fig2

Figure 2. Two alternative routes of MGE inheritance enable them to cross the colonization bottleneck. In the vertical transmission scenario, MGEs are inherited together with their bacterial host during the cell division and this relationship could be beneficial for both (mutualism). In the horizontal transmission scenario, MGEs could move to any successfully colonizing bacterial host, breaking taxonomic associations between the bacterial hosts and their MGEs. This relationship could be considered parasitic as the fitness interests of MGEs are not aligned with those of their host. Most MGEs are capable of both modes of transmission and could hence be considered as facultative mutualists (or parasites)^[25,26]. MGE: Mobile genetic element.