GENETICS AND CYTOLOGY OF SACCHAROMYCES 257 



YEAST GENETICS 



Until 1935, yeasts were considered to be devoid of sex and, therefore, un- 

 suitable for genetical analysis. At that time, Winge showed that the standard 

 yeast cell carried two sets of chromosomes — one contributed from each 

 parent — and was, therefore, a typical hybrid. The hybrid yeast cell produces 

 four spores, each with a single set of chromosomes. Each of these spores is a 

 sex cell. By fusing in pairs they can produce the standard (hybrid) yeast cell 

 and complete the life cycle. In this laboratory it was shown that the spores 

 are of two mating types, and that each spore can produce a culture each cell 

 of which can act as a sex cell, like the original spore. Mass matings between 

 two such spore-cultures result in the production of fusion cells, from which 

 new hybrids are produced by budding. 



This work made it possible to study the inheritance of biochemical de- 

 ficiencies in the organism on which classical enzyme study is based, and to 

 attack the problem of the relation of genes to enzymes in this fruitful mate- 

 rial. We have related specific genes to several of the most thoroughly studied 

 classical enzymes: sucrase, maltase, alpha methyl glucosidase, melibiase, and 

 galactase. 



The principal advantages of yeasts for biochemical genetics are: 



(1) Yeast enzymes have been the subject of intensive biochemical study. 



(2) Techniques for studying respiration and fermentation are based prin- 

 cipally on work with yeast and thus especially adapted to this organism. 

 Yeasts grow as free cells rather than as mycelial matts and, therefore, can be 

 subdivided any number of times without injury, thus simplifying weighing 

 and dilution of the cells. 



(3) Large quantities of cells are available from industrial sources or can be 

 grown cheaply and quickly and are easily stored in living condition. 



(4) A variety of genes concerned with the differential utilization of nu- 

 merous monoses as well as di- and poly-saccharides are available. 



(5) A polyploid series of yeast cultures is now available: (a) haploid cells, 

 each containing a single set of chromosomes, (b) diploid yeast cells, each con- 

 taining the double number of chromosomes, (c) triploid, and (d) tetraploid 

 cells (made available by our recent discovery of diploid gametes [Lindegren 

 and Lindegren, 1951]). 



(6) With free cells it is possible to study competition between genotypes 

 and to observe the advantages or disadvantages in controlled environments. 

 The populations involved are enormous and the life cycles short, so it is pos- 

 sible to simulate natural selection in the laboratory. Experiments of this type 

 have enjoyed an enormous vogue with bacteria, but it has not been possible 

 to distinguish gene-controlled variation from differentiation. Eor this reason, 

 experiments with bacteria cannot be interpreted in terms of the comparison 

 between gene-controlled and other types of inherited characteristics. 



