XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 499 



first clearly demonstrated l)y Mullcr in 1927 {10) in Dmsophtla 

 melanognder (see Chapter XIV). 



This mutagenic action of radiation has been observed in all the 

 organisms investigated, namely, viruses, bacteria, fungi, liverworts, 

 flowering plants, a variety of insects, and mice. X rays have com- 

 monly been used as a source of radiation but sex-linked lethal muta- 

 tions have been produced in Drosophila by exposure to neutrons 

 (11-15) and to a radiation (16,17). The chance that a given dose 

 will produce a visible mutation is not only of the same order for a 

 variety of transitions between different allelomorphs at the same locus 

 in Drosophila, but, as Gowen (18) has pointed out, it is not greatly 

 different in organisms as dissimilar as viruses, bacteria, and in- 

 sects (Drosophila), which contrasts with the fact that estimated 

 spontaneous mutation rates in different species are roughly inversely 

 proportional to life span. 



It is estimated that in Drosophila an exposure of something like 

 200 million r. is needed in order that there should be produced in half 

 the irradiated genes a mutation whose phenotypical expression is a 

 change in eye color from "white" to "eosin," and about 10 million 

 r. for that corresponding to the change from ''wild" type to "forked 

 bristle." These are the extremes listed by Lea (i). The average 

 value for three bacterial mutations affecting colony appearance is 18 

 million r. The figure for delayed development of bacteriophage re- 

 sistance in Escherichia coli is 300 million r. In Drosophila the induc- 

 tion of sterility by the irradiation of the gonads limits the dose to 

 which the germ cells may be exposed by existing techniques to around 

 3000 r. A similar Hmitation would obviously apply to any other 

 type of cell that is rendered completely in viable by irradiation. Bac- 

 teria have the advantage in this respect not only of being among the 

 more resistant organisms but also in that up to the highest X-ray 

 doses examined the proportion of survivors is exponentially related 

 to the dose. For example, in strain B/r of E. coli used by Demerec 

 and Latarjet (19), 1 in 100,000 bacteria survived a dose of 100,000 

 r. and among the survivors there were 0.02% that showed mutation 

 to bacteriophage resistance. Several hundreds of new strains of 

 bacteria, Neurospora, and various other fungi have been produced in 

 this way, manifesting changes in color, enzymic systems, biochemical 

 characteristics, resistance to bacteriophages, drugs, and antibiotics 

 (30). This relatively recent application of radiation has led to a 

 wealth of new information resulting in considerable advances in our 



