2010 Annual Report
An engineered strain of biotype 2 Y. ruckeri was developed which constitutively expresses the green fluorescent protein (GFP). GFP expression was exceptionally stable and the strain retained its ability to colonize and cause disease in immersion-challenged rainbow trout. This strain has been used for detecting and imaging the interactions between this pathogen and its natural salmonid host. A standardized disease challenge model has been established with this strain.
As part of a multifaceted approach to disease control, we determined variation in survival between families, heritability of post-challenge survival, the stability of resistance during growth, and the response to selection. Over 10,000 fish from 200 families were evaluated by 21 day challenge. The heritability of post-challenge survival, an indicator of disease resistance, was estimated to be 0.35 ± 0.09 and there was no genetic or phenotypic correlation with growth rate, or the 9 and 12 month body weights. Fish maintained their relative “resistant” or “susceptible” phenotype as average body weight increased over 300-fold. These results demonstrate that the mechanism(s) responsible for the higher survival are not transitory during fish development and suggest that our findings are relevant to a wide range of rainbow trout sizes. These results demonstrated that selective breeding is an effective approach to improving resistance to bacterial cold water disease challenge in rainbow trout.
A major need in selective breeding is the development of easy-to-measure surrogates of disease resistance. We demonstrated that resistant fish crosses had, on average, a larger spleen size than susceptible fish crosses. Selecting fish crosses solely based on spleen size was found to predict resistance to the bacterial cold water disease agent indicating a link between these two traits. Since spleen size is easy to measure, it may be a useful selection parameter for evaluation in other fish populations.
A number of novel rainbow trout immune genes were discovered and described that belong to the chemokine, tumor necrosis factor, and toll-like receptor families of immune genes. We reported the discovery of two new rainbow trout chemokine genes: CXCd1 and CXCd2. A comprehensive analysis of the TNF superfamily in teleost fish was completed. Forty-two novel genes were identified from database searches and sequencing efforts.