758 INHERITANCE 



goes the third division, yielding in one case two nuclei A and A, in the 

 second case a and a. These two nuclei now reunite (autogamy), yielding 

 in the one case the homozygote dominant AA, in the other case the 

 homozygote recessive aa. Thus after endomixis has occurred in any stock, 

 the clones are all homozygotes, as Sonneborn points out. 



These results are obviously strong genetic evidence for the occurrence 

 of autogamy at endomixis in 'Pdramecium aurelia. 



Certain rare and exceptional conditions in the genetics of the mating 

 types in P. aurelia are discussed in Chapter XIV, "Sexuality in Unicellular 

 Organisms." 



Inheritance of Mating Type in Paramecium bursaria 



In Paramecium bursaria, according to the work of Jennings (1939a, 

 1939b) , there are in one of the varieties four mating types, A, B, C, and 

 D. Another variety has eight mating types (E, F, G, H, J, K, L, and M) , 

 a third variety four types (N, O, P, and Q) distinct from the four of 

 the first variety. Inheritance of mating type has been examined only in 

 the first variety (some of the relations are here published for the first 

 time) . 



In variety 1 (as in the other varieties), clones of any one of the types 

 may conjugate with clones of any of the other types, but not with clones 

 of their own type Thus in variety 1 six different matings occur, yielding 

 pairs AB, AC, AD, BC, BD, CD. 



Very rarely in this species a single clone, in which all the individuals 

 belong originally to the same type, may difi^erentiate into two types, 

 which thereupon conjugate. This phenomenon is parallel to the segrega- 

 tion of difi^erent types from one type at endomixis in P. aurelia. It is 

 presumably the result of endomixis in P. bursaria; in this species endo- 

 mixis is known to occur very rarely (Erdmann, 1927). Thus the occa- 

 sional "self-fertilization" of a clone is in fact the conjugation of two 

 diverse mating types. A clone of the mating type D has been observed 

 to differentiate part of its individuals into the mating type A; these then 

 conjugate with the D individuals, giving the cross AD. The results of 

 "self-fertilization" may therefore be considered with those of other mat- 

 ings between two different types. 



In studies of mating-type inheritance, four descendant clones are ob- 

 tained from each pair, two from the first fission of each ex-conjugant, 



