SPORULATION 3 -6 



asci will produce only 2-spored forms on potato. It has been dif- 

 ficult to understand why this should occur. The first explanation 

 which comes to mind is that a lethal gene has been segregated; 

 however, it is not reasonable for a lethal gamete to exist as a free 

 cell. It seems probable, however, that a gene, having no effect on 

 the free living state, may act as a spore lethal. We have recently 

 encountered an example. One of our haplophase cultures mutated 

 to produce colonies with a brownish tint. These brown colonies 

 proved capable of copulating and hybrids between them and one of 

 our standard stocks were produced. The hybrids produced 4-spored 

 asci, and a number of them were dissected. Only 2 viable spores 

 were obtained from each ascus. This suggests tiiat the brown mut- 

 ant, although capable of growing in the haplophase, was incapable 

 of producing a viable spore and therefore acted as a spore lethal. 



SPORULATION OF LEGITIMATE AND ILLEGITIMATE DIPLOIDS 



The original concept of heterothallism in yeast (Lindegren and 

 Lindegren, 1943) was based on the following argument. Intact 4- 

 spored asci and single ascospores were isolated from a vigorously 

 sporulating diplophase. The colonies which each produced were 

 tested for their ability to produce spores. Each culture produced 

 by an intact 4-spored ascus developed into a large round colony 

 containing large ellipsoidal cells; all were capable of producing a 

 large percentage of 4-spored asci (Fig. 3-1). On the other 

 hand, when 4-spored asci from the original culture were dissected 

 and the four single spores grown separately, each culture produced 

 small rough colonies, and when these cultures sporulated the asci 

 were usually 2-spored and were relatively non -viable. 



