Selection and evolution of mating products after spontaneous zygogenesis in Escherichia coli K-12

Abstract

The fate of products resulting from spontaneous zygogenesis (or Z-mating; complete genetic mixing in the absence of conjugative plasmid F) was investigated in crosses between Escherichia coli K12 strains carrying chromosomally inserted aminoglycoside resistance cassettes. The use of strains carrying genes conferring dominant resistance to drugs at the same site on the bacterial chromosome provided a new means to select diploid Z-mating products. It also allowed the existence of noncomplementing diploids exhibiting one parental or recombinant type to be confirmed. The formation and fate of Z-mating-derived complementing diploids could be investigated under conditions where loss of complementation is lethal, so as to eliminate colony growth due to intraclonal cross-feeding. Results show that the formation of complementing diploids could readily (i.e., within one to two generations) follow infection of either or both parents with an unidentified agent from an induced lysogenic E. coli strain 84SV carrying a presumptive Z-mating factor. The viable population in the mating mixture could be enriched in complementing diploids under some experimental conditions. A large series of subclonal analyses not only allowed the evolution of Z-mating products to be followed but also revealed unusual variations in drug-resistance marker expression in the descent of some doubly drug-resistant clones.