Genetic demonstration of mitotic recombination in cultured Chinese hamster cell hybrids

Chinese hamster ovary cell hybrids were constructed that are heterozygous for two markers, leuS and emtB, linked to the long arm of chromosome 2. In addition, the chromosome 2 carrying the wild-type leuS and emtB alleles contains, on its short arm, a homogeneously staining region (hsr) in which the gene encoding dihydrofolate reductase (dhfr) is amplified approximately 50-fold. This provides a convenient cytogenetic and biochemical means to distinguish the chromosome 2s from the different parents. Analysis of emetine-resistant segregants isolated from such hybrids identified three distinct classes of segregants. One rare class of segregants loses the wild-type leuS and emtB gene functions on the long arm of the hsr chromosome 2 (H-2) but retains the amplified dhfr genes on the opposite arm. Detailed genetic analysis of two such segregants that did not arise by chromosome loss or deletion revealed that new gene linkage relationships had been established on the H-2 chromosome in each, demonstrating that the segregation events in these cell lines involved mitotic recombination.