GENETICS OF SOMATIC CELLS 459 



rather small, they will incorporate very limited amounts of genetic information from the 

 host cell at best; this also speaks for using multiply marked strains of cells. With 

 cells of the mouse, there is an arsenal of isoantigenic markers, determined by known 

 genetic mechanisms, that might be combined with a series of drug-resistance and 

 nutritional markers to build up suitable, multiply marked strains of cells in tissue culture 

 where most if not all transduction experiments will have to be done. 



Possible analogies between viral tumorigenesis and lysogenic conversion have been 

 discussed by Luria; 810, 811 this has been already considered in the section on viral 

 tumors and infective heredity. 



Tumor viruses show many analogies with the episomes of Jacob and co- 

 workers. 653, 655, 657 These are defined as "a class of genetic elements, which are not 

 essential constituents of the cell since they may be absent from it. When present, 

 they may exist in two alternative states, either as autonomous units replicating inde- 

 pendently of the bacterial chromosome, or as integrated units attached to the bacterial 

 chromosome with which they replicate." Temperate bacteriophages, the sex factor 

 of E. coli and the colicinogenic factors of certain Enterobacteriaceae are some examples 

 of episomes and the recent picture given by Vogt and Dulbecco 1350 regarding the intra- 

 cellular behavior of polyoma virus of mice is very reminiscent of episomic elements. 

 It is therefore of great interest and rather hopeful that Jacob and Adelberg 654 have 

 recently discovered that the typically episomal sexual factor F in E. coli can incorporate 

 small segments of the bacterial chromosome and transmit them to recipient cells which 

 become heterogenotic for the segment in question. Jacob and Adelberg suggest that 

 each episomic element may, during its integrated phase, exchange genetic elements 

 with a nearby chromosome segment to which it is attached and that each genetic 

 element of the bacterial chromosome may become part of an episome. 



CONCLUSIONS 



In many respects, the field of somatic-cell genetics is comparable with bacterial 

 genetics some twenty years ago. Marker characteristics are available for the study of 

 phenotypic cellular variation. Some markers can be exploited for the selective con- 

 centration of rare variants and the semiquantitative or quantitative determination of 

 their frequency, but it is impossible to distinguish between phenotype and genotype and 

 genetic mechanisms cannot be separated from epigenetic changes due to altered genie 

 expression. (Cytologically recognizable chromosomal mechanisms represent one 

 exception, but these have been largely excluded from the subject matter of this review.) 

 The most important need is therefore to develop some method to transfer genetic 

 information between somatic cells. It would be surprising if none of the mechanisms 

 now available for genetic transfer in bacteria, such as sexual recombination, DNA- 

 mediated transformation, transduction by phage, lysogenic conversion, and episomal 

 transduction, would be applicable in some form to somatic cells. Appropriate markers 



