CHAPTER XVm 



PLASMAGENES 



IN RECENT years a considerable body of evidence has accumulated for the 

 existence in the cytoplasm of bodies of a more or less gene-hke nature. 

 Geneticists have been particularly active in investigating them and have 

 referred to them by a variety of names, such as plasmagenes, cytogenes, 

 blastogenes, proviruses, etc. The first has been the most commonly 

 accepted, and will be used here in a rather wide sense, to cover several 

 rather different types of gene-hke entities. 



Broadly speaking, plasmagenes are revealed by two different kinds of 

 evidence. On the one hand, breeding experiments may demonstrate that 

 certain characters are inherited through the cytoplasm and not through 

 the nucleus, and thus provide evidence of the existence of cytoplasmic 

 hereditary determinants. Evidence of this kind can be of two grades. We 

 may find that, throughout a number of generations, a certain character 

 follows the transmission of cytoplasm, even when the whole set of 

 chromosomal genes is removed by crossing. For instance, if a female of 

 type A is crossed to a B male, and her female offspring again backcrossed 

 to B males, and so on for several generations, the A chromosomes will 

 gradually be replaced by B ones, and if any characteristics of A still remain 

 in the offspring after several generations, one may conclude that they are 

 dependent on hereditary factors transmitted through the egg cytoplasm 

 and capable of continued multiphcation in the absence of their corre- 

 sponding A genes. On the other hand, it may be found that although a 

 character is transmitted only by a parent which contributes cytoplasm 

 to the offspring, and is thus directly dependent on a cytoplasmic deter- 

 minant, nevertheless this determinant can persist only in an organism pro- 

 vided with the appropriate gene. In such cases (a good example is the 

 'killer' character in Paramecium) we have to do with a gene-dependent 

 plasmagene. 



A different type of evidence for the existence of plasmagenes appears 

 when it can be shown that a character can be transmitted from cell to 

 cell by inoculation or other treatment with extracts which do not contain 

 functional chromosomes; we may then conclude that we are confronted 

 with a determinant, presumably derived from the cytoplasm, which can 

 persist and impress some defuiite character onto the living cells into which 

 it is introduced. The classical examples of such types of behaviour are the 



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