Epistasis in allelic expression at upper temperature tolerance QTL in rainbow trout

We have mapped the location of QTL affecting upper temperature tolerance in three backcross families of rainbow trout (Oncorhynchus mykiss) derived fr om matings between an F-1 male (high (H) X low (L) temperature tolerance se lected lines) and two H and one L line females using variation at 61 micros atellite loci. Physiological epistasis was observed among paternally inheri ted QTL alleles and this depended upon the genomic background in which the alleles were expressed. A marker allele associated with a QTL leading to en hanced thermal resistance in one background (either H or L) showed the oppo site association in the other genomic background. These results suggest tha t the effectiveness of marker assisted selection (MAS) may be limited witho ut prior knowledge of the performance characteristics of QTL alleles in dif ferent genomic backgrounds. (C) 1999 Elsevier Science B.V. All rights reser ved.