Inefficiency of Genetic Recombination in Hybrids Between Escherichia coli and Salmonella typhosa

Abstract

An Escherichia coli Hfr strain in which three negative chromosomal alleles (leu-, arg-, and mtl-) were closely linked to three positive alleles (ara+, rha+, and xyl+, respectively) was employed in matings with a Salmonella typhosa recipient. The detected expression of the negative E. coli alleles in S. typhosa hybrids selected for receipt of an associated positive E. coli marker was used to determine the occurrence of haploid S. typhosa recombinants, as distinguished from stable partial diploid hybrids. At the same time, the inheritance patterns and segregation behavior of the positive alleles provided indicators of the occurrence of partial diploid hybrids. Examination of both positive and negative markers inherited by ara+, rha+ and xyl+ selected S. typhosa hybrid classes indicated that relatively short E. coli chromosomal segments (generally about 4 min or less in length) were involved in recombination (haploidy), whereas rather extensive E. coli genetic segments were conserved in the diploid state. S. typhosa hybrids selected for receipt of the ara+ marker showed a 52% incidence of leuhaploidy, which is probably close to being an accurate measure of recombination at the site of the ara+ allele. S. typhosa hybrids selected for receipt of the rha+ or xyl+ markers showed only a 20% incidence of argor mtlhaploidy, respectively, but both of these hybrid classes exhibited a higher incidence of conservation of extensive E. coil diploid segments than did the ara+ selected class. Remating of haploid S. typhosa hybrids with recombinant xyl+ mtlor rha+ arg-regions resulted in higher frequencies of hybrid recovery than were observed in the initial matings. However, there was a higher incidence of partial diploidy and a lower incidence of haploidy among the hybrids obtained from these rematings.

Recombination of Escherichia coli Chromosomal Segments in Salmonella typhosa

“…Similarly, none of the hybrids derived from E. Coli WR2017 (Table 2) which expressed any negative alleles of that donor were found to be unstable with regard to the selected marker. This stability of the selected positive marker whenever closely linked negative donor markers are expressed in the hybrid was observed also in our previous study (4), and it is probable that, in those instances, the selected marker has been incorporated by recombination along with the neighboring negative marker. On the other hand, all but 16 of the 61 hybrids obtained with E. coli WR2018 which expressed only the rha+ marker (Table 3) were observed to be unstable (diploid) with respect to this character.…”

“…As a genetic recipient in the cross with an Escherichia coli K-12 Hfr strain, Salmonella typhosa is capable of recombining transferred K-12 deoxyribonucleic acid (DNA) with homologous regions of its own chromosome (1,3,4). Frequently, however, the E. coli DNA persists, unrecombined, in S. typhosa as an addition to the Salmonella genome, thus creating a partially diploid hybrid (1)(2)(3)(4). When an inherited E. coli gene determines a positive phenotypic character for which the corresponding Salmonella allele is a negative determinant, its diploid nature may be discovered by its loss, i.e., segregation, from the S. typhosa hybrid.…”

“…Both have the same chromosome transfer orientation, O-proA-thiA-argH-metBrha-metE.. .lac. Matings between these Hfr strains and S. typhosa WR4200 were accomplished on a minimal agar selective medium, as described previously (4), in which rhamnose (utilized by both of the donor strains but not by the S. typhosa recipient) served as the carbon source. In addition to the cysteine and tryptophan required by S. typhosa WR4200, methionine, arginine, and thiamine were included in the selective medium to allow recovery of those hybrids in which recombination of the negative donor alleles metB-, metEf, argH-, or thiAmight occur.…”

“…After this purification, they were examined for haploid inheritance of the negative donor alleles, as determined by inability to grow on minimal selective medium from which either methionine, arginine, or thiamine was omitted. The demonstration, in some of the hybrids, of the diploid state of the positive donor selective marker, rha+, was accomplished by observing the segregation of this marker on MacConkey agar indicator medium containing 1% rhamnose, as described previously (4).…”

Organization and plasticity of enterobacterial genomes

“…Laboratory studies of intergeneric conjugation show that hybrid recombinant chromosomes are generated infrequently and contain only short segments of transferred donor DNA. Most transconjugants formed in such matings are partial diploids carrying the transferred DNA as extrachromosomal material coupled to at least part of the conducting plasmid or, more intriguingly, as a duplication inserted in the chromosome of the recipient species (Johnson et al 1971(Johnson et al , 1975Mojica-a & Middleton 1972;van Gijsegem & Toussaint 1982). Apart from restriction barriers, which are generally not absolute, the low frequency of haploid recombinants formed by intergeneric conjugation is probably due to nucleotide sequence divergence of different genomes impeding homologous recombination.…”