320 MUTATION AND PLANT BREEDING 



of mutagens has been shown to be "geographically" specific but not 

 functionally specific (6). When a mutagen is found more effective in 

 producing one rather than another physiological property, and when 

 the effect is not due to differential sensitivity of one locus primarily 

 affecting synthesis or to plasmid clearing, a selective effect of the 

 system for recovering variants should be suspected. 



Although no specific mutagenic treatment can be singled out as 

 most effective, certain pitfalls of inefficient use must be avoided. Some 

 mutagens require special conditions for maximal effectiveness. Thus, 

 ultraviolet irradiation is most effective when applied to growing cells 

 containing large pools of nucleic acid intermediates, followed by 

 incubation in a medium deficient in these compounds but enriched 

 with amino acids. Conditions may be allele as well as mutagen 

 specific (40, 99, 102, 114, 115). Certainly a mutagen will be ineffective 

 if it or the products of its action cannot penetrate the cell. Pretreat- 

 ment of bacteria is necessary to obtain maximum effectiveness of 

 manganous ion due to its low permeability (38). 



The discovery of "hot spots" within a locus more sensitive to 

 one than another mutagen, as well as the locus specificity of mutagens 

 (probably due to such regions), favors a cyclical application of differ- 

 ent mutagens (9, 37, 52). Continued application of the same mutagen 

 may be fruitless if the most probable mutations inducible by the 

 mutagen have already been recovered. Besides the use of a variety of 

 mutagens and careful design of the techniques of application optimal 

 to induction, detection, and recovery of mutants, the order of appli- 

 cation is important. Thus, X-ray and ultraviolet irradiation may be 

 initially used to obtain radio-kinetic data on the genetic constitution 

 (30, 89), and to obtain auxotrophs for mutagenicity indices and 

 incorporation of mutagenic purine and pyrimidine base analogs. 



Recombination Between Antibiotic-Producing Strains 



An approach to higher yields lies in the use of existing variation 

 among different antibiotic-producing strains. A combination of 

 desirable characteristics and elimination of unwanted properties 

 might occur during joint cultivation of two different strains (80), but 

 such a "mixed fermentation" would be difficult to stabilize. A single 

 strain possessing only the desirable properties should be obtained 

 through some form of genetic recombination (73) between differing 

 isolates, the form of recombination depending upon the plasticity of 



