Selection of horizontal gene transfer through public good production

In bacteria, cooperative genes encoding public good molecules are preferentially located on mobile genetic elements (MGEs), and horizontal transfer of MGEs favours the maintenance of public good cooperation. The rate of horizontal transfer itself can evolve in response to selective pressures acting on both MGEs and bacterial hosts: benefits and costs of infectious spread, but also indirect effects of MGE genes to the host. We show here that carriage of public good genes on MGEs can generate another indirect selection for MGE transfer. Transfer increases public good production and, when relatedness is sufficiently high, public goods benefit preferentially genotypes with high transfer ability. Both our simulations and experiments indicate that transfer is not required to occur among kin, provided that public goods still benefit kin. Public good gene mobility thus aligns the interests of chromosomes and MGEs concerning transfer, promoting gene exchange among bacteria.