SEGREGATION, VARIATION AND RECOMBINATION 13-5 



VARIATION AND COLONIAL TYPES 



On serial plating a large variety of colonial types appeared from 

 each of the four haploid cultures. Plating and selection of the colonies 

 were continued for several weeks and several stable forms were ob- 

 tained. The cells in these colonies were all round-celled like their 

 progenitors and therefore haploid. The slow-growing, small- colonied 

 culture (A) produced more different types of colonies than the other 

 three This observation is in line with those on other strains, 

 which indicate that weak cultures usually produce many more vari- 

 ations than vigorous cultures, probably because the new mutations 

 meet with less competition from the original type. 



Cultures B, C and D show a characteristic feature of the more 

 vigorous haplophase strains. When plated on agar the character- 

 istic large colony appears accompanied by a smaller smooth form. 

 (The cavities in the colonies reveal the places from which trans- 

 fers have been made.) Transfer of the large rough colony produced 

 the original rough type again accompanied by the small smooth form. 

 The original rough type is the primary segregant while the small 

 smooth form is a variant of the original segregant. The new vari- 

 ant forms a small colony because it grows slowly; its smoothness 

 is due to the small size rather than being related to any specific 

 morphological colonial type. Characteristic colonial morphology 

 does not appear in young or small colonies. Subculture of the 

 smooth produces the smooth form along with a few colonies which 

 appear to be directly derived from this variant now adapted to grow 

 on agar and capable of producing a colony large enough to display 

 its specific morphological characteristics. The new variants from 

 the original segregant pass through the small round phase before 

 becoming adapted. These new types may be either true mutations 

 or what we have called depletion mutations. Depletion mutations 

 are variants which can be propagated indefinitely in the vegetation 

 haplophase but which revert to normal in the heterozygous condi- 

 tion. They cannot pass through the sexual cycle. Much of the varia- 

 tion found when haplophase colonies are plated on agar is due to de- 

 pletion mutation rather than true mutation. This is proved by the 

 fact that the characteristic variation does not reappear among the 

 offspring of hybrids produced from matings of the variants. 



The four segregants shown in fig. 13-2 are characteristically 

 different in colony shape and topography. The D culture is the 

 roughest. The C culture produces the largest colonies and sub- 

 sequent testing on a variety of sugars indicated that it grew most 

 rapidly and produced most gas. The A culture, even after long sub- 

 culturing, was much less vigorous. 



