Chapter 13 

 SEGREGATION, VARIATION AND RECOMBINATION 



SEGREGATION OF MATING TYPE ALLELES 



Fig. 13-1 A, B, C, and D show the cells grown from the four 

 separate spores from a single ascus. Each is a haplophase cul- 

 ture. The four haplophase cultures were paired in all combinations 

 as indicated. A and D were arbitrarily designated as belonging to 

 mating type a, while B and C belong to a . Copulation tubes and 

 zygotes were produced when A and B, A and C, B and D, and C and 

 D were paired. When transferred to gypsum the diploid cells pro- 

 duced 4-spored asci, while the A and D, B and C combinations 

 failed to produce either diploid cells or ascospores. No spores 

 were obtained from any of the unmated single ascospore cultures. 



SEGREGATION OF COLONIAL TYPES 



The colonies originating from the four single ascospore cultures 

 are shown in fig. 13-2. In addition, the colonies of two diplophase 

 cultures are shown on the same scale for comparison with colonies 

 of the four haplophase cultures. In fig. 13-3 outline drawings of 

 the cells of these six cultures appear. The diploid cells are long 

 and ellipsoidal in contrast to the small round cells from the haploid 

 cultures. 



Colonies of diploid cultures are uniformly large and smooth 

 provided they are plated on agar before spores form spontaneously. 

 If they are left a few weeks on an agar plate and then plated again 

 on agar, numerous small rough colonies also appear due to the 

 germination of ascospores which have formed in the colony. The 

 large colony size is evidence of the greater vigor of the diplophase. 



The variation in the colonies of the four haplophase cultures 

 shows the degree of heterozygosis of the diplophase from which 

 they were derived. The different colony types are genetically dif- 

 ferent and result from the segregation of genes before spore forma- 

 tion. 



The photographs of the haploid cells (fig. 13-1) reveal that the 

 cells of the roughest colony (D) produce "rosettes" characteristic 

 of most rough colony types. Culture A grows slowest on agar and 

 the first plating produced only small colonies all of which were 

 apparently uniform. In small colonies morphological character- 

 istics are difficult to determine, since diagnostic characters are 

 clearly shown only by relatively large colonies. 



13-1 



