The latest research on the wild population of dogs in Chornobyl reveals that the genetic differences between these dogs and other similarly isolated dog populations is likely evidence of directional selection, not mutation. The work suggests a potential association between genetic selection and environmental variables and could advance understanding of local adaptation and genomic resilience in contaminated environments.
Previously, researchers had looked at genetic differences in two populations of dogs: one located at the Chornobyl Nuclear Power Plant (NPP) and the other in Chornobyl City, approximately 10 miles away. They found that the genetics of Chornobyl City dogs were very similar to dog populations in Russia, Poland and the surrounding areas, whereas those from the NPP had some distinct differences – specifically in regions associated with DNA repair.
“This work ties up some of the loose ends from our previous work, where in characterizing the population’s genetic structure we found that while these two dog groups are geographically very close, they have a very high degree of genetic difference, and those differences aren’t due to mutation,” said Megan Dillon, postdoctoral research scholar at North Carolina State University’s College of Veterinary Medicine.
“With this work we wanted to explore two additional larger-scale evolutionary drivers – are the genetic differences due to selection, or are they due to genetic drift over time in an isolated population?”
Through natural selection, genes that increase an individual’s environmental fitness become more prevalent in a population over time. Directional selection is a more notable type of natural selection in which an entire population shifts toward a small number of favored traits over time. Directional selection is typically associated with environmental changes.
Genetic drift refers to random genetic changes not necessarily associated with fitness that nonetheless become common in a population. Genetic drift is more prevalent in small, isolated populations because the likelihood of those random changes being passed down is greater, and can result in reduced genome-wide diversity.
The research team compared the genomic data of the two Chornobyl populations with that of free-breeding dogs from Eastern Europe and with an isolated dog population from the Galápagos Islands, to try and tease out the effects of isolation versus environment on the NPP dogs.
While the effective population size of the NPP population is small, it didn’t differ significantly from that of the Chornobyl City population. The NPP also didn’t show evidence of reduced genetic variation consistent with genetic drift.
Genetically, the NPP population differed significantly from other geographically isolated dog populations, including those from the Galápagos Islands. In contrast, the dog population in Chornobyl City was found to be genetically more similar to dog populations in the Galápagos Islands than to the nearby NPP population.
Finally, genomic analysis of the NPP dogs identified evidence of directional selection at over 100 locations. The genes located around these loci are associated with cancer development, DNA replication and repair, cell cycle functions, and immune function.
“While we can’t say for certain whether what we’ve found can be directly confirmed as selection, we do find these genomic signatures suggestive of selection,” Dillon said. “When we dig deeper into those genomic regions, we find that these genes are involved in biologically relevant pathways that could allow them to overcome the effects of being exposed to toxicants in this environment.”
“When we consider the toxic environment around Chornobyl, I can see why most people would think, ‘Surely there’s got to have been some consequence of living in that environment,'” said Matthew Breen, Oscar J. Fletcher Distinguished Professor of Comparative Oncology Genetics at NC State’s College of Veterinary Medicine and corresponding author of the study.
“But what we’re saying is the consequence may not have gone as far as changing the dogs’ genome in order to adapt, it could be the ones that were able to survive the initial exposures had this level of underlying genetic diversity and variants to offer a higher degree of fitness,” Breen added. “Obviously that’s speculation, and we don’t know exactly what’s happened. But it is fair to say that we used a very robust analysis and found signatures of selection in biologically relevant regions.”
The study appears in Genes. Dillon is the first author. Other contributors were Reade Roberts, associate professor of biology at NC State; Jennifer Betz, DVM with Visiting Veterinarians International; Timothy Mousseau, professor of Biological Sciences at the University of South Carolina; and Norman Kleiman, associate professor of Environmental Health Sciences at Columbia Mailman School of Public Health.
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