10-5 THE YEAST CELL 



with only seven failures out of forty -eight tests. With this strain 

 also, culture lA produced illegitimate diploids. This highly fertile 

 culture was also able to mate with 2C, 3A, and 4B, which were in- 

 capable of producing fusions with any other culture with which they 

 were tested. When the a haplophases of the Ly and the **(800 x L) 

 X L" hybrid were outcrossed to the other strains of yeast, only 

 three hybrids were produced in several himdred matings, but out- 

 crossings with the a strains were much more successful and resulted 

 in a large number of hybrids. This occurred in spite of the fact 

 that the HD, M, FLD, D and B cultures were apparently quite in- 

 fertile among themselves. It appears, therefore, that the a strains 

 could be successfully outcrossed to produce hybrids with foreign 

 strains. The control of mating type by the a/a alleles has been cor- 

 roborated by the dissection of more than two thousand asci to date. 

 A standard strain is used to test all the haplophase cultures dis- 

 sected. A considerable number of sterile cultures were formerly 

 found but now that our cultures have been selected and inbred, ster- 

 ility and spontaneous illegitimacy occur only rarely. 



The fact that the haplophase culture from a single spore can be 

 used for an indefinitely large number of matings is particularly im- 

 portant when one wishes to introduce new genes into a stock. When 

 an ascospore is isolated from a natural species of Saccharomyces, 

 its haplophase cells are often foimd incapable of copulation when 

 tested against our inbred stocks. However, persistence is some- 

 times rewarded and occasionally one rare individual copulated to 

 produce a hybrid. These rare clones are used to bring new char- 

 acters into the pedigree. The desirable haplophase cultures are 

 preserved by drying. Dried haplophase cultures retain their full 

 copulative strength indefinitely, although vegetative propagation of 

 a haplophase culture may be accompanied by mutations which re- 

 duce mating strength and transform the original type into a sexu- 

 ally impotent haploid. 



Some mutants obtained by continued selection involving numer- 

 ous platings and transfers over a prolonged period (usually more 

 than a year) become sterile. Prolonged competition and selection 

 probably resulted in loss of fertility because the genes which in- 

 sure fertility do not have a high survival value in competition with 

 other rapidly growing mutants. 



These facts suggest that a moderately prolonged haplophase is 

 a distinct advantage in Saccharomyces because it permits compe- 

 tition among the new mutants that arise in the haplophase, leading 

 to selection of the most vigorous before copulations occur. This 

 mechanism may account for the fact that Saccharomyces is the most 

 cosmopolitan species of yeast. If the mating type strength is great, 

 as in Saccharomycodes ludwigii, fusions occur between the gametes 

 in the ascus without selection or competition and the plasticity of 



