Biparental products of bacterial protoplast fusion showing unequal parental chromosome expression

Efficiently regenerated single colonies from mixed multiply auxotrophic Bacillus subtilis protoplasts, fused with polyethylene glycol, reveal colonies carrying each of the parent types (biparentals) and recombinant colonies. The latter appear in high yields (up to 1% of certain recombinant classes); the yield of biparentals may be as large as 10%, in the range of the indicated physical fusion events. Many of the biparentals are diploids although, contrary to expectation, they are not complementing prototrophs, but show precisely the phenotype of one (either one) of the parent strains. Extensive pedigree analysis and subcloning of diploid lines show that they can propagate with varying stability on the appropriate parental selective medium to reproduce diploid progeny, parental segregants, and late-appearing recombinants, including some prototrophs. Thus, the principal product of intertype protoplast fusion is a diploid carrying two chromosomes, only one of which is expressed in each particular clone. The extinction of one parental genome is especially well demonstrated when it includes suppression of a normally dominant antibiotic sensitivity marker. In transformation experiments, DNA made from a selected diploid clone was able to transfer several of the unexpressed genes. The structural or topological character of DNA associated with the chromosome extinction remains unexplained.