Developmental Responses of Amphibians to Solar and Artificial UVB Sources: A Comparative Study

Abstract— Many amphibian species, in widely scattered locations, currently show population declines and/or reductions in range, but other amphibian species show no such declines. There is no known single cause for these declines. Differential sensitivity to UVB radiation among species might be one contributing factor. We have focused on amphibian eggs, potentially the most UVB-sensitive stage, and compared their resistance to UVB components of sunlight with their levels of photolyase, typically the most important enzyme for repair of the major UV photoproducts in DNA, cyclobutane pyrimidine dimers. Photolyase varied 100-fold among eggs/oocytes of 10 species. Among three species–Hyla regilla, Rana cascadae, and Bufo boreas–for which resistance of eggs to solar UVB irradiance in their natural locations was measured, hatching success correlated strongly with photolyase. Two additional species, Rana aurora and Ambystoma gracile, now show similar correlations. Among the low-egg-photolyase species, R. cascadae and B. boreas are showing declines, and the status of A. gracile is not known. Of the two high-photolyase species, populations of H. regilla remain robust, but populations of R. aurora are showing declines. To determine whether levels of photolyase or other repair activities are affected by solar exposures during amphibian development, we have initiated an extended study of H. regilla and R. cascadae, and of Xenopus laevis, laboratory-reared specimens of which previously showed very low photolyase levels. Hyla regilla and R. cascadae tadpoles are being reared to maturity in laboratories supplemented with modest levels of UV light or light filtered to remove UVB wavelengths. Young X. laevis females are being reared indoors and outdoors. Initial observations reveal severe effects of both UVA and UVB light on H. regilla and R. cascadae tadpoles and metamorphs, including developmental abnormalities and high mortalities. Assays of photolyase levels in the skins of young animals roughly parallel previous egg/oocyte photolyase measurements for all three species.     

UVB dose-toxicity thresholds and steady-state DNA-photoproduct levels during chronic irradiation of inbred Xenopus laevis tadpoles

Environmental stressors that severely impact some species more than others can alter ecosystems and threaten biodiversity. Genotoxic stress, such as solar UV-B irradiance, may induce levels of DNA damage at rates that exceed repair capacities in some species but remain below repair capacities in other species. Repair rates would seem to establish toxicity thresholds. We used inbred Xenopus laevis tadpoles in the laboratory to test the hypothesis that balances between rates of induction of cyclobutane pyrimidine dimers (CPDs; the major UV-B photoproduct in DNA) and rates of CPD removal (repair) can determine UV-B toxicity thresholds. As rates of chronic UV-B irradiance were progressively increased by decreased shielding of lamps, survival decreased sharply over a relatively narrow range of dose rates. Apparent toxicity thresholds were associated with large increases in steady-state CPD levels. Induction at twice the measured removal (repair) rate produced sustained high CPDs and 100% mortality. Induction at one-half the removal rate resulted in negligible CPD levels and low mortality. Increased intensity of visible radiation available to drive CPD photoreactivation, mimicking interspecies variation in DNA repair capacity, reduced steady-state CPD levels and increased survival at UV-B dose rates that were previously toxic, resulting in increased thresholds of apparent toxicity. We suggest that threshold effects due to DNA repair should generally be considered in assessments of effects of genotoxic agents on species-specific population decreases and human health risks.