An Analysis of X-Ray Induced Chromosomal Aberrations in Tradescantia.

HE cause and nature of chromosomal alterations has become an imT portant genetic problem since it has been shown that both mutation and speciation often are associated with structural alterations of the chromosomes (DOBZHANSKY 1937). The frequency of spontaneous structural changes is too low in most organisms to permit a statistical analysis of types or frequencies of alterations, although recently GILES (1940) has been able to analyze a considerable number of natural chromosomal aberrations in Tradescantia hybrids. In most respects these natural aberrations resemble those induced by X-rays, and although the spontaneous breaks are not caused by radiation, the same secondary factors seem to be involved in producing the various types and frequencies of aberrations. The X-ray induced aberrations can be produced in large numbers under controlled conditions, permitting adequate statistical analyses of types of aberrations and their frequency in relation to dosage, temperature, time of exposure, and radiation intensity. The microspores of diploid species of Tradescantia provide excellent material for an analysis of X-ray induced chromosomal aberrations. The six chromosomes are large and the nuclear cycle can be timed rather accurately. During the summer months the microspore nucleus remains in the resting stage for about five days after the microspore is formed following meiosis. At about 30 hours before metaphase the chromosomes begin splitting, and a t 23 to 24 hours before metaphase all chromosomes are split to form the sister chromatids. Thus the nuclear cycle from microspore formation to metaphase requires about six days. During the winter months the nuclear cycle is nearly twice as long and the prophase stage may begin at about 6-70 hours before metaphase. For an analysis of chromatid breaks induced a t prophase the cells were fixed 24 hours after raying. Microspores fixed five days after raying were irradiated in the resting stage. Preparations were made a t 48 hours after raying during the winter months, and 30 hours after raying during the summer months, for the analysis of very early prophase stages. The analyses of dosage, the time factor, and temperature effects were based on experiments done during the winter months. The division figures obtained provide a true random sample of aberrations because there is no chance of elimination of chromosomes or chromosome fragments. The microspore cells were smeared, fixed in alcohol-acetic for a few