Nile tilapia (Oreochromis niloticus) resistance to Streptococcus iniae and S. agalactiae Ib is heritable and can be improved through selective breeding

Authors: Shoemaker, Craig; Lafrentz, Benjamin; LOZANO, CARLOS - Akvaforsk Genetic Center As; RYE, MORTEN - Akvaforsk Genetic Center As; OSPINA-ARANGO, JOSE - Akvaforsk Genetic Center As; Garcia, Julio; Mumma, Wendy - Paige; Bader, Troy; Shelley, John; Beck, Benjamin

Submitted to: International Aquaculture Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 8/18/2019
Publication Date: 10/22/2019
Citation: Shoemaker, C.A., Lafrentz, B.R., Lozano, C., Rye, M., Ospina-Arango, J., Garcia, J.C., Mumma, W.P., Bader, T.J., Shelley, J.P., Beck, B.H. 2019. Nile tilapia (Oreochromis niloticus) resistance to Streptococcus iniae and S. agalactiae Ib is heritable and can be improved through selective breeding [abstract]. International Aquaculture Meeting. 3.

Interpretive Summary:

Technical Abstract: Tilapias (Oreochromis sp.) represent an important source of protein with a global ecomonic value approaching US $8 billion yearly. Two major bacterial pathogens of farmed tilapia, Streptococcus iniae and S. agalactiae, have an annual negative economic impact estimated around $1 billion. Because of the difficulty controlling these bacteria in tilapia production, selective breeding for resistance to the pathogens is a potential tool to improve health and welfare of cultured tilapia. Today selection for increased resistance to a range of pathogens constitute a significant part of advanced breeding programs for species like salmonids and shrimp. In a collaborative effort initiated in 2014, our teams have pioneered large scale studies investigating the genetic background for resistance to S. iniae and S. agalactiae capsular type Ib in Nile tilapia (Oreochromis niloticus). The main objectives for this work have been to 1) to assess the level of additive genetic variation in resistance of Nile tilapia to S. iniae and S. agalactiae Ib; 2) to document realized selection response for resistance to S. iniae and S. agalactiae Ib based on assortative mating of parents ranked on estimated breeding values (EBV) derived from data from controlled challenge tests and subsequent testing offspring; and 3) to determine if resistance to S. iniae and S. agalactiae Ib are genetically correlated traits. Up-to 144 full- and paternal half-sib families were challenged intraperitoneally with S. iniae using pit tagged fish in a common tank over multiple generations. For S. agalactiae, up-to 142 full and paternal half-sib families were intramuscularly injected. Cumulative mortality ranged from 30-60 % for S. iniae and 49-68% for S. agalactiae Ib depending on year class tested. Estimated heritabilities (± SE) were in the range of 0.37 (0.11) - 0.52 (0.12) and 0.29 (0.02) - 0.38 (0.11) for resistance to S. iniae and S. agalactiae Ib, respectively, demonstrating significant potential for improvement through selective breeding. Results from subsequent challenge of offspring produced from assortative mating of parents selected based on EBVs demonstrated mean survival of 88 % (range 60 – 100%) for families produced on high EBV (i.e., S. iniae resistant parents), and only 10 % (range 0 - 42%) for families produced on low EBV (i.e., S. iniae susceptible parents) confirming selection responses. Similar results were seen for resistance to S. agalactiae Ib. No statistically significant genetic correlation was found between resistance to S. iniae and S. agalactiae Ib. Based on these results routine selection for increased resistance S. iniae and S. agalactiae was implemented in Spring Genetics’ breeding program for Nile tilapia in 2016. This represents a global milestone for applied selection work in tilapia.