iMATING TYPE DETERMINATION 273 



nrc in sonic unspecified way different. No mention has yet been made 

 of particular gene differences affecting mating type inheritance. One 

 effective gene difference has, however, been reported (Sonneborn, 

 1939). The difference in the mating type phenomena in the two-type 

 stocks fullv discussed above and in the one-type stocks briefly men- 

 tioned at the be^innintr of this section is due to a difference in a 

 single pair of allelic genes. The recessive allele, for which the one- 

 type stocks are homozygous, restricts mating type to type I: the 

 dominant allele (homozygous in two-type stocks) permits develop- 

 ment of either mating type according to the pattern set forth above. 

 The mode of action of these alleles remains unknown. 



The results of previous investigations on mating type deter- 

 mination in stocks of group A may be summarized as follows: 



1 . The differences between the complementary mating types are 

 due to some as yet unknown differences in the macronuclei. 



2. These macronuclear differences arise at the time the macro- 

 nuclei develop from products of the syncaryon. 



3. Temperature increases at this time increase the probabiUty of 

 origin of a macronucleus that will control the even mating type. 



4. The two macronuclei which develop synchronously in the 

 same fertilized cell are independently determined as to which mating 

 type they will control. 



5. A single gene difference determines whether both mating 

 types can be produced or only one, the odd mating type. 



6. In stocks in which both mating types can be produced, there 

 is no effect of the mating type of an individual on the mating type 

 of its sexually produced offspring. 



Group B Pattern 



Many of the features of mating type determination and inherit- 

 ance in group A are also found in group B (Sonneborn, unpub- 

 lished). As a rule mating types are strictly inherited during vegetative 

 reproduction. Stocks of varieties 4 and 8 show a small percentage of 

 selfing caryonides, just as some stocks of variety 1. Varieties 2 and 

 6, like variety 5 in group A, show a much higher frequency of these 

 selfers. With the exception of the selfers, mating types change only 

 at the time of nuclear reorganization. After nuclear reorganization 

 the sister caryonides from a single reorganized cell may show differ- 



