A mobile genetic element increases bacterial host fitness by manipulating development.

Horizontal gene transfer is a major force in bacterial evolution. Mobile genetic elements are responsible for much of horizontal gene transfer and also carry beneficial cargo genes. Uncovering strategies used by mobile genetic elements to benefit host cells is crucial for understanding their stability and spread in populations. We describe a benefit that ICE, an integrative and conjugative element of , provides to its host cells. Activation of ICE conferred a frequency-dependent selective advantage to host cells during two different developmental processes: biofilm formation and sporulation. These benefits were due to inhibition of biofilm-associated gene expression and delayed sporulation by ICE-containing cells, enabling them to exploit their neighbors and grow more prior to development. A single ICE gene, (formerly ), was both necessary and sufficient for inhibition of development. Manipulation of host developmental programs allows ICE to increase host fitness, thereby increasing propagation of the element.