Identification and gene expression of multiple peptidoglycan recognition proteins (PGRPs) in the deep-sea mussel Bathymodiolus azoricus, involvement in symbiosis?

The relationship between the deep-sea mussel Bathymodiolus azoricus and its thiotrophic (SOX) and methanotrophic (MOX) symbionts has been ecologically and functionally well studied. Endosymbiosis is common in deep-sea hydrothermal vent fauna, yet little is known about the molecular mechanisms underlying the regulation of interactions between host and symbionts. In this study we focused on a group of pattern recognition receptors (PRR), called PGRPs that are able to recognize the peptidoglycan of bacterial cell wall. We first characterised the different PGRPs isoforms in B. azoricus gills and identified five paralogs. Among them two displayed a signal peptide. Then, specific probes designed for each paralog were used to perform real-time PCR quantification in gills of individuals showing various bacterial content as a result of in situ experimental procedures. Overall we found a decrease of PGRPs expression when symbionts amount decreases, suggesting an implication of PGRPs in the regulation of symbionts in B. azoricus gills. We therefore hypothesize that secreted proteins could act as cooperation signals to induce colonisation of symbiotic tissue while non-secreted proteins may regulate the density of endosymbionts within the gill tissue.