134 MUTATION AND PLANT BREEDING 



well as other circumstances which it would lead too far to dis- 

 cuss here, all these decide whether the essential step will take place 

 and whether it will result in a detectable mutation. Specificity may 

 occur at any one of these levels; at which one, cannot be decided 

 without special analysis of each individual case. 



In higher organisms, apparent mutagen specificity at individ- 

 ual loci may be an expression of regional specificity along the chro- 

 mosome. In Drosophila the distribution of visible mutations over 

 the X-chromosome depends to some extent on the mutagenic treat- 

 ment (36). In the silkworm, two linked genes determining the color 

 of the embryonic membranes appear to mutate in different ratios 

 after treatment with X-rays and several chemical mutagens (66). 

 Most or all of these apparent mutations are due, however, to defi- 

 ciencies, and the specific responses observed are therefore properties 

 of the chromosome regions in which the genes are located. Also, spec- 

 ificity may occur at some later step in the mutagenic process, such 

 as the rejoining of broken chromosome ends. This is suggested by 

 the observation that the differential response of the two loci to muta- 

 gens varies with the sex of the treated animal (64). 



The mutagen specificities that have been observed in barley 

 give rise to similar ambiguities of interpretation. The mutation 

 spectra of both chlorophyll and erectoides mutations have been 

 shown to vary according to the mutagen used (44). Some of these spec- 

 ificities have been correlated with detectable chromosomal aberra- 

 tions. How many may be due to differences in the frequencies and 

 types of undetected chromosome rearrangements cannot at present 

 be decided. Moreover, in barley as in the silkworm, the mutation 

 spectrum is under the influence of physiological conditions (29), 

 such as hydration or degree of pregermination, and thus is unlikely 

 to reflect specificities of chemical interaction at the level of the 

 gene itself. 



These doubts concerning the origin of specific responses to 

 mutagens do in no way detract from the great theoretical and prac- 

 tical importance of the observed phenomenon of mutagen specificity. 

 Theoretically, the recognition that mutagen specificity need not 

 always result from specific chemical reactions between mutagens and 

 nucleotides should make the geneticist more rather than less inter- 

 ested in chemical mutagens. It should make him realize that chemical 



