Heredity in Protozoa 519 



dence, it has been concluded from genetic data that the two gametic 

 nuclei in P. aurelia are always genotypically identical (81). 



Genetic changes in autogamy have been reported in Paramecium. A 

 clone of P. bursaria may differentiate into two mating types after autog- 

 amy (31). After autogamy in variety 1 of P. aurelia, all of the progeny 

 usually belong to either mating type I or mating type II, and heterozygous 



Fig. 9. 5. Theoretical genetic effects of autogamy in a heterozygous 

 ciliate, based on Diller's (II) description of autogamy in Paramecium 

 aurelia. In the diagram, it is assumed that four nuclei undergo a sec- 

 ond pregamic division and that only one haploid nucleus of each type 

 completes the third division. 



Strains apparently become homozygous (35, 74). A lag has been noted in 

 the transformation of heterozygous type II into type I. The change occurs 

 at different times in different ciliates, so that cultures come to contain 

 organisms of both types and may show mating reactions. Such a lag was 

 not observed in the change from heterozygous type I to type II. In addi- 

 tion to the usual production of all type I or all type II, both mating types 

 sometimes arise after autogamy. These unusual cases are not readily ex- 

 plained since the lines arising from one ciliate all contain micronuclei 

 and macron uclei derived from the same synkaryon. 



Cytogamy 



In addition to autogamy, cytogamy is believed to occur in P. 

 aurelia (81). Cytogamy (Chapter II) is essentially incomplete conjuga- 



