DOI: 10.5061/dryad.34tmpg4kw
title: Ecology drives the evolution of diverse social strategies in Pseudomonas aeruginosa

description: Bacteria often cooperate by secreting molecules that can be shared as
 public goods between cells. Because the production of public goods is
 subject to cheating by mutants that exploit the good without contributing
 to it, there has been great interest in elucidating the evolutionary
 forces that maintain cooperation. However, little is known on how
 bacterial cooperation evolves under conditions where cheating is unlikely
 of importance. Here we use experimental evolution to follow changes in the
 production of a model public good, the iron-scavenging siderophore
 pyoverdine, of the bacterium Pseudomonas aeruginosa. After 1200
 generations of evolution in nine different environments, we observed that
 cheaters only reached high frequency in liquid medium with low iron
 availability. Conversely, when adding iron to reduce the cost of producing
 pyoverdine, we observed selection for pyoverdine hyper-producers.
 Similarly, hyper-producers also spread in populations evolved in highly
 viscous media, where relatedness between interacting individuals is
 increased. Whole-genome sequencing of evolved clones revealed that
 hyper-production is associated with mutations involving genes encoding
 quorum-sensing communication systems, while cheater clones had mutations
 in the iron-starvation sigma factor or in pyoverdine biosynthesis genes.
 Our findings demonstrate that bacterial social traits can evolve rapidly
 in divergent directions, with particularly strong selection for increased
 levels of cooperation occurring in environments where individual dispersal
 is reduced, as predicted by social evolution theory. Moreover, we
 establish a regulatory link between pyoverdine production and
 quorum-sensing, showing that increased cooperation with respect to one
 trait (pyoverdine) can be associated with the loss (quorum-sensing) of
 another social trait.Bacteria often cooperate by secreting molecules that
 can be shared as public goods between cells. Because the production of
 public goods is subject to cheating by mutants that exploit the good
 without contributing to it, there has been great interest in elucidating
 the evolutionary forces that maintain cooperation. However, little is
 known on how bacterial cooperation evolves under conditions where cheating
 is unlikely of importance. Here we use experimental evolution to follow
 changes in the production of a model public good, the iron-scavenging
 siderophore pyoverdine, of the bacterium Pseudomonas aeruginosa. After
 1200 generations of evolution in nine different environments, we observed
 that cheaters only reached high frequency in liquid medium with low iron
 availability. Conversely, when adding iron to reduce the cost of producing
 pyoverdine, we observed selection for pyoverdine hyper-producers.
 Similarly, hyper-producers also spread in populations evolved in highly
 viscous media, where relatedness between interacting individuals is
 increased. Whole-genome sequencing of evolved clones revealed that
 hyper-production is associated with mutations involving genes encoding
 quorum-sensing communication systems, while cheater clones had mutations
 in the iron-starvation sigma factor or in pyoverdine biosynthesis genes.
 Our findings demonstrate that bacterial social traits can evolve rapidly
 in divergent directions, with particularly strong selection for increased
 levels of cooperation occurring in environments where individual dispersal
 is reduced, as predicted by social evolution theory. Moreover, we
 establish a regulatory link between pyoverdine production and
 quorum-sensing, showing that increased cooperation with respect to one
 trait (pyoverdine) can be associated with the loss (quorum-sensing) of
 another social trait.