Background and Significance of Study

Molecular mechanisms of disease resistance
Infection of salmonids by the myxozoan parasite Myxobolus cerebralis can cause whirling disease, which is responsible for high mortalities in rainbow trout hatcheries and natural populations in the United States. Although considerable research has provided insight into disease pathology, host invasion, and inheritance patterns of resistance, the causal genetic variants and molecular mechanisms underlying host resistance or susceptibility remain elusive. To increase knowledge in this area, we have used QTL mapping and gene expression approaches to identify how resistant and susceptible rainbow trout strains differ genetically and in expression patterns during the early stages of infection.

Monitoring disease resistant rainbow trout lineages in Colorado rivers

The focus of this work is to monitor relative proportions of rainbow trout lineages in Colorado waters infected with the pathogen Myxobolus cerebralis, the causative agent of whirling disease. Using RADseq, we have identified a panel of Single Nucleotide Polymorphism (SNP) markers capable of distinguishing pure German from Colorado River Rainbow and Harrison Lake strains along with their hybrid progeny (F1, F2 and first generation backcrosses). We are using these SNP markers to identify the recent ancestry of individual rainbow trout collected from the wild. This work will allow our collaborators at Colorado Parks and Wildlife and Colorado State University to monitor successful reproduction and recruitment of rainbow trout with whirling disease resistant ancestry in locations where several year-classes of these fish have been stocked.

Results

We have made significant progress towards understanding the genetic basis of whirling disease in rainbow trout through the use of gene expression and quantitative trait loci (QTL) mapping. Gene expression results produced the first candidate gene implicated in the whirling disease resistance phenotype (Baerwald et al., 2008) by comparing Hofer and Trout Lodge rainbow trout strains using microarray technology. A time course study identified several additional candidate genes differentially expressed in early host response to pathogen exposure and implicated Th17 cell differentiation as a potential target for future studies (Baerwald, 2013). QTL mapping of progeny from Hofer and Colorado River (CRR) rainbow trout crosses identified a chromosomal region that explained 50 – 86% of the phenotypic variation and 100% of the genetic variation for whirling disease resistance (Baerwald et al., 2011). The study also revealed a specific allele that appears fixed in the Hofer stain and is always present in whirling disease resistant progeny but is very rare in the CRR strain.

Collaborators

Eric Fetherman, Colorado Parks and Wildlife

Publications

Fetherman ER, Winkelman DL, Baerwald MR, Schisler GJ. 2014. Reduced susceptibility and infection severity of age-0 rainbow trout following Myxobolus cerebralis-resistant rainbow trout introduction to the upper Colorado River. PLOS ONE 9(5).

Baerwald MR. 2013. Temporal expression patterns of rainbow trout immune-related genes in response to Myxobolus cerebralis exposure. Fish and Shellfish Immunology 35: 965-971.

Baerwald MR, Petersen JP, Hedrick RP, Schisler GJ, May B. 2011. A major effect quantitative trait locus for whirling disease resistance identified in rainbow trout (Oncorhynchus mykiss). Heredity 106: 920-926.

Baerwald MR, Welsh AB, Hedrick RP, May B. 2008. Discovery of genes implicated in whirling disease infection and resistance in rainbow trout using genome-wide expression profiling. BMC Genomics 9: 37.

For more information contact Melinda Baerwald: mrbaerwald@ucdavis.edu