756 INHERITANCE 



Segregation of mating types at endomixis. — It is of great interest that 

 at the first fission after endomixis segregation of the mating types may 

 occur, just as it does after conjugation. The single individual, of a definite 

 mating type, divides after endomixis into two that may be diverse in 

 mating type. The nuclear processes in endomixis are not yet fully cleared 

 up, but are known to be in many respects similar to those which occur 

 at conjugation. According to the recent work of Diller ( 1936) , the simi- 

 larity goes so far that there is in endomixis a union of two micronuclei 

 (presumably haploid), just as occurs in conjugation. In this autogamy 

 the two micronuclei that unite are of course both from the same indi- 

 vidual. After their union, the single (diploid) nucleus divides into two, 

 then into four. Two of these become macronucei and two remain micro- 

 nuclei, just as in conjugation. The two macronuclei are separated at fis- 

 sion, just as occurs after conjugation, and the same is true of the micro- 

 nuclei. As will be seen later, there is genetic evidence that autogamy does 

 indeed occur at endomixis. 



In variety or group 1 of Paramecium aurelia, Kimball (1937) deter- 

 mined the mating type of the two clones produced by each of 181 indi- 

 viduals that had undergone endomixis. Of the individuals, 96 gave 2 

 clones of the same type (both I or both II) , while 85 gave two clones of 

 different types (one type I, one type II). In other respects also the re- 

 sults of endomixis in relation to the mating types are like those of con- 

 jugation (see Kimball, 1937). 



As a rule, in both endomixis and conjugation the segregation of the 

 mating types occurs at the first fission after completion of the process. 

 But in a certain stock there was a small proportion of cases in which 

 segregation into two mating types occurred at the second fission after 

 conjugation. In this same stock, cytological examination showed that in 

 about the same small proportion of cases the ex-conjugants had three or 

 four macronuclear Anlagen in place of the usual two. It of course re- 

 quires two fissions to separate these into different individuals. Sonneborn 

 is disposed to consider this the cause of the occasional segregation of the 

 mating types at the second fission, in place of the first. 



Single-type clones. — There exist clones in which there is no segrega- 

 tion into different mating types at endomixis. In such clones endomixis 

 makes no change in the mating type; the clone remains throughout of 

 the same type (Sonneborn, 1938c). Of twenty-six clones examined by 



