The Effect of Oxygen Concentration on X-ray Induced Chromosome Breakage in Maize

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Abstract
Pollen carrying the dominant alleles of a and sh (on chromosome 3 with .27% crossing over) was X-rayed at 1000 r, one-half being irradiated in air and the other half in 100% nitrogen. It was used to pollinate tester plants a sh A2CR, and the resulting kernels were scored for entire and sectorial mutations (mosaics) involving a and sh. Approx. 3% of the entire mutations involved only one of the two genes, whereas all of the mosaics involved both genes. It was assumed that loss of both A and Sh was due to chromosome breakage and loss of one gene to gene mutation or internal deletion. Irradiation in N2 (reduced oxygen concn.) compared with irradiation in air gave a decrease in mutations by a factor of 3.2 for both entire and. sectorial mutations, 2.7 for entires and 9.9 for mosaics, with a significant difference between the last two values. This difference suggested that the mechanism of chromosome restitution following breakage was sensitive to oxygen concn. during irradiation, since the mosaic class arose largely from chromosome breakage without reunion (giving a chromatid type of breakage-fusion-bridge cycle), whereas a portion of the entire mutations resulted from chromosome breakage with reunion of broken ends. Assuming that irradiation in nitrogen increased the ability of broken chromosome ends to rejoin, the terminal deletions (mosaics) would be reduced, but not the interstitial deletions (entires). The 3 mosaics out of 4505 kernels examined in the nitrogen treatment were thought to result from the chromosomal type of breakage-fusion-bridge cycle, which depended on fusion of broken ends of centric chromosomes and would be increased by reduced oxygen concn. Entire mutations could also result from terminal deletions including the A and Sh regions and this type would be reduced by the nitrogen treatment. It was believed that the effect of oxygen was immediate at the time of irradiation, and that its effect was on the recovery process rather than on the breakage process.
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