Chornobyl dogs show genomic signs of selection after decades in a contaminated environment

A new genomic study of free-breeding dogs living around the Chornobyl Nuclear Power Plant has identified strong signatures of directional selection after more than 40 years in a contaminated environment. The genomic regions involved include genes associated with DNA repair, cell survival, immune responses and cancer-related pathways, although the researchers caution that radiation cannot be separated from the many other environmental pressures present in the exclusion zone.

Dogs have lived within the Chornobyl Exclusion Zone since the 1986 nuclear disaster. The current populations are thought to descend at least partly from pets left behind during the evacuation and have now persisted in the area for more than 30 generations. Previous genomic studies had already revealed marked genetic differences between dogs living immediately around the nuclear power plant and dogs living in Chornobyl City, only around 16 km away. These differences could not be fully explained by breed composition, geographic isolation, inbreeding or increased mutation rates.

Whole-genome analysis of Chornobyl dogs

Researchers initially analysed whole-genome sequencing data from 106 dogs sampled in 2018 and 2019 (58 dogs from the nuclear power plant population ; 48 dogs from Chornobyl City). After removing closely related animals and individuals genetically assigned to the opposite population, 63 dogs were retained for the main analyses, including 33 from the nuclear power plant and 30 from Chornobyl City.

The dataset contained more than 13.5 million single nucleotide polymorphism (SNPs), allowing the researchers to examine genetic differentiation across the genome and search for regions showing patterns compatible with positive selection.

Nuclear power plant dogs remain genetically distinct

The dogs living near the nuclear power plant showed substantially greater genetic differentiation than expected given their geographic proximity to the Chornobyl City population. Pairwise genetic differentiation between the nuclear power plant and Chornobyl City populations reached an FST (a statistical measure of genetic differentiation between populations) of approximately 0.039 in the reduced-density dataset, while comparisons with other European and Russian free-breeding dog populations produced even higher values.

The researchers also compared the Chornobyl dogs with geographically isolated dog populations from the Galápagos Islands. Despite their physical isolation, the island populations did not show the same degree or genomic pattern of differentiation seen in the nuclear power plant dogs. This argues against geographic isolation and genetic drift alone being sufficient to explain the Chornobyl findings.

More than 1,100 candidate loci linked to the nuclear power plant population

Genome-wide analysis identified 1,574 genetic outlier loci, of which 1,138 were specifically associated with the nuclear power plant population. These loci were distributed across 29 of the dog's 38 autosomes. Most were not isolated genetic signals. The researchers identified 86 genomic regions containing multiple significant outlier loci, while 1,094 of the 1,138 loci were located close to another outlier. In total, 247 genes were found within or near the candidate regions. Chromosomes 8, 26 and 27 were among those showing particularly strong differentiation.

Importantly, the majority of the candidate regions were supported by several independent genomic methods. A total of 111 regions containing 1,093 outlier loci were supported by at least three analytical approaches, strengthening the evidence that they reflect selection rather than random demographic effects.

DNA repair, immune response and cell survival pathways

The biological functions associated with these genomic regions are noteworthy. Genes located near candidate loci were enriched in pathways involving:

• DNA replication, recombination and repair

• cell death and survival

• cellular damage and maintenance

• immune and inflammatory responses

• cell-cycle regulation

• antimicrobial responses

• cancer-associated biological pathways.

Cancer, including melanoma and urogenital cancers, emerged among the strongest disease-related pathway associations.

One outlier on chromosome 8 was also located within a region of the T-cell receptor alpha chain, potentially linking some of the observed selection signals with immune function. These functions are biologically relevant because ionising radiation and several environmental toxicants can cause DNA damage, oxidative stress and inflammatory responses.

However, pathway enrichment does not demonstrate that these genes directly confer radiation resistance.

A population decline around the time of the 1986 disaster

The researchers also reconstructed changes in effective population size across previous generations. The nuclear power plant population showed evidence of a decline beginning approximately 35–40 generations ago, broadly compatible with the timing of the 1986 accident and the subsequent efforts to kill abandoned domestic animals within the exclusion zone.

Several mechanisms could have contributed to this decline, including the initial population bottleneck, acute environmental exposure, culling or selection acting against less-fit individuals.

A comparable historical decline was not detected at the same period in the Chornobyl City dog population.

The effective population size of the two present-day groups, however, was not consistently different enough to explain the genomic divergence through drift alone.

Not evidence that Chornobyl dogs are “radiation-proof”

The findings provide strong evidence that directional selection has contributed to the evolution of the nuclear power plant dog population, according to the authors. They do not demonstrate that these dogs have evolved specific resistance to radiation.

The environment surrounding the Chornobyl Nuclear Power Plant contains a complex mixture of stressors. In addition to radionuclides, animals may be exposed to heavy metals, organic contaminants, pesticides and other toxic substances, while food availability, disease pressures and human interactions may also differ between locations.

It is therefore impossible from the genomic data alone to identify which environmental factor, or combination of factors, produced the selective pressure.

The absence of biological samples from the original dog population present before and immediately after the accident also prevents direct reconstruction of how allele frequencies changed through successive generations.

Dogs as sentinels of long-term environmental exposure

The study nevertheless provides a rare example of a free-breeding mammalian population living for dozens of generations in a heavily modified environment. Dogs are particularly interesting because they share many environmental exposures with humans and are increasingly used as sentinel species for environmental health risks.

The concentration of selection signals around genes involved in DNA maintenance, cell survival and immune function therefore warrants further investigation.

Future work combining genomic data with individual measurements of radiation, heavy metals and other contaminants could help determine which exposures are associated with particular genetic variants and whether these changes translate into measurable differences in health, longevity or reproductive fitness.

For now, the study provides strong evidence that the dogs living closest to the Chornobyl Nuclear Power Plant have followed an unusual evolutionary trajectory, one that appears to involve natural selection rather than genetic isolation or random drift alone.