Effect of Ploidy and Mutagens on Bromodeoxyuridine Resistance in Haploid and Diploid Frog Cells

Abstract

THE frog embryo cell line ICR 2A is the first established haploid vertebrate cell line¹. In haploid cells recessive mutations should be detectable at a frequency 10⁶ to 10⁹ times greater than expected in diploid cells; mutagen treatment should increase the yield further. These predictions are useful to test whether variants arising in culture are the result of gene mutation. To apply this test to frog cells, mutations for thymidine kinase were sought. Such mutants were first obtained by exposing mouse L cells to the thymidine analogue 5-bromodeoxyuridine (BUdR); a loss of thymidine kinase activity prevented the lethal incorporation of BUdR into DNA². The new phenotype was considered to be the result of gene mutation because of its heritability and eventually because of data from Luria-Delbrück fluctuation analyses³ (a test of the spontaneity or non-inducibility of a process, not its cause). The question of origin was further complicated by a number of factors: (1) the necessity of a long, repeated, exposure to BUdR²; (2) the high mutation rate (up to 10⁻³) compared with bacterial mutants (10⁻⁹10⁻⁶)^4,5; and (3) the presence of resistant clones with intermediate enzyme levels^4,5.