238 Cytoplasm as Seat of Genetic Properties 



others (see Sonneborn, 1955) now show that, after all, this strange 

 cytoplasmic property which acts upon the macronuclei is not auton- 

 omous but is itself under nuclear control, just as in Ascaris and other 

 cases we have mentioned. 



In view of these facts Sonneborn draws general conclusions which 

 are much more cautious than those usually presented by the believers 

 in plasmagenes. To quote: "These considerations, like others set forth 

 in recent years by other investigators, render less sharp and simple 

 our concepts of mutation, self-duplication, genetic materials, and the 

 genetic system. The undisputed two-way interaction between nucleus 

 and cytoplasm does no violence to the simple, monistic concept that 

 genetic control resides ultimately in the nucleus alone, so long as the 

 nucleus is the only genetic material in the system. However, if by 

 genetic material we mean any material which controls the formation 

 of more of its own kind and which, if changed, controls the forma- 

 tion of the new kind, then some of the results . . . speak for the inclu- 

 sion of the cytoplasm as genetic material and as part of the genetic 

 system." We have made it sufficiently clear where we stand after con- 

 sidering the entire body of facts. 



One more point may be made in regard to cytoplasmic control of 

 the nucleus. It is well known that in aphids, after many generations 

 of diploid parthenogenesis producing only females, males appear — 

 sometimes from special arrhenotoke mothers — by a strange process of 

 eliminating one X-chromosome from the egg during the equational 

 division of the parthenogenetic diploid egg. Since this process is 

 influenced by the number of preceding parthenogenetic generations, 

 by temperature, and sometimes by the maternal organism ( the females 

 producing only males), there can be hardly any doubt that the 

 causative agent is a cytoplasmic condition. The fact that all imaginable 

 variants are realized in different forms — for example, no males, males 

 after few or many generations, males only from certain females, which 

 produce the elimination of one X in all their eggs, and males and 

 females from the same mothers — suggests the presence of threshold 

 conditions in the cytoplasm and the accumulation of some substance 

 in the course of generations which, when concentrated beyond the 

 threshold, exercises the effect upon chromosomal behavior. The same 

 interpretation must apply to the fact that the males as well as the 

 simultaneous -females suddenly undergo typical meiosis (meaning 

 reductional divisions ) while, thus far, the females had only equational 

 ones. Furthermore, Seiler (1920) could control the movements of the 

 X- and Y-chromosomes in the meiosis of the moth Talaeporia by 



