Toward the Use of Genomics to Study Microevolutionary Change in Bacteria
Figure 2
A schematic illustration of the evolution of the C. jejuni ST-21 clonal complex in cattle and chickens.
The common ancestor of the complex occurred in chickens (red). During evolution, the lineage occasionally switched to a cattle host (indicated by a blue branch) and sometimes back to chicken. The bacteria acquired DNA by homologous recombination from other C. jejuni in the same host. Since recombination is assumed to occur from donors within the same host, the gene pool is determined by the genomic composition of the strains that colonize each host. The gene pools are illustrated for two separate loci (right and left facing arrows) in chickens and cattle. The gene pools contain alleles whose frequencies occur at much higher frequency in one host than another (shown in colour) and others that did not (shown in black). The former are informative about the host in which the recombination event occurred, while the latter are not. The recombination event labelled a introduces the left facing black arrow gene from the cattle gene pool and is phylogenetically informative because it defines a lineage that is largely restricted to cattle. The five recombination events labelled b are not phylogenetically informative, since they only affect a single strain in the sample. These events are nevertheless informative because they introduce alleles that are characteristic of the host species. The event labelled c is both phylogenetically informative and characteristic of host. The event labelled d is noninformative.