POSTZYGOTIC ELIMINATION OF GENETIC FACTORS IN ESCHERICHIA COLI

Abstract

Exceptional diploids from crosses of strain K-12 are hemizygous for the closely linked factors Mal and S. Some diploids hemizygous for a Mal allele from one parent and an S allele from the other parent suggest possible postzygotic elimination of genetic segments. To test this possibility the cross (M-Het F+)x(T-L-Th-F) was compared with cross (M-Het F-)x(T-L-Th-F+). The M-Het parent also carried Lac-Mali- and the other parent carried Lac4-Sr. Close linkage between the Lac markers permitted isolation of balanced diploids (Lac1-Lac4+/Lac1+Lac4-) on cross-plates of EMB lactose agar. With either polarity of F+/F- about of the 618 diploids examined had the markers from the F-parent and were hemizygous for Mai and S. About 20% had one or more markers from the F+ parent in hemizygous condition. These results and the fact that over 5% of the diploids were amphitypic support the hypothesis of postzygotic elimination of chromosome segments. In explanation of the influence of F polarity on elimination, it is postulated that a locus on the chromosome of the F+ parent breaks after meiosis, resulting in loss of the distal segment or interference with normal disjunction. Prior crossing-over between this locus and the markers would result in characteristics of the F+ parent; no crossing-over would give those of the F~ parent.