Evolutionary aspects of high mutation pressure were studied in laboratory populations of Drosophila melanogaster that have irradiation histories up to 600 generations. Dose-response regressions for the x-ray induction of various types of mutation were obtained from six of these populations. The sensitivity of these irradiated populations relative to an unirradiated control population was characterized by dose reduction factors. Sensitivity decreased stepwise with the stepwise increase in irradiation levels to which the populations had been exposed every generation (0 R, 2 kR, 4 kR, 8 kR; 1 R = 0.258 mC/kg) but remained the same over hundreds of generations when the irradiation levels were constant. Resistance is controlled by single genetic factors. Additional factors evolved in subpopulations exposed to increased irradiation levels, and different factors evolved in populations that were kept separate from the beginning of their irradiation histories. Two of three factors persisted in subpopulations no longer irradiated, but one factor disappeared; this last one behaved like a transposon. Factors of relative radioresistance are stage specific (immature oocytes) and some of them are assumed to modify or control mutation-rate genes. The resistance factors enable populations to achieve an equilibrium between the amounts of environmental mutagens and intrinsic mutation rates.The evolutionary regulation of mutation rates toward either a minimal (1) or an optimal level (2) has long been a subject of discussion. Hints of the physiological pathways that may be followed in the course of continuous adjustments of mutation rates are given by the recent knowledge on the complex network of genetically controlled error-proof and error-prone mechanisms of DNA repair (3,4). Theoretical models of adjusted (or optimal) mutation rates have shown that such adjustments cannot be expected with all population structures but rather some specific requirements have to be met (5-8). Experimental evidence on this matter is rare; only the chemostat experiment of Cox and Gibson (9) showed that, in Escherichia coli, genotypes with higher intrinsic mutation rates are better adjusted to fast changes in the environment.In a long-time program to study responses of Mendelian populations to high mutation pressure, I observed an adaptation to the environmental factor x-rays in several experimental populations of Drosophila melanogaster: in the early history of any irradiated population, progeny numbers were drastically reduced but recovered, after various generations, to the carrying capacity of the system. Part of this adaptation is achieved by decreased rates of the germ-cell mutations induced by x-rays (11-14). Studies of the above populations are suitable to examine the general question of adjustment of mutation rates to optimal levels. Adjustment means that radiosensitivity (i) remains constant over several hundred generations in populations continuously kept at the same level of per generation input of x-rays, (it) increases in subpopu...

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