314 MUTATION AND PLANT BREEDING 



within individual colonies. These different morphological types may 

 be "purified" by any of several methods for obtaining single cultures. 

 A spectrum of stable but different morphological types will be 

 obtained along with the unstable prototype. The yield of different 

 types will often be different and a pattern of production associated 

 with morphology may be found. The variability may be due to hetero- 

 karyosis (57). An isolate with stable morphology and high yield is 

 established to supply inocula. The original culture should be retained 

 since some derivatives are found to "degenerate", that is, decrease in 

 yield, to be susceptible to actinophages, or fail to survive mainte- 

 nance techniques such as lyophilization. 



Isolation of Colonies with Different Morphology 



Usually, some variation in morphology can be found in cultures 

 that have been repeatedly "purified". New morphological types can 

 be uncovered by plating on different media. An increase in morpho- 

 logical variation follows mutagenic treatment and may be used as a 

 measure of effectiveness. Increased variation may not be due to muta- 

 tion, however. Dissociation of homokaryotic sectors from hetero- 

 karyotic growth, induction and lysis by temperate phage or bacterio- 

 cins, changes in cytoplasmic determinants of morphology, if such 

 exist, and changes of balance of nuclear types may produce variants 

 that are not mutants. Thus, the use of morphological variation as a 

 measure of induced mutation is not valid unless proved by genetic 

 analysis. Such variation may be a partial explanation of pleomorph- 

 ism found among the actinomycetes as well as the higher fungi (13). 



Experience with morphological variation in antibiotic-producing 

 cultures can be used to predict what types do not produce desirable 

 yields, but not what types will produce higher yields than the cultures 

 established as the norm or "controls". Two types common to both 

 the actinomycetes and higher fungi and producing no or very little 

 yield are aconidial and colorless forms. 



Random Isolation 

 Isolation of large numbers of colonies derived from individual 

 spores surviving various mutagenic treatments is the next phase of a 

 strain selection program. It is essential that these isolates be pure 

 clones and not mixtures of mutant and nonmutant genomes. Muta- 

 genic treatments applied to suspensions of nongerminating conidia 



