262 CELL HEREDITY 



INFECTIOUS AGENTS 



There are a number of striking examples in the hterature of host-cell 

 traits determined bv infectious particles. In the past, it was considered 

 not onlv possible but essential to distinguish between normal cell con- 

 stituents and intracellular parasites of \arious sorts. One criterion gen- 

 erally employed was infectiyity. 



Suppose we define as an infectious agent any hereditary material 

 capable of entering a cell from outside and replicating within it. An 

 example of such an agent is DNA itself, as employed in transformation 

 experiments: and the ultimate example is the sperm nucleus in fertiliza- 

 tion. If instead of replication one takes cell damage as the criterion, the 

 situation is no clearer: consider the lysogenic bacteria, in which the virus, 

 when it is in the prophage state, contributes no damage whatsoever but 

 often some positive properties such as special new antigens. Indeed, 

 all recent work in this field has demonstrated the futility of attempting 

 to distinguish between normal cell constituents and infectious agents. 

 All one can do is to investigate each genetic system for what it is, hoping 

 ultimately to construct a picture of the cell in which the various jig-saw 

 pieces fall into place. In the following discussion, we shall briefly con- 

 sider a few outstanding examples of infectious determinants. 



Kappa 



In 1943, Sonneborn reported cytoplasmic inheritance in Faramecium 

 aurelia of a trait called killer, recognized as the ability of some strains 

 to kill others. To understand the method of genetic analysis employed, 

 it is necessary to consider first a simplified account of the principal 

 features of mating and meiosis in Paramecium, as diagrammed in 

 Figure 9.10. 



Each organism contains two diploid micronuclei and one large macro- 

 nucleus, a cell organelle unique to ciliated protozoa, containing many 

 replicates of the diploid genome. Conjugations involve pairing of two 

 animals at the anterior end and in the paroral cone (mouth) region. 

 After pairing begins, the macronuclei of both mates disintegrate, and 

 the micronuclei undergo meiosis, producing eight haploid nuclei per ani- 

 mal; any one of these nuclei may survive, the other seven degenerate. 

 The remaining nucleus in each animal divides mitotically, and one of 

 the two daughter nuclei passes across the paroral cone into the other 

 mating cell. Each mate then contains two haploid nuclei, which fuse to 

 produce a diploid fusion nucleus containing one chromosome comple- 



