

GENETICS OF SOMATIC CELLS 451 



of distinguishing between reversible changes, inducible at will by certain variations in 

 the composition of the medium, and stable differences, capable of breeding true upon 

 repeated culture. Examination of the chromosomal complement of different clones, 

 isolated from highly aneuploid populations, show that they may differ with regard to 

 their karyotype and these differences tend to remain fairly stable and permanent in the 

 course of serial passage. A semiannual recloning was found sufficient to maintain the 

 stability of a clonal stemline. 1322 



The nutritional requirements of various standard established strains of cells in 

 vitro are surprisingly similar. 314 Only a few of them exhibit some special, unusual 

 requirements; these have been summarized recently by Eagle. 314 For this reason, 

 it is of considerable importance to select clear-cut nutritional variants experimentally 

 on a large scale. Their availability will facilitate the analysis of variation in populations 

 of cultured cells, it will permit the development of a field dealing with their biochemical 

 genetics, and it is a basic prerequisite for the exploration of the feasibility of genetic 

 analysis through somatic crossing over, as discussed in the section on normal cells 

 in vivo. Experiments in this direction are now well in progress. Hsu and Kellogg 603 

 reported recently the isolation of variants adapted to galactose and to xylose, respectively, 

 from a subline of the well-known L strain of murine fibroblasts. While glucose supported 

 the growth of all lines, xylose did not support any line except the xylose-adapted strain. 

 Galactose supported only a very limited growth of the parental line while the galactose- 

 adapted strain was growing much better. Eagle and co-workers, 312 who found that 

 meso-inositol was an essential requirement for all strains of cells they have studied, 

 isolated one inositol-independent line from strain L cultures. This line could utilize 

 glucose for the biosynthesis of inositol, and significant amounts of the latter (of the 

 order of 10 " 6 M) were actually released into the medium. 



A" method for selecting auxotrophic mutants of HeLa cells, that is, variants with at 

 least one more nutritional requirement than the type of cell from which they had been 

 derived, was recently described by De Mars and Hooper. 244 Their procedure was 

 based on the fact that cultivated mammalian cells are usually unable to proliferate 

 in the presence of the folic-acid analogs, aminopterin and amethopterin, this effect 

 being reversible by a mixture of adenine, thymine, and glycine. Also, bacteria which 

 are unable to synthesize thymine, either because of a mutational deficiency or as a 

 result of sulfonamide treatment, die in media containing all factors necessary for 

 growth with the exception of thymine, because of unbalanced growth. 211 On the other 

 hand, if the thymine-free medium also lacks one of the other factors necessary for growth, 

 the bacteria do not undergo unbalanced growth and remain viable. This preferential 

 survival of thymine-starved cells also deprived from an additional growth factor was 

 postulated to permit the selection of auxotrophic variants from large-cell populations 

 made thymine deficient through aminopterin treatment. Accordingly, HeLa cells 

 were treated with aminopterin and their survival in a growth-supporting, complete 

 medium was compared with survival in a medium deficient in a single essential 

 nutrient. 



