UV repair and resistance to solar UV-B in amphibianeggs: a link to population declines?

The populations of many amphibian species, inwidely scattered habitats, appear to be in severe decline; other amphibians showno such declines. There is no known single cause for the declines, but theirwidespread distribution suggests involvement of global agents--increased UV-Bradiation, for example. We addressed the hypothesis that differentialsensitivity among species to UV radiation contributes to these populationdeclines. We focused on species-specific differences in the abilities of eggs torepair UV radiation damage to DNA and differential hatching success of embryosexposed to solar radiation at natural oviposition sites. Quantitativecomparisons of activities of a key UV-damage-specific repair enzyme, photolyase,among oocytes and eggs from 10 amphibian species were reproduciblycharacteristic for a given species but varied > 80-fold among the species.Levels of photolyase generally correlated with expected exposure of eggs tosunlight. Among the frog and toad species studied, the highest activity wasshown by the Pacific treefrog (Hyla regilla), whose populations are not known tobe in decline. The Western toad (Bufo boreas) and the Cascades frog (Ranacascadae), whose populations have declined markedly, showed significantly lowerphotolyase levels. In field experiments, the hatching success of embryos exposedto UV radiation was significantly greater in H. regilla than in R. cascadae andB. boreas. Moreover, in R. cascadae and B. boreas, hatching success was greaterin regimes shielded from UV radiation compared with regimes that allowed UVradiation. These observations are thus consistent with the UV-sensitivityhypothesis.