of the genetic structure indicates strong genetic differentiation at short distances and reduced connectivity, for both E. zosteroides and L. rodriguezii populations. By combining genetic structure and modelling of marine currents, I studied the role of propagule dispersal, using E. zosteroides populations in Provence as a case study. The results 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 for gene flow. RAD-seq was also used to detect outlier loci that could be involved in local adaptation to depth.
In the second part, I looked at 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, interacting with other evolutionary forces such as genetic drift and mutations, controls the evolution of genetic diversity. Here, I used RAD-seq to study the signatures of clonality on a genome-wide scale. The results obtained confirm the impact of clonality on the genomic diversity of L. rodriguezii, with variable levels depending on the population. Comparison with a sexually reproducing 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 a strong short-distance genetic structure for sparse populations has important implications for the conservation of these species.