Using a biological time machine to dissect evolution in Chernobyl

Overview

Evolution shapes phenotypes in numerous ways, and so understanding evolutionary processes is central to understanding the drivers of variation within and among populations. In this PhD project, the successful student will use a biological time machine to study phenotypic evolution in an organism that has been exposed to profound environmental change. The biological time machine is the freshwater crustacean, an organism that can reproduce both asexually, and also generate sexually-produced resting eggs that can survive in lake sediment for decades (even centuries). The environment is the lakes immediately surrounding the Exploded Chernobyl Nuclear Power Plant.

The student will take sediment cores from the lakes across the Chernobyl area (including low radiation sites), split the cores vertically and isolate Daphnia resting eggs from one half of the core. They will maintain hatched Daphnia in a state of asexual replication, thus taking genetic snapshots from various times in the past.

The other half of the cores will be used to conduct radiation dosimetry, in order to date the hatched Daphnia populations with respect to the radioactive history of the area. Specifically, the student will determine the activity concentrations of various radioactive isotopes that have different half-lives (the time takes for half of the isotopes to disintegrate into a secondary isotope) and are of different energies (high energy alpha particles and lower energy gamma rays). This will allow us to pinpoint the construction and initial operation of the Chernobyl nuclear power plant (low-level chronic radiation), the accident (massive discharge of radioactive material), and the decline in radiation since the accident and infer the dose levels experienced by organisms at these times.

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Image Captions

Multiple sediment cores will be taken from various lakes across the Chernobyl area. Daphnia resting eggs will be hatched and radiation dose will be determined from various depths (time points) in the sediment. Hatched Daphnia will be used for phenotypic and population genetic analysis.

Methodology

The student will travel to the Ukraine sample sediment cores from multiple lakes within the Chernobyl Exclusion Zone (CEZ). They will then receive training in radioisotope analysis at the HydroMeteorological Institute, Kiev, under the supervision of Dr Gennady Laptev. The student will conduct the Daphnia hatching, phenotypic experiments, and population genetic study at the University of Stirling, with support from the University of Glasgow.

Project Timeline

Year 1

Year 1: Daphnia distribution and radiation dose over space and time. Three sediment cores will be obtained from each of eight lakes from across the CEZ. The cores will be measured and split longitudinally; Daphnia resting eggs will be isolated from along one half of each sediment core. With the other half of each core, we will conduct cutting-edge analysis using the distributions of Plutonium 139 (high energy alpha radiation) Caesium 137, Lead 210, Americium 241 and Strontium 90 (lower energy gamma radiation) isotopes to determine how radioactive material was deposited in freshwater lakes, and then determine the radiation dose rate experienced by organisms that inhabit those lakes (including Daphnia). The student will then conduct a large-scale hatching programme, following proven methods, to revive the Daphnia eggs from dormancy; these hatched Daphnia will be propagated by asexual reproduction to establish independent genetic lines from different Chernobyl lakes at different historical time points. The project will aim to address the following points, although its exact trajectory will depend on the student’s own interests and initial results:

Q1.1: How patchy was the deposition of radioactive material within lakes before the nuclear reactor was built, during normal operation, and immediately after the accident?

Q1.3: How does the spatio-temporal distribution of Daphnia resting eggs covary with radiation dose rate within and across Chernobyl lakes?

Year 2

Year 2: Radiation and Daphnia trait evolution. Using the genotype lines established in Year 1, the student will conduct a suite of standard Daphnia life history experiments to quantify how key fitness traits such as asexual replication rate, propensity to produce male offspring, lifespan and reproductive senescence vary with respect to radiation dose and lake. The experiments will be blocked according to lake, and there will be a minimum of 100 genotypes per lake (x 3 replicates per genotype). This experiment will allow the student to answer the following questions:

Q2.1: Does radiation dose rate fuel phenotypic evolution in Daphnia?

Q2.2: Can rapid adaptive evolution in Daphnia populations mitigate the negative impacts of exposure to radiation?

Year 3

Year 3: Population genetic consequences of the Chernobyl accident in Daphnia. Using simple and well-established microsatellite genetic tools, the student will be able to examine how genetic diversity and population structure has changed over space and time. S/he will use microsatellites to: (1) quantify the supply of alleles, i.e., the supply of genetic variation, over time; (2) look for evidence that some pre-accident alleles either disappear or become rare, and evaluate whether this is due to a massive population bottleneck, or due to radiation-mediated selection; and (3) test whether there is significant arrival of genetic variants from neighbouring lakes, as would occur if migration of radiation-resistant Daphnia allowed for the recolonization of lakes after the Chernobyl accident. This will allow them to ask:

Q3.1: Is there an association between radiation dose rate and the supply of microsatellite alleles, i.e., the supply of genetic variation, over time
Q3.2: Is there evidence of an extreme population bottleneck or gene flow associated with post-accident recolonization of lakes?

Year 3.5

Year 3.5: Synthesis of phenotypic and population genetic data. In the final six months, the student will have the opportunity to quantify the extent to which phenotypic change and population genetic patterns are linked. S/he can then evaluate these relationships and look for evidence of radiation-mediated selection on the Chernobyl Daphnia populations over time. We envisage that this project has the potential to generate 5-6 first-author papers for the student, produced throughout the PhD with the first potentially being written by the end of the 1st year.

Training
& Skills

The student will develop a broad skills and training portfolio that will relate to both the precise needs of the project, broader communication skills, and their own interests. The student will undertake radioisotope analysis training under the supervision of Dr Gennady Laptev, developing key environmental analytical skills in the premier environmental radiation facility. They will then get both project-specific and general training in a diverse range of skills from Dr Stuart Auld (Stirling) and Prof Neil Metcalfe (Glasgow), in experimental design, life history and evolutionary theory, population genetics, senescence and aquatic biology. The student will also attend workshops to develop skills in code-based statistical analysis of phenotypic and genetic data, and also communicating science to a public audience. These skills will be of use within academia but also much more broadly.

References & further reading

Goodman, J., Copplestone, D., Laptev, G.V., Gaschak, S., Auld, S.K.J.R. (2019) Variation in chronic radiation exposure does not drive life history divergence among Daphnia populations across Chernobyl. Ecology and Evolution 9: 2640-2650.

Mousseau, T. A., Moller, A. P. (2014) Genetic and Ecological Studies of Animals in Chernobyl and Fukushima. Journal of Heredity 105:704-709.

Further Information

For initial enquiries, please contact Dr Stuart Auld, s.k.auld@stir.ac.uk

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