Validation of Linkage Between BCWD Resistance and Spleen Size QTL on Omy19 in Rainbow Trout: Pleiotropy Versus Linkage

Bacterial cold water disease (BCWD) is caused by infection with Flavobacterium psychrophilum, and results in significant economic losses in salmonid aquaculture. Previously, we identified a major QTL for BCWD resistance on Omy19 (h²_q=0.57-0.67) as well as a QTL for surrogate measures of disease resistance (SW=spleen weight and SI=spleen index).  The objectives of this study were to (1) validate the observed linkage of BCWD resistance with spleen size in an additional genetic background; and (2) test the hypothesis of “close linkage” versus “complete pleiotropy”.  We developed four  full-sib families whose offspring were evaluated for BCWD resistance, SW and body weight.  A total of 800 offspring were genotyped with eight STR markers from Omy19.  We estimated genetic correlation between BCWD resistance and spleen size by fitting a multivariate mixed model with software DMU.  We then ran univariate and bivariate linkage analysis using half-sib family regression analysis with software GridQTL and variance components method with software SOLAR.  We estimated positive non-significant genetic correlation r_g(survdays, SW)=0.21 with heritability h²_survdays=0.42 and h²_SW=0.65.  We validated significant and suggestive QTL for BCWD resistance (11-13 cM; h²_q=0.03-0.21) and spleen size (8-33 cM; h²_q=0.06-0.31) on Omy19. The bivariate linkage analysis of survstatus with SW detected a suggestive QTL (LOD=1.77; h²_q=0.07) at 34 cM. None of the “close linkage” and “complete pleiotropy” models were rejected at P=0.05 significance level. Although non-significant, the log likelihood of “complete pleiotropy” models were higher than those of “close linkage” models at 0-25 cM region which provides a weak support for a pleiotropic QTL.