GENOMIC SELECTION EXPLOITS WITHIN-FAMILY GENETIC VARIATION FOR DISEASE RESISTANCE IN RAINBOW TROUT

Flavobacterium psychrophilum is the etiological agent causing bacterial cold water disease (BCWD) in salmonid fish. Previous breeding strategies to reduce losses due to BCWD involve testing and determining phenotypic differences between multiple families of rainbow trout. These results were used to determine an estimated breeding value based on a resistance phenotype. This study used genotypic markers, generated from a training population and associated with improved BCWD survival, to identify individual fish within a family to generate a genomic-estimated breeding value (GEBV).  A single sire was crossed with high-GEBV and low-GEBV dams from the same family of rainbow trout. Progeny were then challenged with F. psychrophilum to determine the improvement made due to differences in the GEBV.  Mean survival of progeny from high-GEBV dams after experimental F. psychrophilum challenge was 19.6 percentage points greater compared to progeny from low-GEBV dams.  After adjusting the data for non-genetic effects, progeny from high-GEBV dams had a greater (P < 0.0001) probability of survival (0.43 ± 0.03) than their half sibs from low-GEBV dams (0.22 ± 0.02).  Mortality kinetics were similar between high- and low-GEBV progeny groups, with peak mortalities occurring between 4 and 9 days post-infection in both groups. However, during this 6-day period of peak mortality, cumulative mortality was smaller in high-GEBV progeny (38.8%) than in low-GEBV progeny (57.5%).  Between 10 and 21 days post-infection, cumulative mortalities were nearly identical between high- and low-GEBV progeny (16.1 versus 16.7%, respectively). Thus, mean days to death, with a value of 21 used for fish surviving the challenge, was greater (P < 0.0001) in high- (14.0 ± 0.4 days) versus low-GEBV progeny (11.1 ± 0.4 days).  Accuracy of GEBV predictions made on the non-phenotyped parents was 0.72 (95% CI = 0.60 - 0.81).  These results demonstrate that using genomic selection, improved BCWD survival can not only be achieved by identifying more resistant families of trout, but also by identifying superior individual fish within a family.