Research Article
Inês Barros, Susana M
Abstract
Mussels belonging to the Bathymodiolus genus are the most abundant species inhabiting hydrothermal vent sites from the Mid-Atlantic Ridge. The presence of endosymbiont bacteria in vent mussel Bathymodiolus azoricus gills is regarded as an evolutionary feature that confers deep-sea vent mussels the ability to adapt to chemosynthesisbased environment while potentially driving host-immune gene expression. In the present study, the functional immunological capabilities of B. azoricus gill tissues were addressed during an acclimatization experiment in aquaria environment at atmospheric pressure, where vent mussels were exposed to Vibrio diabolicus stimulations, for recurrent periods of 6h, alternated with longer sea water incubation intervals, over a 3 weeks’ time course. The effect of V. diabolicus exposures were analyzed, at distinct time points, where mRNA transcript levels from both host-immune and endosymbiont genes potentially revealed gene expression interdependence between host and endosymbionts. qPCR results targeting selected host-immune and endosymbiont genes presented significant gene expression differences between sea water control and V. diabolicus exposed mussels. The effect of time of acclimatization and endosymbiont prevalence, on host-immune gene expression, suggested that distinct timedependent immune gene responses in B. azoricus are tied to endosymbiont bacteria. The results reflect a direct effect of V. diabolicus on endosymbiont gene expression profiles as demonstrated by up-regulation of endosymbiont genes such as ALDH, CA, CBB, MeDH, MMO and SOXB particularly at 2 and 3 weeks’ acclimatization. V. diabolicus stimulations caused up and down regulation of gene expression seen at 72 h and 1 week and 48 h, 2 and 3 weeks’ acclimatization, respectively. These gene expression profile studies supported B. azoricus ability to mobilize its immune system and to react against Vibrio challenges. A putative protective role of endosymbionts was considered in light of the progressive decline of immune gene transcriptional activity beyond 2 weeks of acclimatization coincident with predictable endosymbiont loss in land-based aquaria systems at atmospheric pressure. A yet uncharacterized protective role of endosymbionts is here evoked, for the first time, and may extend to counteracting apoptosis induction resulting from Vibrio infections as suggested by BCL2 and p43 down-regulation gene expression seen for all the acclimatization time-points.