The complete salmonid IGF-IR gene repertoire and its transcriptional response to disease.

The insulin-like growth factor (IGF) receptor (IGF-IR) is necessary for IGF signalling and has essential roles in cellular growth. In teleost fish, two distinct IGF-IR duplicates are conserved called IGF-IRa and IGF-IRb. However, while a salmonid-specific whole genome duplication (ssWGD) is known to have expanded several key genes within the IGF axis, its impact on the IGF-IR repertoire remains unresolved. Using bioinformatic and phylogenetic approaches, we establish that salmonids retain two IGF-IRa paralogues from ssWGD and a single IGF-IRb copy. We measured the tissue-specific and developmental transcriptional regulation of each IGF-IR gene, revealing tight co-expression between the IGF-IRa paralogues, but expression divergence comparing IGF-IRa and IGF-IRb genes. We also examined the regulation of each IGF-IR gene in fish challenged by bacterial and viral infections, adding to recent reports that the IGF axis has roles linking growth and immunity. While whole salmonid fry showed a small upregulation of IGF-IR expression during both types of infection, bacterial challenge caused striking downregulation of IGF-IRa1 and IGF-IRa2 in head kidney and spleen of adult fish, alongside genes coding IGF hormones, highlighting a strong repression of IGF-signalling in primary immune tissues. The reported immune-responsive regulation of IGF-IR genes adds to an emerging body of evidence that supports important cross-talk between master growth and immune pathways in vertebrates.