After a long career as a science writer, textbook author, and genetic counselor, I’ve become an accidental authority on squashes. I began…
In the original Planet of the Apes, the Forbidden Zone is a future radiation-devastated landscape from which hardy new mutants arise, shifting the evolutionary course of humanity.
The “nuclear exclusion zone” sounds similar, but is real, referring to the 3,004-square-kilometer (about 1,160-square-mile) environs of the Chernobyl nuclear power plant that exploded on April 26, 1986, at 1:23:58 am. The Chernobyl Dog Research Initiative has published their analysis of genetic changes among the dogs who live today in what researchers call the aftermath of “an ecological catastrophe of massive proportions.” Gabriella Spatola and Elaine Ostrander of the National Human Genome Research Institute and colleagues report their findings in Science Advances.
Soon after the explosion, as humans fled, workers remained to clean up and to cull the canine population. But some dogs survived, and the workers as well as tourists have cared for them since. Yes, Chernobyl was and remains a tourist destination, by jeep or jet.
An Unnatural Experiment
The area instantly became a genotoxic zone. Damage to DNA from ionizing radiation ranges from single-DNA-base changes to shattered chromosomes, while heavy metal poisoning chops DNA across both strands and strangles repair. With faulty DNA repair, mutations accrue.
Reaction to the continuing exposure is at the population level too. Migration is severely constrained, and so animals trapped with their relatives mate with them, a little like the incest theme of the novels Flowers in the Attic and Middlesex. Genetic diversity declines rapidly in the face of forced inbreeding.
Over time, natural selection favors mutations that provide a reproductive advantage in the new landscape, while weeding out non-adaptive traits. Ultimately, new gene pools emerge from the population bottlenecks that winnow the survivors, with chance sampling favoring some gene variants (aka mutations) while eliminating others.
So far genetic studies of the nuclear exclusion zone have focused on vegetation, small animals, and genome changes in people who developed thyroid cancer following the exposure. The Chernobyl Dog Research Initiative formed in 2017 to follow the ballooning canine population near the reactor remains – some 800 animals, for dogs don’t know they’re living in a hot zone and have no reason to flee as long as the workers continue to care for them.
The researchers collected and preserved blood samples from dogs exposed to different radiation levels in several places:
• The Chernobyl New Safe Confinement structure built to contain radioactivity from the damaged reactor
• A train station in the midst of the contamination
• The dense forest around the city of Pripyat, where many workers lived, about 3.5 km (2.2 miles) from the reactor
• Chernobyl City about 15 kilometers (9.3 miles) away, where a few hundred people remain
• Slavutych, 45 kilometers (28 miles) away in Belarus where most of the exposed families relocated.
Radiation levels differ. For example, radiation in the power plant from the breakdown of cesium-137 is from 10 to 400 times higher than it is in Chernobyl City. The half-life exceeds 30 years, so the contamination will be around for awhile.
DNA Reveals Robust Sex Lives
The researchers compared 129,497single-DNA-base sites in the genome that vary among individuals (SNPs) for 302 free-roaming Chernobyl dogs, and compared the data to those from other canine populations. Novel gene variants among the animals were dubbed “survival loci” (a locus is the site of a gene on its chromosome).
The data revealed three groups of dogs corresponding to the three geographic locations, but with enough overlap to suggest a canine version of cross pollination, what we geneticists dub “gene flow”. The dogs from the train station were the most adept at getting out and mingling, according to their SNPs. In addition, animals from the surrounding areas have more stretches of identical SNPs on both members of chromosome pairs than do other dogs. Such homozygosity is a telltale sign of inbreeding. Parent-offspring matings were the most common.
The researchers compared the Chernobyl dog genomes to those of “free-breeding dogs” – aka mutts – from elsewhere, and also to genomes of 1,324 purebreds. The irradiated animals are most similar to German shepherds and Eastern European shepherds. One dog from Chernobyl City seems to have had recent run-ins with a Siberian huskie and a pinscher. Clearly, mysteries remain.
Overall, the analysis revealed 15 complex family trees unique to the Chernobyl population compared with the genomes of other dogs globally. And the same mutations popped up in dogs from different parts of the exclusion zone. It’s more likely that the animals moved about and mated, sharing their genes, than that the same mutations happened by chance repeatedly. By comparing the DNA data from groups-within-groups, the researchers can even deduce specific matings, like confiscating phones and scrutinizing dating apps to reconstruct possible hookups.
The largest family has 162 dogs from different locations. The seven smallest genetically-distinct family groups consist of only two animals each, a parent and offspring. Most of the animals belong to one of the three large groups, each with more than 10 members. It’s complicated. For example, 10 of the 14 train station dogs are in the same family, but a single animal links them to the Pripyat family.
Odd was the absence of the expected “popular sire effect,” in which a specific male’s genome echoes loudly through a population because he mates more than other males, like Genghis Kahn’s Y chromosome ending up in many males of our own species. This didn’t happen. Perhaps in the curious conditions of the spent reactor, the odd emptiness, mating among the resident dogs was common. Everyone did it. But the apparent absence of a studly canine could reflect biased sampling – researchers might have missed males that contributed the most to the next generation. Capturing females was easier.
The investigators will continue to track and analyze this unique window into what they call the “genomic scarring caused by long-term multigenerational radiation exposure from environmental effects of a human-wrought disaster.“