Macroevolution of plasmids: a model for plasmid speciation.

A new evolutionary model for diversification in plasmid incompatibility groups (plasmid speciation) is suggested. The model is based on the formation of plasmid cointegrates from two compatible plasmids. The existence of plasmid cointegrates is well known, however, their potential key role in plasmid macroevolution has not yet been recognized. In a hypothesis presented here, one of the rep genes is supposed to be relaxed from selection in plasmid cointegrates and thus becomes free to accumulate mutations. These mutations can lead to a change in incompatibility specificity. Evidence supporting this hypothesis comes from the common occurrence of multi-replicon plasmids in nature as well as from experimental studies on plasmid cointegrate formation. A more speculative extension of this model hypothesizes an evolutionary scenario for origin of the eubacterial single-replicon genome and the eukaryotic multi-replicon genome, as well as the place of plasmids and viruses in this picture.