GENETICS OF SOMATIC MAMMALIAN CELLS. IV. PROPERTIES OF CHINESE HAMSTER CELL MUTANTS WITH RESPECT TO THE REQUIREMENT FOR PROLINE

Abstract

HILE the availability of the single cell plating technique has facilitated study of some aspects of the genetic biochemistry of mammalian cells in vitro (PUCK and FISHER 1956;TJIO and PUCK 1958;PUCK, CIECIURA and ROBINSON 1958), other:; like mutagenesis and recombination have progressed slowly, largely because of the difficulty in securing adequate genetic markers.In contrast to the abundance in microorganisms of many stable markers such as those involving nutritional deficiency and virus-, drug-and radiation-resistance, the markers available for genetic studies in mammalian cells have been limited in number; their expression is often variable from day to day; and the spontaneous reversion rate is often so high as to make the resolving power of genetic experiments much less than that attainable with fungi, bacteria and bacteriophage.A program was begun in this laboratory to attempt isolation of nutritional markers in mammalian cells cultivated in vitro.To this end, the molecular requirements for single-cell plating were first defined for the S3 HeLa cell (FISHER, PUCK and SATO 1959).However, the high degree of chromosomal instability and polyploidy characteristic of this heteroploid cell line made it disadvantageous for genetic experiments.Consequently, attention was turned to the Chinese hamster cells which had been introduced into in vitro culture in this laboratory (PUCK, CIECIURA and ROBINSON 1958).While these cells also have a small degree of variability in chromosome number presumably due to spontaneous lag or nondisjunction, the modal number is usually diploid and relatively constant, and karyotypic analysis is possible with only small uncertainty, at least as compared with the HeLa or similar cells.Moreover, the low chromosome number of Chinese hamster cells makes karyotype analysis feasible and their rapid growth (1 0-hour generation time) is favorable for experimentation.A defined medium for single cell plating of Chinese hamster cells was developed (HAM 1962).One subclone derived from a Chinese hamster ovary which had