Mutation frequency at a recessive locus in haploid and diploid strains of a slime mould

Abstract

IT is puzzling that mutants are isolated at a similar frequency in tissue cultures of haploid and diploid strains^1–3, for twice as many mutations have often been assumed necessary to produce a mutant diploid as to produce a mutant haploid. Explanations for the apparent high frequency of mutation in diploids^1–6 include epigenetic effects^1–3, changes in gene dose⁷, dominant mutations⁸, recessive X-linked mutations⁴ and heterozygosity before selection⁹. Most recessive autosomal mutants are assumed to arise because of haploidy for the chromosome containing the mutant locus, since most cell cultures are aneuploid^4,10,11. It is difficult to measure the mutation frequency at recessive loci in cultured haploid and diploid strains because of the paucity of haploid strains and the problems caused by aneuploidy. The slime mould Dictyostelium discoideum, a simple eukaryote, is suitable for this purpose, however, as both haploid and diploid strains are available^12–14 and aneuploids are so unstable or inviable that none has been isolated¹⁴. Using this organism, I have found, contrary to the expectation that mutant diploids should be extremely rare (say ² × 10⁻¹² diploid, 10⁻⁶ haploid )^1–6, that the difference in mutation frequency between haploids and diploids for one recessive locus is only an order of magnitude.