Evolution of Chlamydia trachomatis diversity occurs by widespread interstrain recombination involving hotspots.

Chlamydia trachomatis is an obligate intracellular bacterium of major public health significance, infecting over one-tenth of the world's population and causing blindness and infertility in millions. Mounting evidence supports recombination as a key source of genetic diversity among free-living bacteria. Previous research shows that intracellular bacteria such as Chlamydiaceae may also undergo recombination but whether this plays a significant evolutionary role has not been determined. Here, we examine multiple loci dispersed throughout the chromosome to determine the extent and significance of recombination among 19 laboratory reference strains and 10 present-day ocular and urogenital clinical isolates using phylogenetic reconstructions, compatibility matrices, and statistically based recombination programs. Recombination is widespread; all clinical isolates are recombinant at multiple loci with no two belonging to the same clonal lineage. Several reference strains show nonconcordant phylogenies across loci; one strain is unambiguously identified as recombinantly derived from other reference strain lineages. Frequent recombination contrasts with a low level of point substitution; novel substitutions relative to reference strains occur less than one per kilobase. Hotspots for recombination are identified downstream from ompA, which encodes the major outer membrane protein. This widespread recombination, unexpected for an intracellular bacterium, explains why strain-typing using one or two genes, such as ompA, does not correlate with clinical phenotypes. Our results do not point to specific events that are responsible for different pathogenicities but, instead, suggest a new approach to dissect the genetic basis for clinical strain pathology with implications for evolution, host cell adaptation, and emergence of new chlamydial diseases.