Comparative study of immune responses in the deep-sea hydrothermal vent mussel Bathymodiolus azoricus and the shallow-water mussel Mytilus galloprovincialis challenged with Vibrio bacteria

The deep-sea hydrothermal vent mussel Bathymodiolus azoricus and the continental European coast Mytilus galloprovincialis are two bivalves species living in highly distinct marine habitats. Mussels are filter-feeding animals that may accumulate rapidly bacteria from the environment. Contact with microorganism is thus inevitable during feeding processes where gill tissues assume a strategic importance at the interface between the external milieu and the internal body cavities promoting interactions with potential pathogens during normal filtration and a constant challenge to their immune system. In the present study B. azoricus and M. galloprovincialis were exposed to Vibrio alginolyticus, Vibrio anguillarum and Vibrio splendidus suspensions and to a mixture of these Vibrio suspensions, in order to ascertain the expression level of immune genes in gill samples, from both mussel species. The immune gene expressions were analyzed by means of quantitative-Polymerase Chain Reaction (qPCR). The gene expression results revealed that these bivalve species exhibit significant expression differences between 12 h and 24 h post-challenge times, and between the Vibrio strains used. V. splendidus induced the strongest gene expression level in the two bivalve species whereas the NF-κB and Aggrecan were the most significantly differentially expressed between the two mussel species. When comparing exposure times, both B. azoricus and M. galloprovincialis showed similar percentage of up-regulated genes at 12 h while a marked increased of gene expression was observed at 24 h for the majority of the immune genes in M. galloprovincialis. This contrasts with B. azoricus where the majority of the immune genes were down-regulated at 24 h. The 24 h post-challenge gene expression results clearly bring new evidence supporting time-dependent transcriptional activities resembling acute phase-like responses and different immune responses build-up in these two mussel species when challenged with Vibrio bacteria. High Pressure Liquid Chromatography (HPLC)-Electrospray ionization mass spectrometry (ESI-MS/MS) analyses resulted in different peptide sequences from B. azoricus and M. galloprovincialis gill tissues suggesting that naïve animals present differences, at the protein synthesis level, in their natural environment. B. azoricus proteins sequences, mostly of endosymbiont origin, were related to metabolic, energy production, protein synthesis processes and nutritional demands whereas in M. galloprovincialis putative protein functions were assumed to be related to structural and cellular integrity and signaling functions.