EVOLUTION OF THREE TANDEM COPIES OF THE INTERLEUKIN-1 RECEPTOR-LIKE 1 GENE IN SALMONID FISH AND THEIR CONTRIBUTION TO RAINBOW TROUT BACTERIAL COLD WATER DISEASE RESISTANT PHENOTYPE
Cassidy Shaw, National Center for Cool and Cold Water Aquaculture, USDA
Cassidy H. Shaw1, Sixin Liu1, Guangtu Gao1, Yniv Palti1, and Gregory D. Wiens1
1. National Center for Cool and Cold Water Aquaculture, Agricultural Research Service, United States Department of Agriculture, 11861 Leetown Road, Kearneysville, WV 25430, USA
Rainbow trout exhibit extensive phenotypic variation in innate disease resistance. Five generations of family based selection has resulted in rainbow trout lines with either increased or reduced survival following exposure to the gram-negative bacterium, Flavobacterium psychrophilum (Fp), the causative agent of bacterial cold water disease (BCWD). Whole body RNA-seq analysis and RT-PCR of spleen tissue samples identified lower basal level expression of interleukin-1 receptor-like 1 (il1rl1) in susceptible line fish as well as decreased expression in response to infection. Genomic analyses identified three tandem, il1rl1 genes on chromosome 3 located in a previously identified, BCWD QTL region. We report the genomic organization and evolution of three il1rl1 genes, and develop gene specific assays to measure the expression of 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 tandem il1rl1 genes and one pseudogene are present in Atlantic salmon (Salmo salar) and Coho salmon (Oncorhynchus kisutch) genomes with 86-94% sequence identity to rainbow trout il1rl1 genes. Low homology (20-25%) was observed between trout il1rl1 genes and those of higher vertebrates. Phylogenetic analyses grouped trout il1rl1 with il1rl1 sequences from other species and apart from other genes of the interleukin 1 receptor family and suggest that the salmonid il1rl1 genes expanded by an ancestral tandem duplication in the salmonid linage. Differential expression analyses will be used to determine the contribution of il1rl1 gene copy number to the disease resistant phenotype.