MATING TYPE DKTFRAIINATION 275 



4. This pnrcnr-progcny correlation is due to cytoplasmic con- 

 ditions ciiaractcristic of the two mating types which determine the 

 manner in which the new nuclei develop. 



NATURE OF MACRONUCLEAR DIFFERENCES 



1 he facts presented in the previous section show a consistent 

 pattern of mating type determination and inheritance in both groups 

 of varieties, hut leave unanswered a central question: In what ways 

 do the macronuclei in cells of different mating types differ? A pos- 

 sible clue to these differences comes from a study by Chao (1953) on 

 the killer character in variety 4. Sonneborn (1947) showed that the 

 killer character is determined by the presence of self-duplicating par- 

 ticles, called kappa, in the cytoplasm. He further showed that the 

 presence of kappa is conditioned by the presence of a gene, K, in 

 the macronucleus. Freer (1950) developed technics for visualizing the 

 particles cytologically, and these technics have been used by Chao to 

 study directly the factors influencing kappa concentration. Two of 

 his observations are pertinent here. ( 1 ) Cells of a given mating type 

 w^ill support twice as many kappa particles when they have the 

 genotype KK as when they have the genotype Kk. This observation 

 shows a direct numerical relationship between the number of K genes 

 in the nucleus and the number of kappa particles in the cytoplasm. 

 (2) Cells of a given genotype will support twice as much kappa 

 when they show the odd mating type as when they show the even 

 mating type. These observations suggested (Nanney, 1953) that 

 macronuclei in cells of the odd mating type contain twice as many 

 K genes as those of the even mating type. Alternatively, it is possible 

 that certain genes in one mating type are precisely twice as active 

 as those in the other mating type, but this explanation seems less 

 reasonable. 



Chao's observations form the basis for a gene-dosage hypothesis 

 of mating type determination. This hypothesis can take a number of 

 different forms. Differences in the number of K genes could be 

 achieved though a doubling or halving of the chromosomes bearing 

 the K gene, thus yielding an aneuploid constitution for one of the 

 mating types. Similarly, differences in the number of K genes could 

 be brought about by a doubling or halving of the entire chromosome 

 complement. This interpretation would ascribe significance to dif- 



