Yniv Palti ^1* ROGER L. VALLEJO ² Maureen K. Purcell ³ Guangtu Gao ² Kristy Shewbridge ² Roseanna Long ² Christopher Setzke ⁴ Breno D. Fragomeni ⁵ Hao Cheng ⁶ Kyle E. Martin ⁷ Kerry Naish ⁴

• ¹ Agricultural Research Service, Cool and Cold Water Aquaculture Research, Agricultural Research Service (USDA), Leetown, WV, United States
• ² Cool and Cold Water Aquaculture Research, Agricultural Research Service (USDA), Leetown, West Virginia, United States
• ³ Western Fisheries Research Center, United States Geological Survey (USGS), Seattle, Washington, United States
• ⁴ School of Aquatic and Fishery Sciences, College of the Environment, University of Washington, Seattle, Washington, United States
• ⁵ Department of Animal Science, University of Connecticut, Storrs, United States
• ⁶ Department of Animal Science, College of Agricultural and Environmental Sciences, University of California, Davis, Davis, California, United States
• ⁷ Hendrix Genetics (USA), Sumner, United States

Infectious hematopoietic necrosis (IHN) is a disease of salmonid fish that is caused by the IHN virus (IHNV), which can cause substantial mortality and economic losses in rainbow trout aquaculture and fisheries enhancement hatchery programs. In a previous study on a commercial rainbow trout breeding line that has undergone selection, we found that genetic resistance to IHNV is controlled by the oligogenic inheritance of several moderate and many small effect quantitative trait loci (QTL). Here we used genome wide association analyses in two different commercial aquaculture lines that were naïve to previous exposure to IHNV to determine whether QTL were shared across lines, and to investigate whether there were major effect loci that were still segregating in the naïve lines. A total of 1,859 and 1,768 offspring from two commercial aquaculture strains were phenotyped for resistance to IHNV and genotyped with the rainbow trout Axiom 57K SNP array. Moderate heritability values (0.15-0.25) were estimated.Two statistical methods were used for genome wide association analyses in the two populations.No major QTL were detected despite the naïve status of the two lines. Further, our analyses confirmed an oligogenic architecture for genetic resistance to IHNV in rainbow trout. Overall, 17 QTL with notable effect (≥1.9% of the additive genetic variance) were detected in at least one of the two rainbow trout lines with at least one of the two statistical methods. Five of those QTL were mapped to overlapping or adjacent chromosomal regions in both lines, suggesting that some loci may be shared across commercial lines. Although some of the loci detected in this GWAS merit further investigation to better understand the biological basis of IHNV disease resistance across populations, the overall genetic architecture of IHNV resistance in the two rainbow trout lines suggests that genomic selection may be a more effective strategy for genetic improvement in this trait.