Heredity in Protozoa 515 



was believed to indicate that mating types are controlled by the macro- 

 nucleus (74), although it is not clear how this can explain the production 

 of two types from one exconjugant. The possible action of cytoplasmic 

 factors {plasmagenes) in variety 1 seems to have been dismissed in the 

 statement that "mating is determined by the genes and is not affected by 

 whatever initial differences in cytoplasm may have existed" (86). 



Mendel ian inheritance has been reported in matings between a per- 

 manently type I strain and a "two-type" strain in which both types I and 

 II appear within a clone (74). The hybrid exconjugant lines usually 

 showed the two-type condition, indicating its dominance over the one- 

 type trait. In back-crosses between the recessive (one-type) and the hybrid 



Fig. 9. 2. Inheritance of mating types in conjugation of Paramecium 

 aurelia, group A, mating types I (solid black) and II. 



(two-type) progeny, about half of the exconjugant lines showed the one- 

 type and the rest the two-type condition, as in a mendelian back-cross. 

 Certain exceptional results were later attributed to cytogamy, which may 

 occur in about 60 per cent of the conjugating pairs at 27° (81), a tem- 

 perature within the optimal range (25-30°) for conjugation of variety 1 

 (87). The method of inheritance, in this "first discovery of inheritance in 

 Mendelian ratios in the ciliate Protozoa" (74), is not altogether clear. 

 Individual matings, in crosses between the one-type (type I) and the two- 

 type strains, doubtless involved type I and type II ciliates. These type II 

 ciliates presumably were hybrids because the exconjugant lines, instead 

 of belonging only to type II or type I, included both mating types. The 

 progeny, in back-crosses to the parental type I (one-type) stock, must have 



