Fig. 2. Host control stabilises the evolution of cooperation in the microbiome.
a Schematic of the model: Both hosts and microbiota can invest in cooperation and, in addition, hosts can invest in control mechanisms that favour more cooperative symbionts over less cooperative ones. Hosts control also negatively effects all symbionts at cost (f) and hosts pay a direct cost for control (g). b Within-host evolution of symbiont cooperation (shown here for the first host generation, as an illustration). Increasing symbiont generations per host generation (generation ratio) promotes symbiont cooperation when there is host control, but hinders cooperation when there is not. c Effect of relatedness and benefit to cost ratio on the evolution of cooperation. Cooperation evolves across broad parameter ranges with host control, where increasing the symbiont to host generation ratio only increases the range of conditions where cooperation is stable. The regions where cooperation evolves for hosts and symbiont overlap perfectly and so we show only a single plot for cooperation. d Cooperation collapses when symbionts can evolve cooperation independently of the trait that is the target of host control. Mutualism is stable while the trait and cooperation are fixed (original model) but when symbionts are allowed to evolve the trait-cooperation link, cooperation and control are quickly lost. Reinstating the relationship again renders host control effective and restores cooperation. Unless stated, parameters are: x = y = 2, f = 0.02, g = 0.1, m = 1 × 10−6, M = 0.05.