pnas | Many bacterial species are social, producing costly secreted “public
good” molecules that enhance the growth of neighboring
cells. The genes coding for these
cooperative traits are often propagated via mobile genetic elements and
can be virulence
factors from a biomedical perspective.
Here, we present an experimental framework that links genetic
information exchange
and the selection of cooperative traits.
Using simulations and experiments based on a synthetic bacterial system
to control
public good secretion and plasmid
conjugation, we demonstrate that horizontal gene transfer can favor
cooperation. In a well-mixed
environment, horizontal transfer brings a
direct infectious advantage to any gene, regardless of its cooperation
properties.
However, in a structured population
transfer selects specifically for cooperation by increasing the
assortment among cooperative
alleles. Conjugation allows cooperative
alleles to overcome rarity thresholds and invade bacterial populations
structured
purely by stochastic dilution effects. Our
results provide an explanation for the prevalence of cooperative genes
on mobile
elements, and suggest a previously
unidentified benefit of horizontal gene transfer for bacteria.
Bacteria often cooperate through the production of public goods that
change their environment. These processes can affect
human health by increasing virulence or
antibiotic resistance. Public good production is costly, making
cooperation susceptible
to invasion by nonproducing “cheater”
individuals. Bacteria also readily share genes, even among distinct
species. Our experiments
and models converge to show that when both
cheating and cooperative genes are transferred, cooperators win against
cheaters
because transfer increases assortment
among alleles, favoring cooperation. This can explain why genes for
cooperation are
often mobile, and suggests that, in
addition to reducing antibiotic resistance spread, preventing gene
mobility could reduce
cooperative virulence.
