“…Extensive studies carried out on the location of the trpEG and, mainly, on the leuABCD genes in several aphid subfamilies, either on plasmids or on the main chromosome (Lai et al, 1995Rouhbakhsh et al, 1996;van Ham et al, 1997van Ham et al, , 1999van Ham et al, , 2000Sabater-Muñ oz et al, 2002, revealed a great plasticity throughout B. aphidicola evolution, showing that the genomic stasis hypothesis is not supported when plasmids are taken into account. In fact, the evolutionary history of plasmids is puzzling, since not all of the bacterial lineages carry plasmids, and not all plasmids have the same gene content and/or gene order.…”