Cargo capacity of phages and plasmids and other factors influencing horizontal transfers of prokaryote transposable elements

Horizontal transfer of transposable elements (TEs) plays a key role in prokaryote genome evolution. Most TEs do not encode the enzymatic machinery allowing them to transfer between host cells and it is widely assumed in the literature that horizontal transfer of prokaryote TEs is mediated by other mobile genetic elements such as phages and plasmids. In a recent study, we have shown that phages are less tolerant to insertion sequences (IS, the most frequent class of prokaryote TEs) and therefore have a lower cargo capacity than plasmids. Consequently, while our analysis confirmed the crucial role of plasmids as efficient vehicles of IS horizontal transfer, we concluded that phages are unlikely to efficiently shuttle IS elements between prokaryotes. Here, we discuss whether or not the distribution pattern observed for IS elements in phages and plasmids also holds for other TEs, such as transposons and mobile introns. We also further explore various factors that may impact the relative capacity of phages and plasmids to mediate TE horizontal transfer among prokaryotes.