11-5 THE YEAST CELL 



plate one finds an outer fringe of extremely rough, large colonies 

 surrounding a central group of small, smooth ones. All belong to 

 the same genotype but the rough character cannot come into expres- 

 sion until the colony attains considerable size. 



We have observed several hundred different types of rough - 

 colonied yeasts, and although each one is distinctive and recogniz- 

 able and can be duplicated and recognized when transplanted, we 

 seldom encoimter duplicates except in closely inbred cultures. The 

 range of variation is extremely great. In addition to the fundamental 

 "rosette" or budding pattern, differences in shapes and sizes of the 

 cells affect the colonial form. In all colonies the variation in cell 

 size increases with age, generally in the direction of producing 

 larger, longer cells. As a rule, extremely rough colonies contain 

 more elongate cells than smoother colonies, and part of the basis 

 for extreme roughness is probably the maintenance of end-to-end 

 connections after cell division which has been described so fre- 

 quently in the genus Bacillus. 



All four cultures from the single ascospores isolated from a 

 4-spored ascus originating from stable, smooth-colonied, wild-type 

 diploid cultures of Saccharomyces cerevisiae are usually rough- 

 colonied. Although we have dissected many asci from the same 

 diploid culture, duplicate cultures are seldom found among the colo- 

 nies grown from the single ascospores, indicating that the diploid 

 cell is heterozygous for many loci which affect "roughness." There 

 are some pedigrees, however, in which one allele apparently con- 

 trols such an extremely coherent growth that the haplophases car- 

 rying it often settle out rapidly on growth in broth, making it pos- 

 sible to differentiate this culture from its allelic type by the speed 

 with which flocculation (autoagglutination) occurs. However, since 

 most haplophase cultures are slowly autoagglutinable, it is diffi- 

 cult to get good Mendelian segregation. In the pedigrees described 

 in Chapters 19 and 26, the symbols F and f (flaky) are used to de- 

 signate rapidly autoagglutinable and less rapidly autoagglutinable 

 cultures. *F" cultures are always very rough. Since many factors 

 apparently affect roughness or autoagglutination, we have not at- 

 tempted to work out linkage relations. 



QUANTITATIVE CHARACTERS 



Quantitative characters such as weight and height (in man, for 

 examDle) are under genetic control but are also influenced consid- 

 erably by the environment. Similarly, quantitative characters such 

 as the dry weight of yeast produced in a specified medium are under 

 genetic control but are dependent on environmental conditions while 

 a character like mating type is relatively independent. Rate of growth 

 is a character of great economic importance but it is also very dif- 

 ficult to measure accurately because of the influence of the environ- 



