Phylogeny and expression of tetraspanin CD9 paralogues in rainbow trout (Oncorhynchus mykiss)

CD9 is a member of the tetraspanin family, which is characterised by a unique domain structure and conserved motifs. In mammals, CD9 is found in tetraspanin-enriched microdomains (TEMs) on the surface of virtually every cell type. CD9 has a wide variety of roles, including functions within the immune system. Here we show the first in-depth analysis of the cd9 gene family in salmonids, showing that this gene has expanded to six paralogues in three groups (cd9a, cd9b, cd9c) through whole genome duplication events. We suggest that through genome duplications, cd9 has undergone subfunctionalisation in the paralogues and that cd9c1 and cd9c2 in particular are involved in antiviral responses in salmonid fish. We show that these paralogues are significantly upregulated in parallel to classic interferon-stimulated genes (ISGs) active in the antiviral response. Expression analysis of cd9 may therefore become an interesting target to assess teleost responses to viruses.