Evolution of interleukin-1 receptor-like 1 and its role in rainbow trout (Oncorhynchus mykiss) resistance to Flavobacterium psychrophilum

Abstract

Abstract Rainbow trout exhibit extensive phenotypic variation in innate disease resistance and we have divergently selected lines with either increased or reduced survival following exposure to the gram-negative bacterium, Flavobacterium psychrophilum (Fp). Following five generations of selection, gene expression was analyzed between the resistant and susceptible lines by RNA-seq and RT-PCR in either whole body or spleen tissue samples. Lower base-line expression of interleukin-1 receptor-like 1 (il1rl1) was identified in susceptible line fish as well as decreased expression in response to infection. Genomic analyses identified three tandem, il1rl1 genes on chromosome 3 located within a previously identified, bacterial cold water disease QTL. Here, we report the genomic organization and evolution of the three il1rl1 genes, and develop gene specific assays to measure the expression of the individual il1rl1 genes in susceptible and resistant rainbow trout lines. Sequence homology between these three putative genes is approximately 95%. Each gene contains a signal peptide, three IG/IG-like domains, a transmembrane region and a TIR domain. Two il1rl1 genes and one pseudogene are present in Atlantic salmon (Salmo salar) genome with 90–94% sequence identity. Phylogenetic analyses suggest that the salmonid il1rl1 genes expanded by tandem duplication within the salmonid linage. Our working model is that il1rl1 gene copy number or differential expression may contribute to specific disease resistance.