Hijackers, hitchhikers, or co-drivers? The mysteries of microbial mobilizable genetic elements

Mobile genetic elements shape microbial gene repertoires and population dynamics, but their mechanisms of horizontal transmission are often unknown. Recent results reveal that many, possibly most, bacterial mobile genetic elements require helper elements to transfer between (or within) genomes. We refer to these non-autonomous, albeit mobile, elements as Hitcher Genetic Elements (hitchers or HGEs). They could constitute a large fraction of pathogenicity and resistance genomic islands, whose mechanisms of transfer have remained enigmatic for decades. Together with their helper elements and their bacterial hosts, hitchers are in tripartite networks of interactions that evolve within a parasitism-mutualism continuum, with advantages and costs to each party. The emerging view of microbial genomes as networks of interacting mobile genetic elements brings to the fore many mysteries. Which elements are being moved, by whom, and how? How often are hitchers costly hyper-parasites or instead beneficial mutualists to their helpers and to the bacterial hosts? What is the evolutionary origin of hitchers? Are there key advantages associated with hitchers' lifestyle that justify their unexpected abundance across genomes? Or is their frequency largely the result of selfish spread across communities? Understanding the principles, origin, mechanisms, and impact of Hitcher Genetic Elements will lead to key insights in bacterial ecology and evolution.