T. M. SONNEBORN 165 



there are, however, a number of exceptions. The rule is that two 

 mating types belonging to different varieties do not conjugate 

 with each other. Indeed, they do not even react to each other 

 in any observed way. Exceptions to this rule are shown only by 

 certain combinations of mating types belonging to the eight 

 varieties 1, 3, 4, 5, 7, 8, 10, and 14, as shown in Table I. These 

 exceptions are of two kinds. First, certain combinations of mating 

 types of different varieties react sexually to each other and form 

 conjugating pairs. Second, other combinations give a weaker sex- 

 ual reaction, adhering to each other briefly, but they never pro- 

 ceed to form conjugating pairs. These intervarietal reactions are 

 often difficult to obtain except when the cultures are in optimal 

 condition for conjugation. It is therefore possible that some com- 

 binations which have never been observed to react might do so 

 and that others which have been seen to react only weakly 

 might yield some conjugant pairs. 



The system of intervarietal mating reactions leads clearly to 

 the conclusion that, at least for the eight varieties listed in 

 Table I, the two mating types of one variety are related to the 

 two in other varieties. In the two pairs of varieties 1 and 5, and 

 4 and 8, the odd-numbered mating type of each variety mates 

 with the even-numbered mating type of the other : both I and 

 IX mate with II and with X; and both VII and XV mate with 

 VIII and with XVI. Hence, similarities exist between mating 

 types I and IX, between II and X, between VII and XV, and 

 between VIII and XVI. Types V and VI can be brought into the 

 system of similarities because V mates with both II and VI; and 

 II mates with both I and V. Hence I is similar to V and II is 

 similar to VI. The groups of similarities are enlarged by the facts 

 that V mates with II, VI, and XVI; and XVI mates with V, VII, 

 and XV. Hence V, VII, and XV form a similar set; and II, VI, 

 and XVI form another similar set. By adding the other reactions 

 in Table I, it can easily be shown that the odd-numbered mating 

 types in these eight varieties form one similar group which may 

 be designated the minus group; while the even-numbered mat- 

 ing types form another similar group, which may be called the 

 plus group (Sonnebom and Dippell, 1946). Thus, each of these 



