Electrophoretic shift mutants in Chinese hamster ovary cells: evidence for genetic diploidy.

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Abstract
Chinese hamster ovary (CHO) cells are not extensively functionally hemizygotic. Effective haploidy is unsatisfactory as a general theory to explain the frequency of recessive mutants in this cell line. CHO cells were screened for electrophoretic shift variants of enzymes coded by .apprx. 40 genetic loci. Clones isolated after exposure to UV radiation were examined by starch gel and Cellogel electrophoresis. Shift variants were recovered for enzymes representing 11 different loci. Variant clones were subcloned to demonstrate the heritability of the variations. Mutants at 9 loci produced multiple-banded patterns consistent with the patterns expected of genes at loci represented twice (diploid). Chromosome localization of these diploid loci in other mammalian species where they have been mapped suggest that they represent a random sample of CHO genes. The chromosomal variation did not take place in tetraploid cells. The quasi-diploid CHO cells appear only as functionally hemizygous as is expected of a slightly hypodiploid cell line derived from an organism in which the haploid number is 11.