292 SEX IN MICROORGANISMS 



Ordinarily the mating reaction and conjugation occur only between 

 the two mating types in a variety. P. aiirelia (Sonneborn, 1937, 1938a, 

 1939a, 1942a, 1947), P. caudatimj (Gilman, 1939, 1941, 1950; Y. T. 

 Chen, 1944; Hiwatashi, 1949a), P. ivoodmffl (Woodruff, cited by 

 Sonneborn, 1939b), and P. calkijisi (Sonneborn, 1939b; Wichterman, 

 1951) follow this type of behavior. 



In the second group of animals, sexually isolated varieties are 

 again found, but in this group several mating types are present in 

 certain varieties and any one of these will mate with all others in the 

 variety. P. bursaria (Jennings, 1938, 1939a; Jennings and Opitz, 1944; 

 Chen, 1946a), P. trichhmi (Sonneborn, 1938a, 1939b), and P. imil- 

 timicromicleattim (Giese, 1941) show this type of behavior. 



Ajirelia-Type Systems. Among the aurelia-type species (two 

 mating types per variety) Paramecium aurelia has been examined 

 most thoroughly. The fifteen known mating types of this species fall 

 into eight sexually isolated varieties. With the exception of variety 

 7, which contains one (type XIII), all varieties contain two mating 

 types (Table I). On the basis of experiments on the breeding systems 

 and certain genetic studies, Sonneborn and Dippell (1946a) have 

 separated the several varieties into two distinct groups, the group A 

 and the group B varieties (Table I). In some respects the group A 

 varieties (Sonneborn's odd-numbered varieties 1, 3, 5, 7) are the more 

 interesting, for in this group intervarietal mating can occur in certain 

 combinations of types. Indeed every one of the group A mating types 

 gives at least one such reaction. These intervarietal mating reactions 

 are exceptional. They are never as intense as their intravarietal count- 

 erparts, they occur only under unusually favorable conditions, and 

 only four of them (types I x X; II x V; II x IX; II x XIII) ever lead 

 to complete conjugation (Sonneborn and Dippell, 1946a). Sonneborn 

 and Dippell have noted the following interesting relationships in these 

 group A reactions: (1) all seven group A mating types give a unique 

 set of reactions and are therefore different (types I and IX react with 

 different intensities with types II and X); (2) types I, V, IX, and 

 XIII are similar since they all react with type II; likewise types II, VI, 

 and X all react with type XIII; (3) mating reactions and conjugation 

 only occur between odd- and even-numbered types, never between 

 two odd- or two even-numbered types. From these observations 

 Sonneborn and Dippell (1946a) conclude that the several group A 

 mating types consist of two general types, or two series of homologous 



