Radiation-mediated supply of genetic variation outweighs the effects of
selection and drift in Chernobyl Daphnia populations
Abstract
Populations experiencing varying levels of ionising radiation provide an
excellent opportunity to study the fundamental drivers of evolution.
Radiation can cause mutations, and thus supply genetic variation; it can
also select against individuals that are unable to cope with the
physiological stresses associated with radiation exposure. Since the
nuclear power plant explosion in 1986, the Chernobyl area has
experienced a spatially heterogeneous exposure to varying levels of
ionising radiation. We sampled Daphnia pulex (a freshwater crustacean)
from lakes across the Chernobyl area, genotyped them at eleven
microsatellite loci, and also calculated the current radiation dose
rates. We then investigated whether the pattern of genetic diversity was
shaped primarily by radiation-mediated supply of variation consistent
with increased supply of de novo mutations, or by greater
radiation-mediated selection at higher dose rates. We found that
measures of genetic diversity, including expected heterozygosity (an
unbiased indicator of diversity) were significantly higher in lakes that
experienced higher radiation dose rates; this is consistent with
mutation outweighing selection as the key evolutionary force in
populations experiencing high radiation dose rates. We also found
significant but weak population structure, and clear evidence for
isolation by distance between populations. This evidence suggests that
gene flow between nearby populations is eroding population structure,
and that mutational input in high radiation lakes could, ultimately,
supply genetic variation to lower radiation sites.