The characterization of toll-like receptor repertoire in Pinna nobilis after mass mortality events suggests adaptive introgression.

The fan mussel Pinna nobilis is currently on the brink of extinction due to a multifactorial disease mainly caused to the highly pathogenic parasite Haplosporidium pinnae, meaning that the selection pressure outweighs the adaptive potential of the species. Hopefully, rare individuals have been observed somehow resistant to the parasite, stretching the need to identify the traits underlying this better fitness. Among the candidate to explore at first intention are fast-evolving immune genes, of which toll-like receptor (TLR). In this study, we examined the genetic diversity at 14 TLR loci across P. nobilis, Pinna rudis and P. nobilis × P. rudis hybrid genomes, collected at four physically distant regions, that were found to be either resistant or sensitive to the parasite H. pinnae. We report a high genetic diversity, mainly observed at cell surface TLRs compared with that of endosomal TLRs. However, the endosomal TLR-7 exhibited unexpected level of diversity and haplotype phylogeny. The lack of population structure, associated with a high genetic diversity and elevated dN/dS ratio, was interpreted as balancing selection, though both directional and purifying selection were detected. Interestingly, roughly 40% of the P. nobilis identified as resistant to H. pinnae were introgressed with P. rudis TLR. Specifically, they all carried a TLR-7 of P. rudis origin, whereas sensitive P. nobilis were not introgressed, at least at TLR loci. Small contributions of TLR-6 and TLR-4 single-nucleotide polymorphisms to the clustering of resistant and susceptible individuals could be detected, but their specific role in resistance remains highly speculative. This study provides new information on the diversity of TLR genes within the P. nobilis species after MME and additional insights into adaptation to H. pinnae that should contribute to the conservation of this Mediterranean endemic species.

Underwater acoustic positioning systems as tool for Posidonia oceanica beds survey.

The study of the dynamics of Posidonia oceanica beds, and more particularly, the position of their lower limit, are usually carried out by the installation and monitoring of markers set on the sea bottom. Acoustic telemetry could constitute an interesting alternative solution to the marking of the lower limit seagrass beds. In order to test the potentialities of this system, an experiment was conducted in the Natural Marine Reserve of Cerbère-Banyuls (eastern Pyrenees, France). The possibility of very precise mapping offers many potentialities with regards to the work of monitoring networks in the marine environment (species and assemblages of importance), but also in the monitoring of competition dynamics between the species.