13-6 THE YEAST CELL 



The colonies of the four primary types obtained by subculture 

 six months after the original isolation are shown in fig. 13-4. The 

 three photographs labelled "A" all show variants of the A culture. 

 Two of these are relatively stable while considerable variation is 

 visible in the third. The large colony size shows that some of these 

 new variants have become fairly well adapted to the medium. The 

 primary segregants from ascospores B, C and D are still clearly 

 recognizable in spite of several serial transfers. They are gener- 

 ally much more stable than the A culture although many new vari- 

 ants were obtained from each of them. However, the slow growing 

 A culture produced so many variants during the serial transfers 

 that neither the original segregant nor the lines of descent can be 

 traced. 



VARIATIONS IN RELATION TO LIFE CYCLE 



Haploid yeast cells are much smaller and more variable than 

 diploid cells, varying more both from culture to culture and with- 

 in a single culture than diploid cells. These differences are also 

 reflected in the colonies. 



Segregation of genes occurs when the chromosomes of the dip- 

 loid cells are segregated at the reduction division just prior to 

 sporulation. The haplophase originates by the reduction of the dip- 

 lophase at spore formation, and the segregation of a heterozygote 

 produces segregants of different genotypes. Yeasts are extraor- 

 dinarily heterozygous, and a great variation of colonial forms is 

 obtained by the isolation of single ascospore cultures. Each of the 

 four spores formed in a single ascus is usually genetically differ- 

 ent. The haploid segregants are usually rough -colonied. The seg- 

 regant cultures also vary in fermentative ability, in ability to syn- 

 thesize amino acids, vitamins, and nucleic acid components, in 

 color, and in the size and shape of the haploid cells. The type of 

 cell aggregation is also characteristically different. Haplophase 

 clones generally tend to produce aggregated or agglutinated cells 

 much more frequently than diploid clones. 



Haplophase yeasts are nearly always inferior in synthetic abili- 

 ity when compared quantitatively, or qualitatively, to the diploid 

 parent from which they originated; many of them have lost certain 

 specific characteristics. For example, a single ascospore culture 

 originating from S. cerevisiae may be unable to ferment sucrose, 

 or maltose, or galactose, or even glucose, although the original 

 cultures fermented these sugars successfully. 



Variation in the haplophase enormously increases the number 

 of colonial forms, but the original segregant can generally be dis- 

 tinguished from the secondary variant when the culture is plated. 

 At first, the variants are usually slow-growing and produce small, 

 round colonies, but on transfer they become adapted and stabilized 



