418 MUTATION AND PLANT BREEDING 



there appears to be a specific relation between the chemical agent 

 and the position of breakage of the chromosome. 



Microorganisms 



The most productive and illuminating of recent studies on muta- 

 genic specificity are those carried out with microorganisms: fungi, 

 bacteria, and bacteriophages. The outstanding advantage of the 

 experimental methods used with these materials is the extraordinarily 

 high resolving power which permits localization of genetic events with 

 sufficient accuracy to approach a correlation with macromolecular 

 substructure. These investigations differ from those on higher plants 

 and animals in that inter-allelic or intra-locus rather than inter-locus 

 specificity is tested, and also, back-mutation analysis is used almost 

 exclusively. 



In Neurospora mutagenic specificity has been reported for two 

 particular alleles at two loci, one adenineless, the other inositolless, 

 which have been combined in a double mutant. These alleles have 

 mutated at different relative rates under the action of eight different 

 agents tested, i.e., six chemical mutagens of the alkylating type, ultra- 

 violet light, and X-rays (38, 69, 70). The proportion of act : inos + 

 mutants recovered ranged from 1.5 (ethyl methane sulphonate) to 

 3,800 (bromoethyl methane sulphonate) among the chemical muta- 

 gens. With ultraviolet light the proportion was 0.5, for X-rays 16, and 

 for spontaneous mutations 10. The widely different results with the 

 two methane sulphonates, ethyl and bromoethyl, demonstrate 

 the pronounced specificity which related chemicals may have for the 

 genetic material. A further indication of differential mutagenic effect 

 of these two alleles is that, although ultraviolet induced only about 

 half as many reversions to act as inos + , if the UV exposure is preceded 

 by a 20-minute treatment with formaldehyde then about four times as 

 many act as inos + mutants are recovered (45). 



Preliminary evidence of gene-specific effects, i.e., inter-locus 

 specificity, for 8-ethoxycaffeine, dimethyl sulfate, and ultraviolet 

 light, has been obtained recently in the ascomycete Ophiostoma mul- 

 tiannulatum (75) and in Schizosaccharomyces pom be (32). 



In bacteria early evidence indicating that not only different loci 

 but also different alleles of the same locus may react differently to the 

 same mutagen was reported by Demerec (13) in 1953. Thirty-five 



