Chapter VI 

 VARIATION 



The Aspergilli are a variable and mutable group of fungi. They are 

 characterized by great diversity and variability as they are isolated from 

 nature, and an increasing amount of evidence shows that they can be made 

 to vary, or mutate, in the laboratory by subjecting them to a number of 

 different imposed stimuli. Frequently the same types of mutants or vari- 

 ants ultimately result under both natural and artificial conditions. Never- 

 theless, it is believed desirable to consider somewhat separately variations 

 and mutations resulting from natural causes and those resulting from 

 imposed stimuli. 



Definition of Terms 



Before entering upon a discussion of variation, either natural or induced, 

 it is important to define certain terminology which is to be employed. It 

 is recognized that our definitions will not agree in all cases with those of 

 earlier workers, nor do we expect that all subsequent investigators will 

 accept those which we propose. If the meaning in the present discussion 

 is clear, our purpose will have been served. 



(1) The term mutant, or mutation, is used to designate a strain whose 

 source is actually known and can be verified. Furthermore, it is limited to 

 those substrains which originated as sharp breaks from parent cultures 

 (usually interpreted as gene mutations), and in successive culture genera- 

 tions retain their distinguishing characteristics unaltered. This may or 

 may not have a taxonomic connotation. Upon occasion it is used in essen- 

 tially the same sense as "variety". To illustrate, A. nidulans mut. albus, 

 A. fumigatus mut. helvola, A. niger mut. cinnamomeus, etc., are used as 

 Latin names to designate forms which differ from the parent species in 

 certain striking details. In other instances it is used to identify a type of 

 change, rather than to designate a particular and isolated strain resulting 

 from such change. It is considered correct to refer to artificially produced 

 albino, yellow, and buff-colored strains of A. terreus as mutations (see p. 

 75), since they are constant in character and are known to have originated 

 from a cinnamon-colored parent culture, wholly representative of the 

 species; and we believe it represents good judgment to refrain from assign- 

 ing Latin designations to each of them. The term mutation, then, refers 

 to altered strains of constant character and known lineage, whether or not 

 the y are given Latin designations. 



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