Compensation for solar UV damage by solar radiation of longer wavelengths (Smittia spec., Chironomidae, Diptera).

Photobiological experiments and laboratory observations on the egg deposition behaviour of Smittia were undertaken to assess the effect of different spectral ranges of solar radiation on inactivation and photoreactivation of the eggs.The egg deposition of Smittia is not confined to certain periods of the daily cycle, nor are the eggs hidden away even from bright laboratory light. Both the egg shells and the jelly of the clusters absorb UV poorly.An action spectrum was established for inactivation of eggs by exposition to ultraviolet radiation (UV) during intravitelline cleavage. It shows a pronounced peak at 295 nm and overlaps with the spectrum of the sunlight between 300 and 310 nm wavelength.The inactivating range of solar UV was substituted for by 305 nm radiation of 5 nm band with and a dose rate of 0.28 W/m2, obtained from a monochromator. This "solar UV equivalent" caused inactivation of 50% of exposed eggs within 12 min, whereas exposition to full overhead sunlight produced the same effect only within 3 hrs.It is concluded that the inactivating effect of solar UV (300-310 nm) is counteracted by repair mechanisms requiring light of longer wavelengths (320-480 nm) which are also emitted by the sun.The type of photoreactivation described is difficult to understand within the established framework of nucleic acid repair. However, it seams to play a vital rôle in the survival of Smittia eggs under sunlight without need for pigmentation or shading.