512 Heredity in Protozoa 



Syngamy in diploid Protozoa 



A number of Protozoa undergo gametic meiosis and are diploid 

 throughout most of the life-cycle (Chapter II). Except for the ciliates, 

 which carry on conjugation instead of syngamy, the genetics of diploid 

 species is yet to be investigated. 



GENETIC EFFECTS OF 

 CONJUGATION 



The significance of conjugation in heredity was discussed by 

 Biitschli, R. Hertwig, and Maupas long before adequate experimental 

 data were available. It was suggested that conjugation, in bringing about 

 biparental inheritance, forms new combinations and thus increases varia- 

 tion. At the same time, conjugation was believed to level out major dif- 

 ferences arising in other ways, and in this sense, to limit the range of 

 variation. 



General effects of conjugation 



The work of Pearl (57) indicated that exconjugants are less vari- 

 able than non-conjugants, and that conjugation tends to prevent extreme 

 variation instead of inducing variation. However, Jennings (26) found 

 that exconjugants were more variable than non-conjugant lines with re- 

 spect to fission-rate. Since these differences were inherited, conjugation 

 in a population apparently gave rise to new biotypes, although the de- 

 scendants of a single pair were closely similar as a result of biparental 

 inheritance. The appearance of new combinations after conjugation 

 within a population was reported also in later investigations (7, 32, 64). 

 The effects may vary with the strain of Paramecium aurelia, variation 

 being increased in some strains but not in others (90). 



The other general effect of conjugation is the production of similarities 

 through biparental inheritance in single pairs of exconjugant lines. In 

 tracing the effects of hybridization on viability, body-length, and fission- 

 rate of Paramecin??! aurelia, Sonneborn and Lynch (92) found that some 

 lines resembled one parental type, some resembled the second, and others 

 were intermediate. Inbreeding showed that the intermediate types were 

 heterozygous; the others were apparently homozygous. It was concluded 

 that the inheritance of these traits in P. aurelia is basically mendelian. 



Cytoplasmic lag in biparental inheritance 



An unusual feature of hybridization has been the occurrence of a 

 "cytoplasmic lag" in the exconjugant phenotypes of P. aurelia. In the 

 experiments of Sonneborn and Lvnch (91), the two lines from each pair 

 of conjugants did not become phenotypically identical until ten genera- 

 tions or so had passed. A similar lag characterizes inheritance of body- 



