Maria Hansson

In vertebrates, studies of the aryl hydrocarbon receptor (AhR) have been of particular interest due to its direct involvement in the toxic effects exerted by several environmental pollutants, such as PCB and dioxin. After binding to the toxic molecule, this receptor initiates transcription of several genes that are part of the biotransformation battery but most notably the cytochrome P450 (CYP) genes, which results in an excessive release of oxidative enzymes and free radicals. The AhR is represented by a single gene in mammals (AhR1) but two distinct genes reside in other vertebrates like birds and fish (AhR1 and AhR2). The Atlantic salmon (Salmo salar L.) is a common top-predator to the Baltic Sea, a region that is one of the most contaminated bodies of water within Europe after a long history of high pollution exposures. In this thesis, I describe the identification and characterization of six AhR genes in the Atlantic salmon, two AhR1 and four AhR2 genes. I have investigated if AhR genotypes interact differently with individual load of PCBs so mediating differences in AhR/CYP1A1 transcription levels and thereby also affecting tissue levels of antioxidants in foraging individuals from natural populations of Atlantic salmon in the Baltic Sea. I have identified allelic variation in the AhR2 5´-flanking region and measured the levels of PCB and astaxanthin in muscle tissue. The transcription levels of the AhR2 genes and the CYP1A1 gene were measured by real-time quantitative PCR. The analyses revealed that the relative transcription levels of the AhR2a, AhR2g and AhR2d genes were significantly correlated with the relative transcription levels of the CYP1A1 gene and that the transcription of CYP1A1 was negatively correlated with antioxidant levels (i.e. astaxanthin) in muscle. The transcription level of one of the salmon AhR genes, AhRa, was significantly associated with the AhR2a 5´-flanking region genotype and with the interaction of the genotype and individual PCB level. There was no correlation between the levels of PCB in muscle and the transcription levels of CYP1A1. These results suggest that biotransformation activity in wild Atlantic salmon individuals from the Baltic Sea may be affected by genetic polymorphisms at the AhR loci. Finally, I have investigated the evolution of AhR genes in Atlantic salmon and vertebrates by searching for AhR homologs in the database containing the complete genome of the pufferfish (Fugu rubripes). I have described the evolution of vertebrates and fish in an attempt to explain the incidence of multiple AhR genes in the salmon genome, concluding that these genes probably arose from several whole-genome duplication events in the ancestors of salmonid fish.