The purpose of this study was to compare fluence-response relationships for the production of cyclobutane pyrimidine dimers in epidermal or dermal DNA of platyfish Xiphophorus hybrids irradiated with UVB, and to determine photoreactivation from black light on dimers produced in situ. This was accomplished by quantitative gel electrophoresis of unlabeled DNA following extraction of the DNA and treatment with an enzyme specific for the detection of pyrimidine dimers. The dermis was the target tissue for UV-induced DNA damage in Xiphophorus hybrid fish skin. Shapes of dimer-fluence response data following filtered sunlamp irradiation (lambda > 290 nm) or monochromatic wavelength 302 nm in the epidermis or dermis were different. In the epidermis there was an initial steep upward slope followed by a plateau, whereas in the dermis a linear relationship was observed. The final values of dimers at the high doses were, however, nearly equal in the epidermis and dermis exposed to either radiation. These differences in fluence-response relationships are probably attributable to the intertwining of the epidermis and to the shielding effect of the epidermal layer, with scales leading to a heterogenous population of cells which are exposed to different UV doses. Photoreversal of dimers was readily observed by black light irradiation in both epidermis and dermis irradiated with either lambda > 290 nm or 302 nm.
