High individual variability in the transcriptomic response of Mediterranean mussels to Vibrio reveals the involvement of myticins in tissue injury

Mediterranean mussels (Mytilus galloprovincialis) are sessile filter feeders that live in close contact with numerous marine microorganisms. As all invertebrates, they lack an adaptive immune response and how these animals are able to respond to a bacterial infection and discriminate it from their normal microbiome is difficult to understand. In this work, we conducted Illumina sequencing of the transcriptome of individual mussels before and after being infected with Vibrio splendidus. The control mussels were injected with filtered seawater. We demonstrate that a great variability exists among individual transcriptomes and that each animal showed an exclusive repertoire of genes not shared with other individuals. The regulated genes in both the control and infected mussels were also analyzed and, unexpectedly, the sampling before the injection was considered a stress stimulus strong enough to trigger and modulate the response in hemocytes, promoting cell migration and proliferation. We found a clear response against the injection of filtered seawater, suggesting a reaction against a tissue injury in which the myticins, the most expressed antimicrobial peptides in mussel, appeared significantly up regulated. Functional experiments with flow cytometry confirmed the transcriptomic results since a significant alteration of hemocyte structures and a decrease in the number of hemocytes positive for myticin C were found only after a Vibrio infection and not observed when mussels were bled before, generating a tissue injury. Therefore, we report the involvement of myticins in the response to a danger signal such as a simple injection in the adductor muscle.