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International audience; Environmentally induced reductions in fitness components (survival, fecundity) are generally considered as passive, maladaptive responses to stress. However, there is also mounting evidence for active, programmed forms of environmentally induced cell death in unicellular organisms. While conceptual work has questioned how such programmed cell death (PCD) might be maintained by natural selection, few experimental studies have investigated how PCD influences genetic differences in longer-term fitness across environments. Here, we tracked the population dynamics of two closely related strains of the halotolerant microalga Dunaliella salina, following transfers across salinities. We showed that after a salinity rise, only one of these strains displayed a massive population decline (-69% in one hour), largely attenuated by exposure to a PCD inhibitor. However, this decline was followed by a rapid demographic rebound, characterized by faster growth than the non-declining strain, such that sharper decline was correlated with faster subsequent growth across experiments and conditions. Strikingly, the decline was more pronounced in conditions more favourable to growth (more light, more nutrients, less competition), further suggesting that it was not simply passive. We explored several hypotheses that could explain this decline-rebound pattern, which suggests that successive stresses could select for higher environmentally induced death in this system.

Previously confused with the small giant clam Tridacna maxima, the recently resurrected Noah's giant clam, Tridacna noae, has been reported from the Taiwanese and the Ryukyu archipelagoes. Our recent underwater observations now extend its distribution to Dongsha (northern South China Sea), Bunaken (Sulawesi Sea), Madang and Kavieng (Bismarck Sea), the Alor archipelago (Sawu Sea), Kosrae (Caroline Islands), New Caledonia, the Loyalty Islands and Vanuatu (Coral Sea), Viti-Levu (Fiji), Wallis Island, and Kiritimati (Northern Line Islands). Published mitochondrial DNA sequences retrieved from open-access databases also indicate its presence in eastern Negros (Philippines), in the Molucca Sea, at Ningaloo Reef (Western Australia), and in the Solomon Islands. Noah's giant clam is thus a widely distributed Indo-West Pacific species. Wherever research has been done on small giant clams throughout T. noae's range, the inadvertent confusion of T. noae with T. maxima might have led to overestimating actual T. maxima densities and to errors in estimating demographic parameters.

International audience; Macroalgae can represent a significant component of biological introductions in coastal areas. One such example is the Asian kelp Undaria pinnatifida, which has been both intentionally and unintentionally introduced throughout the world. The alga was first introduced in the Bay of St-Malo (English Channel, France) for cultivation in the early 1980s, where it subsequently escaped and established a number of spontaneous satellite populations in the wild. We here document the present-day distribution of U. pinnatifida in this bay based on a grid sampling design. The alga was observed in most of the surveyed sites (84%). To test if this broad distribution of U. pinnatifida could be at all associated with the annual supply of propagules released from the local farm, we evaluated population genetic diversity and connectivity throughout the bay using nine microsatellite loci. We detected limited gene flow that could not be attributed to any isolation-by-distance mechanism, or to habitat type. In further support of a lack of direct connection between farms and satellite populations, spontaneous populations in the open bay were strongly differentiated from farmed populations. Future studies should investigate environmental disturbance, including commercial and recreational nautical activities, which may promote further spread of this pioneer alga.

