Effects of Combined Radiations on Crossing over I N Drosophila Melanogaster ’

N attempts a t an experimental approach to the problem of the mechanism of I crossing over many modifying agents have been used : temperature (PLOUGH 1917), aging (BERGNER 1928), inversions (STEINBERG 1936), mineral content (BROWNING 1949), and radiation (MULLER 1925). As diverse as are these agents, they all have one effect in common. They modify the rate of crossing over in either the region of the kinetochore or the region very distal to it. In most cases there is observed a pattern of change which KIKKAWA (1934) called, “the proximal increase and compensatory distal decrease”. This similarity of effect suggests a common path of action by all the agents; but few attempts to construct a unifying hypothesis have been presented. Three which deserve mention are: the mechanical hypothesis of KIKKAWA (1934), one involving coiling modifications by MATSUURA (1940), and one stressing the role of heterochromatic areas (SCRULTZ AND REDFIELD 1951). These have much in common, and offer a good starting point for a generalized hypothesis. In recent experimental investigations, much emphasis has been laid upon the effect of radiations. WHITTINGHILL and his associates (WHITTINGHILL 1954; HINTON and WHITTINGHILL 1950) have done a thorough analysis of this aspect; but they have devoted most of their discussion to a consideration of the time at which crossing over occurs rather than the mechanism whereby the radiation affects the process. However, since much stress is put upon the role of recombination in the gonia1 cells, one has the feeling that these authors believe that the radiation acts to cause increase in the number of breakage points. Such a role of modifying agents is also apparent in the recent publication of LEVINE (1955), in which it is suggested that the correlation between mineral deficiency and crossing over increase can be explained by rupture of the chromosomal strands at points of calcium binding. Both agents (X-ray and mineral deficiency) are known to disintegrate the chromosome structure (STEFFENSEN 1955) and both increase crossing over in certain regions. Although there are a number of objections to this idea, it is a natural assumption and one which merits further investigation. It was the increased breakage hypothesis which these experiments were designed to test. It has been demonstrated that following treatment with infrared radiation chromosomes of Drosophila are broken more frequently by exposure to X-rays (KAUFMANN, HOLLAENDER and GAY 1946). It was felt that this potentiating effect of the infrared might apply to a normal chromosome breakage process in the course of crossing over. A test of this idea was made with negative results (IVES, FENTON, YOST and LEVINE 1953), and it was concluded that the type of recombination