Chromosomes of L-M Mouse Cells and Variants

The L-M (199P) mouse cell strain grown in modified medium 199 plus 0.5% peptone (199P) and several derived substrains were analyzed to determine their specific chromosomal patterns for purposes of characterization. The modal numbers of chromosomes and biarmed chromosomes were determined for each cell strain. One of the substrains, known as L-M (2 × E), resulted from the transfer of L-M (199P) cells to a double-strength solution of Eagle's basal medium amino acids, vitamins, and glutamine in Hanks' balanced salt solution (2 × E). Strains L-Ma (199P) and L-MI (2 × E) were derived, respectively, from in vivo passage in C3H mice of L-M (199P) and L-M (2 × E) cells. Each strain presented a distinctive chromosomal pattern, apparently due to the different selection pressures to which these cells were subjected. The frequency and distribution of marker chromosomes were tabulated for the E, F, G, and H markers. The L-M (2 × E) strain showed a marked difference in the marker chromosome pattern compared to the L-M (199P) and L-Ma(199P) strains. From the morphological relationships between the marker chromosomes and their frequency and distribution, it is suggested that the ability of these cell strains to grow continuously on protein-free and chemically defined media may be related to gene dosage. The population dynamics of L-M cells in vitro is discussed in the light of these observations. The possible origins of the chromatid gaps observed in both the strains carried in 199P and in 2 × E medium are also discussed.