27-9 THE YEAST CELL 



results. This was supposed at first to be the result of statistical 

 sampling; in the enormous population of cells, the chances that a 

 given zygote had been derived from the two preponderant genotypes 

 is presumably very great. However, the demonstration of depletion 

 mutation indicates that many of the variants which appear on the 

 plates are depletion mutations which cannot be transmitted through 

 the zygote. Therefore, the reliability of the method of mass mating 

 is greater than we had at first supposed. As a matter of fact, when 

 the mating types were first discovered, we suspected that it would 

 not be possible to use mass matings for genetical analyses, because 

 the individual cultures appeared to be so "mutable". We thought that 

 it might be preferable to use Winge and Laustsen's original method 

 of spore-to-spore matings and rely on the characteristics of the dip- 

 lophase for genetical analysis. However, numerous trials have con- 

 firmed the reliability of the mass mating method, although at the 

 time, it seemed curious that such apparently unstable gametes could 

 produce such surprisingly stable zygotes. 



Each of these examples could be explained by assuming the ex- 

 istence of autonomous cytoplasmic entities. However, an alternative 

 explanation will be presented after a discussion of the plasmagene 

 hypothesis. 



THE PLASMAGENE HYPOTHESIS 



Plasmagenes are hypothetical hereditary units supposed to re- 

 side in the cytoplasm which perpetuate themselves independently of 

 the genome in cells in which they find a "favorable" environment* 

 They were invented to explain maternal or cytoplasmic inheritance 

 and have been exploited by Sonneborn to explain the "killer" phenom- 

 enon in Paramecium and by Spiegelman to explain the mechanism 

 of adaptive enzyme formation in yeasts. They are differentiated from 

 viruses in being transmitted by grafts and by maternal inheritance 

 rather than by infection. Darlington believes that viruses have been 

 evolved from plasmagenes, and that plasmagenes themselves have 

 originated from cytoplasmic proteins which have acquired the prop- 

 erty of self -duplication. 



A. The Paracrinkle Virus 



Darlington chose as a prime example of a plasmagene the para- 

 crinkle virus which is without symptom in the King Edward potato, 

 but can be transmitted by graft to Arran Victory. The distinguishing 

 characteristic par excellence of plant viruses is the fact that they 

 are not transmitted through the egg. However, when Carson, Howard, 

 Markham, and Smith made hybrids of King Edward (?) by flourball 

 (o* ) they foimd the seedlings were not infected with the virus al- 