Biological invasions are the second cause of worldwide biodiversity loss, behind habitat destruction. However, introduced species appeared to be excellent models for studying micro-evolutionnary and ecological processes related to colonization and adaptation in new environments on short time scales. Using spatial population genetics, this PhD aimed at analysing the patterns of dispersal and recruitment at different spatial scales by the Japanese kelp U. pinnatifida, introduced 30 years ago in Northern Europe, to determine the vectors of dispersal acting at these different scales. First, a large scale connectivity study was conducted using 19 populations distributed in 3 bays in Northern Brittany (628 individuals genotyped at 9 microsatellite loci). The main results show that despite a shared history of introduction and common genetic background due to aquaculture activities, populations of U. pinnatifida are found to be genetically diversified and structured over small spatial scales (a few km), suggesting a rapid genetic divergence of populations established in the wild and a limited influence of human-mediated transports. Secondly, 30 sites were sampled within a bay (river Rance Estuary and the Bay of St Malo, 995 individuals). The results at this intermediate scale show a mosaic genetic structure independent of geographic location or habitats. This suggests that multiples factors, human transports and long-distance natural dispersal events, interacted to shape the patterns of settlement of this species its initial introduction 20-30 years ago. Third, a fine scale study conducted in 2 populations over 2 years (1021 individuals) show that Undaria pinnatifida disperse effectively over short spatial distances (a few tens of meters). Some longer distance dispersal also occurs, probably through drifting by mature sporophytes. Fourth, monitoring the colonization of a newly established marina (monitored during 7 months; > 400 individuals sampled) allowed us to highlight that U. pinnatifida was able to colonize a new artificial substrate within a few months. These observations were in agreement with the results of an experiment where rocks in natural habitat were cleared and surveyed for the re-establishment of species, showing that U. pinnatifida was transiently favoured by newly available habitats (thereby confirming its pioneer species status). Finally, the microcospic stages of the life cycle of U. pinnatifida were investigated. In particular, we designed 7 oligonucleotide probes to achieve fluorescent in situ hybridizations for the detection of spores in natural habitat and showed that gametophytes might act as a new potential vector of dispersal. The above mentioned results underlined that the sustainable establishment of invasive populations of U. pinnatifida established in Brittany benefitted from multiple dispersal pathways. Its spread is thus limited by its natural dispersal abilities but these limitations are balanced by human-mediated transports and availability of artificial habitats.; Les invasions biologiques sont la deuxième cause de diminution de la biodiversité mondiale, après la destruction des habitats. Les espèces introduites se sont révélées d’excellents modèles d’étude pour appréhender les processus micro-évolutifs et écologiques liés à la colonisation et à l’adaptation rapide d’espèces dans de nouveaux milieux. Par des approches spatiales basées sur la génétique des populations, l’objectif de cette thèse a été de décrire les modalités de dispersion agissant à différentes échelles spatiales chez la macro-algue Undaria pinnatifida, introduite en Europe du Nord depuis une trentaine d'années. Dans un premier temps, une étude génétique de la connectivité a été menée sur 19 populations distribuées dans 3 baies de Bretagne Nord (628 individus génotypés à 9 loci microsatellites). Les principaux résultats montrent qu'en dépit d'une origine commune due à l'aquaculture, les populations d'U. pinnatifida sont génétiquement diversifiées même à une échelle spatiale fine (quelques kilomètres), suggérant une divergence génétique rapide des populations établies dans le milieu naturel et une influence limitée des activités anthropiques. L'étude détaillée d'une des baies (Estuaire de la Rance et de la Baie de St Malo, 955 individus échantillonnés sur 30 sites) montre une structure en mosaïque, indépendante de la situation géographique ou de l'habitat. Cette structure hétérogène suggère que de multiples facteurs, dont les transports anthropiques et des évènements de dispersion naturelle à longue distance, ont interagi pour façonner les schémas actuels d'installation de l'espèce depuis son introduction initiale il y a 35 ans. Une étude menée à une échelle encore plus fine au sein de 2 populations pendant 2 années (1021 individus) indique cependant qu'U. pinnatifida se disperse majoritairement sur de courtes distances (quelques dizaines de mètres), notamment par le biais de spores, et minoritairement sur des distances plus grandes (> 30m) probablement par des fragments de sporophytes matures. Par ailleurs, le suivi temporel (7 mois ; > 400 individus) de la colonisation du port de l'Aber Wrac'h a opermis de mettre en évidence qu'U. pinnatifida était capable de coloniser en l'espace de quelques mois un substrat artificiel vierge à partir des populations naturelles établies. Cette expansion rapide de la population semble être due à la reproduction des individus fondateurs déjà en place les mois précédents. Ces observations ont été soutenues par une expérience de perturbation (mise à nu) d'un substrat rocheux en milieu naturel (3 réplicats) qui a montré que la disponibilité d'un substrat vierge pouvait faciliter de manière transitoire l'établissement d'U. pinnatifida, confirmant ainsi son statut d'espèce pionnière. Finalement, des études ont été menées sur les stades microscopiques d'U. pinnatifida. En particulier afin de détecter spécifiquement des spores dans la colonne d'eau, 7 sondes nucléotidiques ont été optimisées afin de réaliser des hybridations de type "FISH". Des études expérimentales sur les gamétophytes ont de plus permis de montrer qu'ils pouvait être un agent de dispersion. Ces différents résultats soulignent l'interaction entre de multiples modalités de dispersion, dans l'établissement durable d'U. pinnatifida. Par ailleurs, bien qu'étant une espèce invasive à une échelle mondiale, ses capacités de dispersion naturelle apparaissent comme un facteur limitant de son expansion à une échelle locale et régionale, contrebalancée principalement par les activités anthropiques et la disponibilité en habitats artificiels.

Macroalgae can represent a significant component of biological introductions in coastal areas. One such example is the Asian kelp Undaria pinnatifida, which has been both intentionally and unintentionally introduced throughout the world. The alga was first introduced in the Bay of St-Malo (English Channel, France) for cultivation in the early 1980s, where it subsequently escaped and established a number of spontaneous satellite populations in the wild. We here document the present-day distribution of U. pinnatifida in this bay based on a grid sampling design. The alga was observed in most of the surveyed sites (84%). To test if this broad distribution of U. pinnatifida could be at all associated with the annual supply of propagules released from the local farm, we evaluated population genetic diversity and connectivity throughout the bay using nine microsatellite loci. We detected limited gene flow that could not be attributed to any isolation-by- distance mechanism, or to habitat type. In further support of a lack of direct connection between farms and satellite populations, spontaneous populations in the open bay were strongly differentiated from farmed populations. Future studies should investigate environmental disturbance, including commercial and recreational nautical activities, which may promote further spread of this pioneer alga.