T. M. SONNEBORN 225 



probability of each type may not be far from 0.5; and with this 

 probability only one-eighth of the synclones would be uniform 

 for mating type. However, this is the minimal proportion of syn- 

 clones uniform for mating type; all other probabilities give higher 

 proportions. Since the minimum is virtually zero in group B varie- 

 ties under some conditions, the group A method of mating type 

 determination is, under these extreme conditions, more efficient 

 in promoting outbreeding than the group B method. In agree- 

 ment, the most extreme outbreeder in which the method of 

 mating type inheritance is known ( variety 15 ) occurs in group A 

 and the most extreme inbreeders (varieties 10 and 14) in group 

 B. 



The potentiality of the group A method for promoting out- 

 breeding is further indicated by some other facts. Different strains 

 of the same variety, e.g., variety 1, have characteristically differ- 

 ent proportions of the two mating types under the same tempera- 

 ture conditions (Sonneborn, unpublished; some data quoted by 

 Butzel, 1955). Butzel (1955) presented evidence that these dif- 

 ferences are due to genes other than the mating type genes. This 

 probably explains why hybrids between certain strains show in 

 the Fl a much higher frequency of type II than either parent 

 strain ( Sonneborn, 1942a ) , while hybrids between other strains do 

 not ( Butzel, 1955 ) . If the group A method of mating type inheri- 

 tance is ever a strong factor for promoting outbreeding, one might 

 expect outbreeding strains or varieties to yield high frequencies 

 of one mating type under the conditions in which they conjugate 

 in nature. As this discussion has attempted to bring out, there are 

 aspects of mating type inheritance which bear on the breeding 

 system, but little is known as to whether they do in fact operate 

 this way in nature. 



With due allowance for the points on which we are still regret- 

 tably ignorant, the mass of information now in hand on the varie- 

 ties of P. aurelia has acquired for the first time coherence and 

 significance by the perception of its bearing on systems of breed- 

 ing. Instead of being merely so many interesting but isolated facts 

 of natural history or experimental research, they are now the 

 meaningful elements of the grand design by which this organism 



