234 Cytoplasm as Seat of Genetic Properties 



cc. Primary plastid differences 



While in Correns' and his successors' material of hereditary varie- 

 gation the facts were in favor of the absence of genetic differences 

 between the plastids, a considerable group of experiments show in 

 other cases that actually the plastids themselves can be genetically 

 different. The classic work is that of Renner on Oenothera crosses 

 ( see Renner, 1936 ) in which a most penetrating theoretical discussion 

 is found. The type of experimentation is made possible by the complex- 

 heterozygotic composition of Oenothera which easily permits the 

 combining of entire genomes of one type with different cytoplasms, 

 just as if only a single Mendelian difference were involved. The 

 procedure is to produce, by crossing, combinations of the homozygous 

 and heterozygous genomes with the reciprocal cytoplasms containing 

 the respective plastids, including also possible transfer of plastids by 

 the pollen. In some cases, the plastids from one species cannot form 

 chlorophyll in the presence of a hybrid or foreign genome. If plastids 

 are transferred with the pollen, a plastid mixture can be obtained 

 which later segregates and produces variegation. Thus it is shown that 

 plastids may be physiologically different. We may say that this dif- 

 ference is a different norm of reaction with their environment (e.g., 

 success or failure in chlorophyll synthesis). Not only a substrate type 

 difference between the two plasmons is demonstrated, but also a 

 genetic difference between the plastids from different species. Proof of 

 this may be derived from the fact that plastids which have gone 

 through many hybrid generations in the wrong cytoplasm act normally 

 at once when inserted into the proper cytoplasm. Furthermore, in such 

 crosses, cases are found in which both types of plastids may function 

 in the cytoplasm of a heterozygote. If such hybrids are crossed to a 

 third species, the genome of which permits only one of the plastid 

 types to function, the result is variegation, demonstrating again the 

 plastid difference. From an immense body of such facts Renner con- 

 cluded that plastids may be genetically different as autonomous, self- 

 duplicating bodies and that therefore one might speak of a separate 

 "plastidome." 



Though there can be no doubt that these conclusions are correct 

 per se, it does not mean that all comparable cases of variegation are 

 necessarily based upon the same principle. Thus it may be imagined 

 that both types, plastid difference and segregation and a labile cyto- 

 plasm influencing plastid function, may occur together, which leads to 

 complicated results and may account for basic differences in inter- 



