520 Heredity in Protozoa 



tion in which the exchange of pronuclei is inhibited and each "conju- 

 gant" undergoes autogamy (99). The occasional occurrence of cytogamy 

 in Euplotes patella also is suggested by genetic data (38, 60); autogamy, 

 as described for other ciliates, does not occur in this species (36). Cytog- 

 amy has the same genetic significance as autogamy. 



GENETIC SIGNIFICANCE OF 

 THE MACRONUCLEUS 



The role of the macronucleus in heredity remains problematical, 

 although Sonneborn at one time believed that in P. aurelia, "the pheno- 

 type is controlled exclusively by the macronuclear genes" (81). In addi- 

 tion, the detection of macronuclear mutations has been discussed and 

 conditions for their appearance have been postulated (81). However, 

 such mutations apparently have not been demonstrated. 



The supposed macronuclear control of phenotypes in P. aurelia seems 

 to be based upon the correlation of macronuclear "regeneration" with 

 certain results in crosses of killer and no?i-killer strains (discussed below). 

 Macronuclear regeneration was induced by exposing conjugants (variety 

 1) to temperatures of 38.0-38.5° for not less than 3-5 hours following 

 "fertilization" (75). Such treatment seems rather rigorous, since P. cauda- 

 tum may be killed in nine seconds at 40° (59). Among various abnor- 

 malities (81), there was a retarded division of differentiating macronuclei. 

 In the postconjugant fissions, some ciliates received new macronuclei and 

 others only the fragments of the old macronuclei. The latter developed 

 macronuclei from the fragments, each of which became a complete nu- 

 cleus. The resulting macronuclei were distributed in subsequent fissions 

 until the normal nuclear situation was restored. 



In applying this process to the study of genetic problems, Sonneborn 

 (80) crossed non-killers (kk) with homozygous killers {KK). Macronuclear 

 regeneration was induced in the exconjugants derived from the non- 

 killers. These were supposed to have received from their mates an excess 

 of kappa, a cytoplasmic factor essential to development of the killer con- 

 dition. The zygotic nucleus of each exconjugant was a Kk genotype, and 

 the macronuclei derived from the synkaryon had the same genotype. The 

 non-killers which regenerated their macronuclei had to use fraginents of 

 the old kk macronucleus. Only ciliates with new Kk macronuclei de- 

 veloped into killers. Furthermore, non-killers {Kk micronuclei and kk 

 macronuclei) produced no killers after autogamy, although some must 

 have developed KK micronuclei and macronuclei. Their supply of kappa 

 presumably was exhausted before autogamy occurred. Accordingly, the 

 presence of gene K in the macronucleus was considered essential to the 

 continued production of kappa in the cytoplasm. It might be interesting 

 to extend these observations to such a combination as Kk micronuclei 



