Possible mechanisms explaining dimorphic and homeologous loci. (a) Dimorphic loci. Incoming DNA (the shorter, black and grey molecule above) may recombine by illegitimate recombination assisted by homology with the resident bacterial chromosome G1. HR, homologous recombination; IR, illegitimate recombination; G1 and G2, genomes 1 and 2; VS, variable segment. (b) Three possible scenarios to explain the origin of microdiversity at homeologous loci in bacterial genomes (see text for details).