Non-self recognition with the consequent tolerance or immune reaction is a crucial process to succeed as
living organisms. At the same time the interactions between host species and their microbiome,
including potential pathogens and parasites, significantly contribute to animal life diversity. Marine
filter-feeding bivalves, mussels in particular, can survive also in heavily anthropized coastal waters
despite being constantly surrounded by microorganisms. Based on the first outline of the Mytilus galloprovincialis
immunome dated 2011, the continuously growing transcript data and the recent release of a
draft mussel genome, we explored the available sequence data and scientific literature to reinforce our
knowledge on the main gene-encoded elements of the mussel immune responses, from the pathogen
recognition to its clearance. We carefully investigated molecules specialized in the sensing and targeting
of potential aggressors, expected to show greater molecular diversification, and outlined, whenever
relevant, the interconnected cascades of the intracellular signal transduction.
Aiming to explore the diversity of extracellular, membrane-bound and intracellular pattern recognition
receptors in mussel, we updated a highly complex immune system, comprising molecules which are
described here in detail for the first time (e.g. NOD-like receptors) or which had only been partially
characterized in bivalves (e.g. RIG-like receptors). Overall, our comparative sequence analysis supported
the identification of over 70 novel full-length immunity-related transcripts in M. galloprovincialis.
Nevertheless, the multiplicity of gene functions relevant to immunity, the involvement of part of them in
other vital processes, and also the lack of a refined mussel genome make this work still not-exhaustive
and support the development of more specific studies.
