Heredity in Protozoa 523 



said to depend upon transfer of cytoplasm from the killer to the non- 

 killer conjugant (86). When autogamy occurs in such lines, some of the 

 descendants remain killers, presumably as KK-kappa genotypes, while 

 the rest become non-killers after kappa has disappeared from the cyto- 

 plasm. 



The status of the killer and non-killer traits is further complicated by 

 the conclusion that another pair of genes, S and s, and a corresponding 

 plasmagene, sigina, are involved. The relationships between sigma and 

 its homologous alleles are comparable to those between kappa and its 

 related genes. Sigma supposedly has the ability to compete with kappa 

 and to replace it under certain conditions, but is "not actually a factor 

 for sensitivity" (82). It now appears that a homozygous killer (KKSS) 

 strain can yield pure killer and pure sensitive strains differing only in 

 cytoplasmic factors. 



Mating types and cytoplasmic inheritance 



The difficulty of explaining inheritance of mating types in variety 

 4 of P. aurelia (Fig. 9. 3) is responsible for the conclusion that plasmagenes 

 are involved (47, 84). The usual appearance of the original mating type 

 in each exconjugant line wotdd have to be explained on the basis of 

 cytogamy rather than interchange of gametic nuclei, if nuclear genes are 

 responsible. The production of type VIII from conjugants belonging 

 originally to types VII and VIII would imply that type VIII is dominant 

 to VII, if nuclear control exists. Other matings, in which the results are 

 exactly reversed, would indicate that type VII is dominant to type VIII. 

 Furthermore, the origin of both mating types from one exconjugant can 

 scarcely be explained on a micronuclear basis, although a similar phe- 

 nomenon has been attributed tentatively to macronuclear mutation in 

 group A (81). 



These peculiarities in the inheritance of group B mating types are 

 attributed to the occurrence or the lack of cytoplasmic transfer during 

 conjugation (84), although such a process has not been detected in cyto- 

 logical studies on P. aurelia (11) and P. bursaria (100). According to this 

 hypothesis, no transfer of cytoplasm has occurred when type VII conju- 

 gants produce only type VII descendants, and type VIII only type VIII. 

 If the exconjugant lines are all type VIII, in a VII x VIII mating, 

 cytoplasm has been transferred from the type VIII to the VII conjugant. 

 If the results are reversed, cytoplasm has been transferred from type VII 

 to type VIII. If one exconjugant produces two mating types, interchange 

 of cytoplasm has been followed by segregation of plasmagenes in post- 

 conjugant fissions. Sonneborn assumes that there are two kinds of plasma- 

 genes in variety 4, one controlling type VII and the other type VIII. The 

 same explanation is believed to hold for other varieties of group B. 



