City life can be difficult for many organisms. Cities are louder, brighter, hotter, and often more contaminated with toxins than nearby less-developed areas. Ever-increasing research is showing that organisms are coping with the urban environment in many ways. Urban great tits (Parus major) living in areas with high amounts of noise pollution sing at a higher minimum frequency, which prevents their songs from being masked by low-frequency city noise. Orb spiders (Larinioides sclopetarius) in Vienna, Austria build more webs on artificially lit handrails than on handrails without artificial lighting. The artificial lighting attracts more insects, meaning that spiders who build their nests near the artificial light capture more prey. And killifish (Fundulus heteroclitus) living in polluted waters have had repeated selection on their aryl hydrocarbon receptor-based signaling pathways allowing them to survive in waters polluted with PCBs.
One of the better studied environmental contaminants is lead (Pb) since it is known to be toxic and can have severe human health consequences. Lead levels are known to be elevated in many cities and lead can be found in dust, soil, and water. Soil metal concentrations are usually regulated by the federal government and in Australia, national guidelines dictate that soil lead levels are under 300 mg/kg. This guideline was put in place to ensure that blood lead levels remain below 7.5ug/dL. In the United States, the Environmental Protection Agency has a standard of 400 ppm of lead by weight in play areas and 1200 ppm of lead by weight for non-play areas and the Center for Disease Control (CDC) recommends that children have blood lead levels under 5ug/dL. However, the National Institute of Health notes that blood lead levels as low as 2ug/dL can be considered harmful to humans. All this to say, lead is incredibly toxic to humans, and organisms that are constantly exposed to high lead levels are likely forced to adapt.
Using house sparrows (Passer domesticus) as a focal species, Dr. Samuel Andrew from Macquarie University and colleagues investigated signs of adaptation to trace lead contamination across Eastern Australia. Andrew et al. collected 176 total individuals from 11 sites and genotyped them using the house sparrow Affymetrix 200 K SNP array. The collection localities included two mining locations. Soil lead levels at nonmining sites ranged from 46-210 mg/kg while at mining sites lead levels ranges from 638-1500 mg/kg.
After sequencing the DNA and SNP filtering, Andrew et al. had 162,299 SNPs that could be used in downstream analysis. With these 162K SNPs and the estimates of environmental lead levels, the authors used the program BayeScEnv to do ecological association analysis. This type of analysis allows researchers to test if there is a significant association between the different allele frequencies in their samples and the environmental lead contamination. The BayeScEnv models found 60 SNPs that were significant and likely associated with environmental lead levels. These 60 SNPs were physically linked to 39 genes and 12 of these genes are likely relevant to lead exposure. Two of the SNPs were significant in all three of the models that were run in BayeScEnv. One of these was on chromosome 3 and linked to a gene associated with endoplasmic reticulum membrane structure. Going back to general biology, the endoplasmic reticulum is a cell organelle that forms a network of flattened sacs that fold proteins and transports them to the Golgi apparatus. One of the SNPs in the predicted model was linked to a transmembrane metal ion transporter which is known to transport zinc and other metals.
Further research and experiments are needed to understand these candidate genes and how house sparrows are adapting to high amounts of lead exposure. Anthropogenic related pollution and contamination occur quickly on the evolutionary time scale and research like this can help scientists gain a better understanding of how populations are able to respond to extreme environmental degradation. Moreover, this type of research can help urban evolutionary biologists begin to understand if selection acts upon similar pathways in taxonomically diverse species exposed to the same environmental pollutants. With this information, we will be able to better predict the ways in which species cope with the multitude of pollutants they are exposed to within cities.
Featured Image: “House Sparrow” by anuradhac is licensed under CC BY-NC 2.0
