A resurrection ecology test for the effects of ionizing radiation and other environmental correlates on Daphnia evolution

Acute exposure to ionizing radiation has well-documented, immediate negative consequences for individuals. However, the evolutionary consequences for populations exposed to ionizing radiation is unclear. For example, a meta-analysis of taxa exposed to Chernobyl fallout found some evidence for elevated mutation rates in animal and plant taxa; however, in people, de novo mutation rates in offspring of parents exposed to radiation during and after the Chernobyl accident were no higher than controls. Furthermore, whether irradiation and increased mutation rates drive adaptation to radiation also has mixed support. Ambiguity in both cases likely arises from the difficulty of studying mutation rates and adaptation after rare nuclear events whose ionizing radiation is distributed heterogeneously in time and space. Here, we report an attempt to better address this difficulty with a "resurrection ecology" study of Daphnia spp. in Utah lakes that experienced nuclear fallout from US Department of Energy weapons testing in the 1950s and 1960s. The idea was to recover dormant Daphnia eggs from sediment cores that spanned the nuclear testing era in the American West. We predicted that survival and fecundity of eggs hatched in the lab would show fitness declines correlated with ionizing radiation fallout and a potential recovery once nuclear testing stopped. We successfully obtained multiple cores from three lakes that dated back to the 1800s. We isolated >4700 dormant eggs from those cores, spanning the nuclear era, but were only able to hatch a single egg in the lab. Thus, we could not conduct life history experiments to test our prediction. The purpose of this manuscript, therefore, is to describe the study and make our radioisotope core dating and sedimentation data available to other paleolimnological researchers. We also report a side study of stable isotope change through time measured from dormant eggs and the sediment.