Additive genetic variation in resistance of Nile tilapia (Oreochromis niloticus) to Streptococcus iniae and S. agalactiae capsular type Ib: Is genetic resistance correlated?

Streptococcus iniae and S. agalactiae are both economically important Gram positive bacterial pathogens affecting the globally farmed tilapia (Oreochromis spp.). Historically, control of these bacteria in tilapia culture has included biosecurity, therapeutants and vaccination strategies. Genetic gains in performance traits have been realized for Nile tilapia (Oreochromis niloticus) and interest in breeding for disease resistance has recently received attention. The goal of this study was three fold: 1) to verify previous results demonstrating heritability of S. iniae resistance in Nile tilapia families using increased numbers of fish per family; 2) to determine if realized genetic gain in resistance and/or susceptibility to S. iniae can be obtained following positive assortative mating between parents with high or low estimated breeding values (EBV); and 3) to determine if resistance to S. iniae and S. agalactiae capsular type Ib is genetically correlated. A total of 144 and 130 full sib families were challenged intraperitoneally with S. iniae and intramuscularly with S. agalactiae Ib, respectively. Cumulative mortality at test end was 46% for S. iniae and 68% for S. agalactiae. There was a high additive genetic component found for survival in fish injected with S. iniae (estimated heritability 0.52±0.12) and with S. agalactiae (estimated heritability 0.38±0.11). The S. iniae challenge results confirmed additive genetic variation in resistance of Nile tilapia to S. iniae. We also demonstrated via assortative mating that genetic gain for survival to S. iniae is possible. The genetic correlation between resistance to S. iniae and S. agalactiae Ib was not significantly different from zero (rg=−0.30±0.19). The lack of correlation suggests if resistance to both Streptococcus sp. is desired, selection for both traits must be simultaneous. Selection of fish to improve survival to Streptococcus sp. may require a thorough understanding of the type of pathogen prevalent in the region so that custom genetic material may be tailored to meet the needs of the individual farm and/or region.