EFFECTS OF ULTRAVIOLET-LIGHT AND METHOPRENE ON SURVIVAL AND DEVELOPMENT OF RANA-PIPIENS

Recently a suite of relatively specific hindlimb deformities have been observed in several anuran species in North America. These deformitie s include ectopic and supernumerary limbs and missing limbs, limb segm ents, or digits. The objective of this study was to assess two stresso rs hypothesized as responsible for limb malformations in amphibians: m ethoprene, an insect growth regulator that, through interaction with t he retinoic acid signaling system, could possibly cause limb deformiti es, and ultraviolet (UV) light. Northern leopard frogs (Rana pipiens) were exposed to several different concentrations of methoprene both in the absence and presence of UV light designed to mimic the UV wavelen gth spectrum present in sunlight. Exposures were initiated at early em bryonic stages (newly fertilized eggs) and continued through emergence of the forelimbs of the frogs. At the highest methoprene concentratio n tested, both in the absence and presence of UV light, severe develop mental effects were observed, with all organisms dying within 12 to 16 d of test initiation. However, exposure to the pesticide did not caus e limb malformations. Irrespective of methoprene treatment, a very hig h percentage (similar to 50%) of animals held under the UV light for 2 4 d developed hindlimb malformations. These malformations usually were bilateral and sometimes completely symmetrical, and consisted of miss ing limb segments and missing or reduced digits. A complete proximal t o distal representation of the deficiencies occurred, ranging from mis sing or malformed femurs to the absence of single digits or digit segm ents. The developmental period of greatest sensitivity to UV light occ urred during very early limb bud development, corresponding with forma tion of the apical ectodermal ridge. The significance of these finding s in terms of deformed frogs in the held is uncertain. Although the de formity types observed (i.e., missing limb segments and digits) were s imilar to those seen in some field specimens, the UV light treatment d id not cause the full range of malformations observed in animals from the field (e.g., supernumerary limbs, nonbilateral deformities). Furth ermore, although the artificial light spectrum utilized mimicked the r elative UV spectrum present in sunlight, it did not match full sunligh t intensity, and did not accurately mimic visible wavelengths. Finally , the relationship of the UV light dose used in the laboratory to that actually experienced by amphibians in the field is uncertain. Despite these questions, our findings suggest that UV light should be further considered as a plausible factor contributing to amphibian malformati ons in field settings.