The production of chromosome interchanges in Drosophila virilis.

ULLER'S original discovery that ionizing radiations are capable of 141 breaking the chromosome thread was based on investigations carried out with Drosophila melanogaster. But in spite of the numerous studies which have been undertaken using this animal, comparatively little progress has been made in the last twenty years in determining the precise manner in which these breaks are formed and the mechanisms involved in the subsequent rejoining of the broken chromosome ends. The cause of this rather slow advance resides in the fact that Drosophila chromosomes can be observed satisfactorily neither during the breakage or joining process nor for a considerable interval of time thereafter. Consequently, any information regarding these processes must be obtained by inference from events taking place a t a later time when the ultimate results of the radiation can be accurately determined by genetic analysis or by a cytological study of the salivary gland chromosomes. The results of the present investigation are derived from experiments in which the sperm of Drosophila virilis males are subjected to different dosages of X-radiation. Only a particular type of chromosome rearrangement is studied; namely, exchanges between different chromosomes. The frequency with which these interchanges occur a t the different dosage levels provides some insight into the manner in which the breaks are formed. The information obtained has been collected in such a way that both the relative sensitivity of the different chromosomes to breakage and the relative frequency with which certain numbers of chromosomes are involved in the recovered interchanges are revealed. I t is possible to gain some insight into the course of events taking place during the joining of the broken chromosome ends by use of these data. Two different groups of experiments of this type have been conducted: in the one group the temperature a t the time of irradiation was held a t 28" &1"C, while in the other a temperature of 3"+ 1°C was maintained. The finding of a significant difference in the results of these two groups merely emphasizes the inadequate state of our knowledge concerning the biological action of ionizing radiations on Drosophila chromosomes.