The Effect of Intensity of Infrared on X-Ray Induced Chromosome Aberrations in Tradescantia.

REVIOUS investigations have shown that infrared is effective in increasping the number of chromosomal aberrations induced by X-radiation ( KAUFMANN, HOLLAENDER and GAY 1946; SWANSON and HOLLAENDER 1946; YOST 1951). There has been little agreement at the interpretive level, however, on the mode of action of the infrared in producing this increase. Two hypotheses have been advanced. One proposes that infrared radiation affects only the process of recombination of broken ends of the chromosomes, i.e., that recombination is favored at the expense of restitution (KAUFMAKN, HOLLAENDER and GAY 1946; KAUFMANN and WILSON 1949). The other postulates that the effect is to increase the total number of primary breaks ( SWANSON and HOLLAENDER 1946). Recently, evidence has been presented which favors the latter of these two hypotheses (YOST 1951). In an attempt to explain the mechanism whereby the infrared radiations exert their effect on Tradescantia chromosomes, it was proposed that, in addition to complete breaks realized at the time of radiation, the X-rays also induce potential breaks which become complete and available for recombination upon the addition of infrared energy in posttreatment. It was further postulated that the effect of pretreatment is to weaken the chromosome structure, thus making it more susceptible to fragmentation by X-radiation ( SWANSON 1949 ; YOST 1951). If the preceding series of postulates be true, a decrease. in the intewity of the infrared treatments, keeping the total dose constant, should result in a decrease in the number of exchange aberrations recovered in posttreatment, with no decrease in the number of terminal deletions and isochromatid aberrations. This follows from the assumption that the breaks involved in producing the increase are not complete until the addition of the infrared ; and since the lower the intensity, the lower the probability that any two complete breaks will be coincident in time in the same cell,.the possibilities of an exchange taking place in any cell are lowered. On the other hand, there should be no effect of intensity of infrared in pretreatment, since there is no disjunction of the chromosome structure which can undergo recombination, with infrared treatment alone. These tests imply that recombination can take place whenever broken ends are available, but the length of time a broken end can remain open is limited in Tradescantia, Previous data have shown that infrared posttreatments are effective in Tradescantia as long as 96 hours after X-radiation