Validation of linked QTL for bacterial cold water disease resistance and spleen size on rainbow trout chromosome Omy19

• We confirmed the presence of QTL for BCWD and spleen size on Omy19.
• BCWD resistance and spleen size are not influenced by a single pleiotropic QTL.
• BCWD and spleen size loci are distinctly separated from each other on Omy19.

Bacterial cold water disease (BCWD) is caused by infection with Flavobacterium psychrophilum, and outbreaks cause significant economic losses in salmonid aquaculture. Previously, we identified a major QTL for BCWD resistance on rainbow trout chromosome Omy19 as well as a QTL for spleen size (SPLW = spleen weight and SPLI = spleen index) which in naïve fish is a surrogate indicator of BCWD resistance. The objective of this study was to validate the BCWD resistance and spleen size QTL in a subsequent generation and investigate whether the correlation between BCWD resistance and spleen size loci on Omy19 was due to close linkage or pleiotropy. We designed four full-sib matings whose offspring were evaluated for BCWD resistance, body weight, SPLW and SPLI. A total of 800 offspring were genotyped with seven microsatellite markers from Omy19. Our analyses confirmed the presence of QTL for BCWD and spleen size on Omy19. Our findings also suggest that BCWD resistance and spleen size are not influenced by a single pleiotropic QTL on Omy19 as the QTL were separately inherited in the families studied. These data increase our understanding of the genetic basis of rainbow trout BCWD resistance.