Additive genetic variation in resistance of Nile tilapia (Oreochromis niloticus) to Francisella orientalis and its genetic (co)variation to both harvest weight and resistance to Streptococcus agalactiae Ib

An understanding of the genetic relationship between traits of economic importance for a widely cultured species such as Nile tilapia (Oreochromis niloticus) is important, in particular the relationship between disease resistance and growth. Francisellosis results in economic losses for the industry, and we confirmed that Francisellosis resistance is under additive genetic control in our population. Analysing four generations, significant additive genetic variation was found for survival to F. orientalis challenge (P < 0.001; Log-likelihood-ratio test) with an estimated heritability of h(2 )= 0.31 +/- 0.05 across all generations. The random effect common to full-sibs was also significant, but of lower magnitude (c2 = 0.075 +/- 0.02) (P < 0.001; Log-likelihood-ratio test). Results of as-sortative mating groups confirmed that genetic gain could be obtained for survival to F. orientalis challenge since offspring of fish selected with high estimated breeding values exhibited higher survival to F. orientalis challenge. The genetic correlation between harvest weight and survival to F. orientalis challenge was very low (r(g) =-0.02 +/- 0.13) and not significantly different from zero (Log-likelihood-ratio test p > 0.05). The genetic correlation between survival to F. orientalis challenge and survival to S. agalactiae Ib challenge was positive but not significantly different from zero (r(g )= 0.14 +/- 0.11; Log-likelihood-ratio test p > 0.05). The genetic correlation between harvest weight and survival to S. agalactiae challenge was negative but also not significantly different (r(g) = -0.19 +/- 0.10; Log-likelihood-ratio test p > 0.05). Lack of significant favourable genetic relationships between economically important traits favours multi-trait selection since selection for growth alone does not improve survival to F. orientalis challenge or survival to S. agalactiae challenge.