Location: Cool and Cold Water Aquaculture Research
Title: Comparison of disease resistance between diploid, induced-triploid, and intercross-triploid rainbow trout including trout selected for resistance to Flavobacterium psychrophilum Authors
Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 18, 2013
Publication Date: June 25, 2013
Citation: Weber, G.M., Wiens, G.D., Welch, T.J., Hostuttler, M.A., Leeds, T.D. 2013. Comparison of disease resistance between diploid, induced-triploid, and intercross-triploid rainbow trout including trout selected for resistance to Flavobacterium psychrophilum. Aquaculture. 410-411, 66-71. Interpretive Summary: The rainbow trout industry is increasing the use of all-female triploid fish. Triploid fish have three sets of chromosomes instead of the normal two sets (diploid). All-female triploid fish are advantageous in rainbow trout aquaculture due to sterility in which only the female fish fail to produce gonads. As a result the triploid fish have superior fillet quality and higher growth and feed conversion rate once they reach reproductive size. Triploid fish are commonly produced by pressure or temperature shock of the eggs (induced-triploids, 3NP). We have recently improved procedures for an alternative strategy of intercross mating of a tetraploid parent with a diploid (2N) parent (referred to as intercross-triploids, 3NC). Tetraploids are produced in a similar manner as 3NPs but have four sets of chromosomes and are fertile. While little is known about the performance or disease resistance of 3NC fish, we hypothesized that they may be superior to triploids produced by pressure treatment (3NP) as they would not have the collateral damage inflicted by high pressure exposure. In this study, we compared disease resistance of genetically related 2N, 3NP and 3NC families and measured post-challenge survival following pathogen exposure. Furthermore, we have recently developed a line of rainbow trout that have improved resistance to a bacterial pathogen, Flavobacterium psychrophilum, the cause of bacterial cold water disease (BCWD) which is one of the most problematic diseases of cultured rainbow trout. It is not known if fish selected for disease resistance as diploids will be resistant as triploids. We therefore compared the resistance of 2N, 3NP, and 3NC rainbow trout to BCWD. We conducted studies using fish from our BCWD resistant line, a BCWD high susceptible line, and a control line. Overall, survival was slightly better for diploids than triploids generated by either triploid production technique and survival was slightly better for 3NC than 3NP fish. Furthermore, the families that were more resistant to BCWD as 2Ns were also more resistant as 3NPs and 3NCs suggesting selection for disease resistance as 2Ns is maintained when the fish are used to make triploids. In summary, while triploidy in general may have a modest and inconsistent detrimental effect on disease resistance, our results suggest that progress in family-based selective breeding of diploid fish for BCWD resistance will translate into improved survival when triploids are derived either by shock treatment or intercross breeding.
Technical Abstract: All-female triploid fish are advantageous in rainbow trout (Oncorhynchus mykiss) aquaculture due to sterility and the consequent superior fillet quality, growth, and feed conversion achieved at reproductive size. Triploid fish are commonly produced by pressure or temperature shock of the zygote (induced-triploids, 3NP), but can also be produced by mating a tetraploid parent with a diploid (2N) parent (intercross-triploids, 3NC). Little is known about the disease resistance of 3NC fish. In this study, we measured post-challenge survival of genetically-related 2N, 3NP and 3NC families after exposure to Flavobacterium psychrophilum the etiological agent of bacterial cold water disease (BCWD). The families used in this study originated from either an unselected commercial stock or BCWD resistant or susceptible lines of rainbow trout. The overall survival of 2N fish was slightly higher than the respective 3NP and 3NC fish. Although 3NC fish tended to show higher survival than 3NP fish, this response was not sufficiently consistent to promote intercross production of triploids for increasing disease resistance. The survival of ten sets of 3NP, 3NC and 2N families, created from BCWD resistant and susceptible line parents, demonstrated overall similar inherent relative survival differences among the ploidy types both among families and between lines, although family differences in response to triploidization and triploidization strategy were observed. In summary, minimal survival difference between 3NP and 3NC families following experimental challenge suggests that innate resistance to F. psychrophilum is not substantially altered by the triploidization strategy and progress in family-based selective breeding of diploid fish for BCWD resistance will substantially translate into improved survival when triploids are derived either by shock treatment or intercross breeding.