Recombination frequencies were determined for 15 independently isolated auxotrophs of C. crescentus crossed pairwise in all possible combinations. The results indicate that the mutants may be grouped into at least two types: "fertile" strains, which recombine with all other mutants at frequencies ranging from less than 10-6 to 3 × 10-2, and "nonfertile" strains which recombine with fertile strains at high frequencies and with other nonfertile strains at low or negligible frequencies. Several

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... es of evidence indicate a polarized inheritance of markers. Two of these are (1) the preferential inheritance of unselected markers from the nonfertile parent in fertile × nonfertile crosses, and (2) the consistent ordering of markers based on the frequency at which the mutants recombine with each of the three fertile strains. Although the evidence is not conclusive at this point, the results are most consistent with conjugation as the mechanism of gene transfer in these bacteria.