Quantitative trait loci for upper thermal tolerance in outbred strains of rainbow trout (Oncorhynchus mykiss)

The expression of three putative QTL for upper thermal tolerance (UTT) was examined in two strains of outbred rainbow trout unselected for this trait using simple-sequence repeat (SSR; microsatellite) markers associated with UTT in backcrosses of lines selected on this trait. Two-way diallel lots in the third generation of an outbred pedigree were exposed to an acute therm al challenge. QTL detection was performed separately by each second-generat ion parent within each diallel lot, incorporating the effects of full sib f amilies and correlated traits. Inheritance of different alleles at the SSR Ssa20.19NUIG from the sire 93-32-1 was strongly associated with the thermal tolerance of his half sib progeny, explaining 7.5% of their phenotypic var iance in this trait. A hierarchical linear model incorporating allelic inhe ritance from all four grandsires of the experimental diallels (in addition to family specific and covariate trait effects) was also used to detect ass ociations between the SSR and thermal tolerance in their third-generation g randprogeny. Ssa20.19NUIG was strongly associated with thermal tolerance in the grandprogeny of the grandsire G(0)SVM2. The generally stronger marker- trait associations found in male parents may be partially due to reduced ch romosomal recombination rates in male salmonids compared to females. These results indicate the effects of a QTL on a fitness-related trait in unselec ted populations of rainbow trout.