Chromosomal Recombination Targets in Interspecies Lateral Gene Transfer

Lateral gene transfer (LGT) among strains is common, in both isolates generated in the laboratory and those examined directly from patients. In contrast, there are very few examples of recent acquisition of DNA by any spp. from any other species. Interspecies LGT in this system was analyzed using crosses of tetracycline (Tc)-resistant L2/434 and chloramphenicol (Cam)-resistant VR-123. Parental strains were created using a plasmid-based Himar transposition system, which led to integration of the Cam marker randomly across the chromosome. Fragments encompassing 79% of the chromosome were introduced into a background, with the total coverage contained on 142 independent recombinant clones. Genome sequence analysis of progeny strains identified candidate recombination hot spots, a property not consistent with × (intraspecies) crosses. In both interspecies and intraspecies crosses, there were examples of duplications, mosaic recombination endpoints, and recombined sequences that were not linked to the selection marker. Quantitative analysis of the distribution and constitution of inserted sequences indicated that there are different constraints on interspecies LGT than on intraspecies crosses. These constraints may help explain why there is so little evidence of interspecies genetic exchange in this system, which is in contrast to very widespread intraspecies exchange in Genome sequence analysis has demonstrated that there is widespread lateral gene transfer among strains within the species and with other closely related species in laboratory experiments. This is in contrast to the complete absence of foreign DNA in the genomes of sequenced clinical strains. There is no understanding of any mechanisms of genetic transfer in this important group of pathogens. In this report, we demonstrate that interspecies genetic exchange can occur but that the nature of the fragments exchanged is different than those observed in intraspecies crosses. We also generated a large hybrid strain library that can be exploited to examine important aspects of chlamydial disease.