Transcriptional and biochemical markers in transplanted Perca flavescens to characterize cadmium- and copper-induced oxidative stress in the field

Despite recent progress achieved in elucidating the mechanisms underlying local adaptation to pollution, little is known about the evolutionary change that may be occurring at the molecular level. The goal of this study was to examine patterns of gene transcription and biochemical responses induced by metal accumulation in clean yellow perch (Perca flavescens) and metal depuration in contaminated fish in a mining and smelting region of Canada. Fish were collected from a reference lake (lake Opasatica) and a Cd, Cu and Zn contaminated lake (lake Dufault) located in the Rouyn-Noranda region (Qc, Canada) and caged for one or four weeks in their own lake or transplanted in the other lake. Free-ranging fish from the same lakes were also collected. Kidney Cd and Cu concentrations in clean fish caged in the contaminated lake increased with the time of exposure, but metal depuration did not occur in contaminated fish caged in the clean lake. After 4 weeks, the major retinoid metabolites analysed, the percentage of free dehydroretinol (dROH) and the retinol dehydrogenase-2 (rdh-2) transcription level in liver decreased in clean fish transplanted into the metal-contaminated lake, suggesting that metal exposure negatively impacted retinoid metabolism. However, we observed an increase in almost all of the retinoid parameters analysed in fish from the metal-impacted lake caged in the same lake, which we interpret as an adaptation response to higher ambient metal concentration. In support of this hypothesis, liver transcription levels of microsomal glutathione-S-transferase-3 (mgst-3) and glucose-6-phosphate dehydrogenase (g6pdh) were enhanced in clean fish transplanted into the metal-contaminated lake and this up-regulation was accompanied by an increase in the activities of corresponding enzymes, involved in antioxidant response. However, although in the same fish the transcription level of Cu/Zn superoxide dismutase (Cu/Zn sod) was also increased, this did not lead to a change in the activity of the SOD enzyme, suggesting that this upregulation was aimed at maintaining SOD-related antioxidant capacities. In contrast, the transcription level of the cat gene, which did not change in contaminated fish, did not compensate for the decrease of CAT activity. After 4 weeks of exposure, some plastic responses of the clean fish were observed when they were transplanted in the metal-contaminated lake. However, probably as a consequence of the prior 80 years of exposure to metals, the contaminated population showed a limited plastic response in the expression of the majority of the candidate genes tested, when they were transplanted in the reference lake. The overall findings of this field investigation highlight how yellow perch molecularly and biochemically responded to a sudden or relatively long-term exposure (4 weeks) to a cocktail of metals.