To determine if synergistic interaction occurred between Mu and ultraviolet light the pollen of both Mu plants and control plants was irradiated for 0', 30', 35', 40 and 45' and then applied to the silks of standard plants. The frequency of plants in these outcross progeny that segregated for seedling mutants equals the mutation frequency.
To determine if U.V. had a synergistic effect at each U.V. dose, the mutation frequency for standard plants at each U.V. dose was calculated. The number of plants tested in the 0' Mu population was multiplied by the mutation frequency determined for irradiated standards for each U.V. dose to determine how many U.V. induced mutant bearing plants would be expected in the 0' Mu population if it had been irradiated by that particular dosage of U.V. Thus, a calculated mutation frequency was determined for the combined effect of Mu and U.V. for each dosage based on an additive (non-synergistic) model. If a synergistic effect is occurring for any particular dose of U.V. then the observed mutation frequency in irradiated Mu outcrosses should be greater than the calculated Mu value for a given U.V. dose.
Thus, for each dose, the ratio of observed to calculated mutation frequency was used to calculate synergism. The results of these calculations are plotted in Figure 1.
Synergistic effects were observed for 30', 40', and 45' of irradiation. The effect at 40' was significant (at 5% level).
These results suggest a synergistic effect is occurring. Most of the populations so far tested are small, which probably accounts for the up and down nature of the curve. The 30' point had the largest test populations (i.e., 526 for Mu crosses and 213 for control crosses). The numbers for the other irradiation times were: 35' Mu 155, control 131; 40' Mu 139, control 130; 45' Mu 161, control 138. The 0' Mu population was 525 for 30' and 178 for 35', 40' and 45'.
U.V. irradiation of lysogenic bacteria is known to induce the replication of the temperate virus. If Mu is viral in nature and is inserted into the chromosome in a lysogenic state, it also may be activated by U.V. and thus there may be more viruses available for insertion (resulting in mutations) in irradiated pollen than in nonirradiated Mu pollen, thus producing a synergistic effect. I am not aware of studies on the effect of U.V. on the mutagenic effect of the various known transposable DNA systems. It would be interesting to know if they respond in a manner similar to Mu in this regard.
If Mu interferes with the DNA repair systems involved in correcting the thymine dimers induced by U.V., such a synergistic response might be expected.
Next summer larger populations of the U.V. experiments will be grown and the dosage curve will be extended.
Mutator and standard pollen were treated with EMS following the techniques developed by Neuffer. This pollen was used in crosses to standard lines to test for a synergistic effect of this mutagen in Mu plants. There does not appear to be any such effect although, again, the populations are small: Mu controls = 189, Mu-EMS = 128 and Standard EMS = 256. No mutants were found in 139 0' U.V. standard plants or in 280 no-EMS standard plants.
Donald S. Robertson
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