516 Heredity in Protozoa 



been represented only by type II phenotypes which were type II/I hy- 

 brids, since the back-cross produced both mating types. However, such 

 assumptions do not explain the occasional origin of two mating types 

 from a single exconjugant, as reported in the back-crosses. 



In group B of P. aurelia — types III and IV (variety 2), VII and VIII 

 (variety 4), XI and XII (variety 6), and XV and XVI (variety 8) — mating 

 types usually do not change at conjugation (87). In crosses between types 

 VII and VIII (Fig. 9. 3), the type VII exconjugant usually produces type 

 VII, and the other exconjugant type VIII lines. Cytogamy, which seems 

 to occur occasionally in variety 4 (77), might account for such behavior 



C"<0 



or 



R E O R G A/N 1 Z A T I 0/N 



M M M 



Fig. 9. 3. Inheritance of mating types in conjugation of Paramecium 

 aurelia, group B, mating types \^II (solid black) and VIII. 



of mating types. However, both exconjugants may sometimes produce 

 type VII lines, or both may give rise to type VIII lines. At present, the 

 combined results cannot be explained logically on the basis of nuclear 

 behavior in conjugation. Consequently, Metz (47) has insisted that cyto- 

 plasmic factors afford the only mechanism which can account for the 

 behavior of mating types in group B. 



The inheritance of mating types in Euplotes patella (types I-VI) has 

 been explained by a system of triple alleles in which the genotypes are 

 represented as follows: type I, mtimt^; type II, mtiints; type III, mtgrntg; 

 type IV, mtjTntj; type V, mtzmts; type VI, mt^mt^. Crosses between types 

 I and II yield types I, II, IV, and V, while IV x VI crosses yield only type 

 I (38). The crosses I x III, II x VI, and III x IV, also have produced the 

 expected results (60). An interesting feature of E. patella is that amicro- 



