A major effect quantitative trait locus for whirling disease resistance identified in rainbow trout (Oncorhynchus mykiss)

Abstract

Whirling disease, caused by the pathogen Myxobolus cerebralis, leads to skeletal deformation, neurological impairment and under certain conditions, mortality of juvenile salmonid fishes. The disease has impacted the propagation and survival of many salmonid species over six continents, with particularly negative consequences for rainbow trout. To assess the genetic basis of whirling disease resistance in rainbow trout, genome-wide mapping was initiated using a large outbred F(2) rainbow trout family (n=480) and results were confirmed in three additional outbred F(2) families (n=96 per family). A single quantitative trait locus (QTL) region on chromosome Omy9 was identified in the large mapping family and confirmed in all additional families. This region explains 50-86% of the phenotypic variance across families. Therefore, these data establish that a single QTL region is capable of explaining a large percentage of the phenotypic variance contributing to whirling disease resistance. This is the first genetic region discovered that contributes directly to the whirling disease phenotype and the finding moves the field closer to a mechanistic understanding of resistance to this important disease of salmonid fish.

Figure 1

Pathogen load distributions for five rainbow trout families exposed to M. cerebralis. Pathogen loads are the log₂-transformed number of M. cerebralis 18S rDNA gene copies per 10⁶ rainbow trout cells. All families demonstrate a bimodal frequency distribution with the majority of individuals either completely resistant (that is, no pathogen load) or very susceptible (that is, very high pathogen load).

Figure 2

Localization of whirling disease resistance QTL on rainbow trout chromosome Omy9. Map positions and LOD scores are based on combining four outbred F₂ mapping families into a single interval mapping QTL analysis using the software MapQTL 6. The combined family 95% genome-wide LOD significance threshold value is 6.6 (dashed constant line). All markers with LOD scores>20 are shown in bold. The QTL (termed WDRES-9) has a LOD peak of 123.3 and the 95% confidence interval is a 1.85 cM region including marker BX310634.