Nile tilapia (Oreochromis niloticus) resistance to Francisella orientalis is heritable and not genetically correlated to growth performance

Authors: Shoemaker, Craig; LOZANO, CARLOS - Benchmark Genetics; Lafrentz, Benjamin; Mumma, Wendy - Paige; VELA-AVITÚA, SERGIO - Benchmark Genetics; OSPINA-ARANGO, JOSE FERNANDO - Spring Genetics; YAZDI, M. HOSSEIN - Benchmark Genetics; RYE, MORTEN - Benchmark Genetics

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 6/19/2023
Publication Date: 9/11/2023
Citation: Shoemaker, C.A., Lozano, C.A., Lafrentz, B.R., Mumma, W.P., Vela-Avitúa, S., Ospina-Arango, J., Yazdi, M., Rye, M. 2023. Nile tilapia (Oreochromis niloticus) resistance to Francisella orientalis is heritable and not genetically correlated to growth performance [ABSTRACT]. 21st International Conference on Diseases of Fish and Shellfish, September 11-14, 2023, Aberdeen, UK. 31.

Interpretive Summary: Global tilapia aquaculture is valued at about $11 billion, annually. Francisellosis caused by a gram-negative bacterium (Francisella (F.) orientalis) result in large monetary losses to the the tilapia industry. Losses result not only from fish mortality but also from poor growth and feed conversion due to chronic F. orientalis infection. Collaborative research was undertaken with commercial partners to confirm the potential to selectively breed tilapia for resistance to F. orientalis and to examine the relationship between growth, a major economic trait. Significant additive genetic variation was found for F. orientalis survival across four generations (estimated heritability = 0.31 ± 0.05). Results of assortative mating groups confirmed that genetic gain could be obtained for F. orientalis survival since offspring of fish selected with high estimated breeding values (EBV) exhibited higher F. orientalis survival. The genetic correlation between harvest weight and F. orientalis survival was low and not different from zero. Lack of significant favorable genetic relationships between economically important traits favors multi-trait selection since selection for growth alone does not improve F. orientalis survival.

Technical Abstract: Introduction: Tilapias (Oreochromis sp.) are among the top farmed freshwater fish in the world with an estimated global production value of US $11 billion. Bacterial diseases result in major losses and control relies on health management, vaccination (if available), antibiotics and more recently selective breeding. An understanding of the genetic relationship between traits of economic importance for a widely cultured species such as Nile tilapia (Oreochromis niloticus) is needed, especially the relationship between disease resistance and growth. Francisellosis caused by intracellular Francisella orientalis, results in high economic losses for the tilapia industry not only due to death loss but also due to poor growth and feed conversion in chronically infected fish. Methodology: Nile tilapia (Spring Genetics Strain; 10,167 fish records) were included in the analysis across four generations (G7B1-G10B1) following intramuscular challenge with F. orientalis doses ranging from 9.2 X 103 to 2.3 x 106 CFU/fish depending on generation. Pit tagged fish challenged for each generation were stocked into a single 5000 L tank and mortality pattern, date of death and time of death recorded. 103,685 harvest weight records (G1B1-G10B1) were used to simultaneously obtain estimates of variance components for estimating genetic correlations between traits. Results: Analyzing 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 h2 = 0.31 ± 0.05. Results of assortative 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 (EBV) for survival exhibited higher survival to F. orientalis challenge (e.g., G10 High EBV mean survival = 80 % vs G10 Low EBV mean survival = 14 %). The genetic correlation between harvest weight and survival to F. orientalis challenge was low (rg= -0.02 ± 0.13) and not significantly different from zero. Conclusion: Survival to F. orientalis challenge was heritable and confirmed via assortative mating. Lack of correlation between F. orientalis resistance and growth suggests multi-trait selection is required to improve both traits.