Population dynamics and genome-wide selection scan for dogs in Chernobyl

Is increased mutation driving genetic diversity in dogs within the Chornobyl exclusion zone?

Environmental contamination can have lasting impacts on surrounding communities, though the long-term impacts can be difficult to ascertain. The disaster at the Chornobyl Nuclear Power Plant in 1986 and subsequent remediation efforts resulted in contamination of the local environment with radioactive material, heavy metals, and additional environmental toxicants. Many of these are mutagenic in nature, and the full effect of these exposures on local flora and fauna has yet to be understood. Several hundred free-roaming dogs occupy the contaminated area surrounding the Chornobyl Nuclear Power Plant, and previous studies have highlighted a striking level of genetic differentiation between two geographically close populations of these dogs. With this work, we investigate mutation as a possible driver of this genetic differentiation. First, we consider large-scale mutation by assessing the karyotypic architecture of these dogs. We then search for evidence of mutation through short tandem repeat/microsatellite diversity analyses and by calculating the proportion of recently derived alleles in individuals in both populations. Through these analyses, we do not find evidence of differential mutation accumulation for these populations. Thus, we find no evidence that an increased mutation rate is driving the genetic differentiation between these two Chornobyl populations. The dog populations at Chornobyl present a unique opportunity for studying the genetic effects of the long-term exposures they have encountered, and this study expands and builds on previous work done in the area.

Contrasting pathogen prevalence between tick and dog populations at Chornobyl

BACKGROUND

The 1986 disaster at the Chornobyl Nuclear Power Plant released massive amounts of radioactive material into the local environment. In addition to radiation, remediation efforts and abandonment of military-industrial complexes contributed to contamination with heavy metals, organics, pesticides and other toxic chemicals. Numerous studies have evaluated the effects of this contamination on the local ecology. However, few studies have reported the effect of this contamination on vector-borne pathogens and their hosts. In this manuscript, we characterize tick-borne pathogen presence at two sample locations within the Chornobyl Exclusion Zone, one at the Nuclear Power Plant (NPP) and another 16 km away in Chornobyl City (CC).

METHODS

Ticks and whole-blood samples were collected from free-breeding dogs captured at the NPP and CC. Endpoint PCR and quantitative PCR were used to identify tick species and to assess the presence of specific tick-borne pathogens, including Anaplasma phagocytophilum, Borrelia burgdorferi sensu lato, Babesia spp., Bartonella spp., Francisella tularensis and general Anaplasmataceae. A droplet digital PCR assay was developed for Babesia canis and A. phagocytophilum to evaluate their presence in dogs from the two populations. Pathogen prevalences between the two sample populations were compared by calculating Z-scores.

RESULTS

Ticks were identified as Ixodes ricinus (n = 102) and Dermacentor reticulatus (n = 4). Overall, 56.9% of I. ricinus ticks were positive for at least one pathogen. A significantly higher prevalence of A. phagocytophilum and B. burgdorferi was found in ticks at the NPP (44.0% and 42.0%, respectively) compared to CC (23.1% and 19.2%, respectively). Babesia spp. (including B. canis and B. caballi) were detected in 8.8% ticks at similar proportions for both populations. Interestingly, we found a significantly lower level of A. phagocytophilum in dogs at the NPP (1.8%) than in dogs at CC (11.7%). In total, 24.3% of dogs were positive for B. canis, evenly distributed across the two populations.

CONCLUSIONS

The results of this study show contrasting pathogen prevalence in both ticks and dogs at the NPP and CC, which may reflect the differential exposures at the two locations. This work adds an important new component to our understanding of the consequences of prolonged exposure to environmental contamination on the wildlife and ecology within the Chornobyl Exclusion Zone.

Evolutionary approach for pollution study: The case of ionizing radiation

Estimating the consequences of environmental changes, specifically in a global change context, is essential for conservation issues. In the case of pollutants, the interest in using an evolutionary approach to investigate their consequences has been emphasized since the 2000s, but these studies remain rare compared to the characterization of direct effects on individual features. We focused on the study case of anthropogenic ionizing radiation because, despite its potential strong impact on evolution, the scarcity of evolutionary approaches to study the biological consequences of this stressor is particularly true. In this study, by investigating some particular features of the biological effects of this stressor, and by reviewing existing studies on evolution under ionizing radiation, we suggest that evolutionary approach may help provide an integrative view on the biological consequences of ionizing radiation. We focused on three topics: (i) the mutagenic properties of ionizing radiation and its disruption of evolutionary processes, (ii) exposures at different time scales, leading to an interaction between past and contemporary evolution, and (iii) the special features of contaminated areas called exclusion zones and how evolution could match field and laboratory observed effects. This approach can contribute to answering several key issues in radioecology: to explain species differences in the sensitivity to ionizing radiation, to improve our estimation of the impacts of ionizing radiation on populations, and to help identify the environmental features impacting organisms (e.g., interaction with other pollution, migration of populations, anthropogenic environmental changes). Evolutionary approach would benefit from being integrated to the ecological risk assessment process.

Environmental radiation exposure at Chornobyl has not systematically affected the genomes or chemical mutagen tolerance phenotypes of local worms

The 1986 disaster at the Chornobyl Nuclear Power Plant transformed the surrounding region into the most radioactive landscape known on the planet. Whether or not this sudden environmental shift selected for species, or even individuals within a species, that are naturally more resistant to mutagen exposure remains an open question. In this study, we collected, cultured, and cryopreserved 298 wild nematode isolates from areas varying in radioactivity within the Chornobyl Exclusion Zone. We sequenced and assembled genomes de novo for 20 Oscheius tipulae strains, analyzed their genomes for evidence of recent mutation acquisition in the field, and observed no evidence of an association between mutation and radioactivity at the sites of collection. Multigenerational exposure of each of these strains to several chemical mutagens in the lab revealed that strains vary heritably in tolerance to each mutagen, but mutagen tolerance cannot be predicted based on the radiation levels at collection sites, and Chornobyl isolates were not systematically more resistant than strains from undisturbed habitats. In sum, the absence of mutational signatures does not reflect unique capacity for tolerating DNA damage.

Environmental radiation exposure at Chornobyl has not systematically affected the genomes or mutagen tolerance phenotypes of local worms

The 1986 disaster at the Chornobyl Nuclear Power Plant transformed the surrounding region into the most radioactive landscape known on the planet. Questions remain regarding whether this sudden environmental shift selected for species, or even individuals within a species, that are naturally more resistant to radiation exposure. We collected, cultured, and cryopreserved 298 wild nematodes isolates from areas varying in radioactivity within the Chornobyl Exclusion Zone. We sequenced and assembled genomes de novo for 20 Oschieus tipulae strains, analyzed their genomes for evidence of recent mutation acquisition in the field and saw no evidence of an association between mutation and radiation level at the sites of collection. Multigenerational exposure of each of these strains to several mutagens in the lab revealed that strains vary heritably in tolerance to each mutagen, but mutagen tolerance cannot be predicted based on the radiation levels at collection sites.