Our work included collecting the samples in the Amazon jungle, processing all the samples collected and analyzing the data. During the project, some of our highlights included developing a novel technique for capturing birds in a challenging set-up, generating at least three new reference genomes, successfully sequencing whole genomes of historic specimens and identifying genes involved in the evolutionary response to mercury contamination in three species of birds.
Our results demonstrate the link between the environmental pollution of mercury (as a result of goldmining activities), elevated mercury levels in birds, the cost to the birds’ fitness (i.e. health) and the genomic mechanism generating those responses. We found that mercury levels in lake sediments were higher in areas with artisanal goldmining activities (i.e. unprotected areas), corresponding with known biochemical processes of ionic mercury methylation by sediment bacteria. Similarly, mercury levels in birds were significantly highest in individuals captured in the unprotected areas. Moreover, levels of the stress hormone corticosterone were significantly lower in birds with higher mercury loads, suggesting that fitness is decreased. Despite not finding evidence of allele frequency changes correlating with high mercury loads (i.e. genome level), we successfully identified genes that are being activated differently in birds with high mercury levels (i.e. transcriptome level). Many of these genes were linked to enzymatic activity and cellular functions, and some had been previously identified in connection with responses to toxicity. In addition, our analysis of the historic specimens indicated that pre-goldmining levels were significantly lower than contemporary levels, suggesting that the source of the excess mercury in the environment is human induced.
One of the strongest patterns that emerged from the integration of all these results is the effect of guild (i.e. diet) on the evolutionary response experienced by birds in response to mercury contamination. Piscivores (fish eaters) were affected the most by environmental mercury pollution; mercury loads in their tissues were the highest, their stress hormone levels changed the most, and the difference in patterns of gene expression was the most noticeable, compared to insectivores, followed by granivores. These results reflect known biomagnification of mercury through the food chain corresponding with higher loads in bigger food items (fish) relative to smaller food items (insects) or food items not associated directly with water (seeds). The repetition of this pattern in all the biological aspects measured reinforces the significance of the role of biomagnification of mercury up the food chain, and emphasizes the importance of integrating relevant biological information in exploring toxicological processes and their impact on animal species, including humans.
