Identification of a novel allele of peroxisome proliferator-activated receptor gamma (PPARG) and its association with resistance to Aeromonas salmonicida in Atlantic salmon (Salmo salar).

Bacterial and viral diseases are major problems in Atlantic salmon aquaculture, but may be challenged through selection of brood stock with enhanced survival to diseases. Today's selection strategy is based on controlled challenge tests using siblings of the breeding candidates, and is thus indirect. Direct trait records on breeding candidates can potentially be provided through identification of genetic variation linked to the susceptibility to the disease. Peroxisome proliferator-activated receptor gamma (PPARG) is a lipid-sensing transcription factor primarily known for inducing fat-accumulation in adipocytes, but also in lipid-accumulating macrophages, in mammalian species. Here we report a novel allele of PPARG, pparg-2, in Atlantic salmon. pparg-2 has an insertion of sixty nucleotides that encodes two additional copies of the almost perfect decapeptide motif, (F/C/Y)NHSPDR(S/N)HS, compared to the previously described pparg-1. pparg-1 contains six copies of this repeat unit whereas eight copies are present in the novel pparg-2 allele. pparg-2 mRNA was detectable in kidney and spleen of random Atlantic salmon samples. Here, we studied the effect of pparg-1 and pparg-2 on survival upon challenge to a highly virulent bacterium, Aeromonas salmonicida, causing furunculosis, and the virus causing infectious salmon anaemia (ISA), respectively, in a Norwegian aquaculture population of Atlantic salmon. ppar alleles were found to be significantly associated with survival upon challenge to A. salmonicida, but not to ISA. pparg-2 was the better allele in terms of survival in the challenge test for furunculosis, survival rates being 0.32, 0.40 and 0.42 for animals with the pparg-1,-1, pparg-1, -2 and pparg-2, -2 genotypes, respectively. We conclude that pparg-2 is in linkage disequilibrium (LD) with, or identical to, a locus contributing to different susceptibility to furunculosis in Atlantic salmon. PPARG was mapped to linkage group eight (LG8) but could only be positioned on the male linkage map since all the informative parents in the mapping families were males. This is the first report showing an association between pparg alleles and an enhanced immune response in fish.