The evolutionary particularity of expanded immune gene families in mussel and their relationship with functional specificity

Abstract

Transcriptomic studies performed under different infection models have revealed the implication of conserved innate immune gene families in the defensive response of mussels. Many of those families are however greatly expanded in comparison with phylogenetically close species. We have deepened in the evolutionary and selection processes that gave rise to these gene families, particularly the pro-inflammatory interleukin IL-17 and the immune Toll-like receptors (TLRs ). Comparative genomics was applied to examine the evolution of these genes from poriferans to higher vertebrates. Common evolutionary patterns were revealed. Cnidaria was highlighted as the most ancient, diverged phylum and expansions of different magnitude were found as well in other marine bivalves and echinoderm species. Concerning mussels, the different clusters obtained in phylogenetic analyses were conserved by the three analyzed species (Mytilus galloprovincialis, M. edulis and M. coruscus). Selection analyses were performed on those clusters/isoforms and the inter-genomic variation was studied to reveal the degree of conservation of these genes or their subjection to presence/absence variation. Former transcriptomic studies had revealed the implication of TLRs in recognizing bacterial infections in hemocytes and gills. and subsequent activation of IL-17 as immune mediators leading to the canonical pathway of NF-kB inflammation. For the current study, a massive expression dataset was built using all the transcriptomic data available for mussel (M. gal/oprovincialis). Clusters of correlated genes were obtained from this dataset. Different expression trends were obtained for different members of the studied gene families. The expression analysis revealed different correlated transcripts that could interact with specific forms of our target genes in response to different stimuli. These results would point towards a clear functional specificity that would be allowed by the great underlying variability