Interaction effects of cell diffusion, cell density and public goods properties on the evolution of cooperation in digital microbes

Abstract

Microbial cooperation typically consists in the sharing of secreted metabolites (referred to as public goods) within the community. Although public goods generally promote population growth, they are also vulnerable to exploitation by cheating mutants, which no longer contribute, but still benefit from the public goods produced by others. Although previous studies have identified a number of key factors that prevent the spreading of cheaters, little is known about how these factors interact and jointly shape the evolution of microbial cooperation. Here, we address this issue by investigating the interaction effects of cell diffusion, cell density, public good diffusion and durability (factors known to individually influence costs and benefits of public goods production) on selection for cooperation. To be able to quantify these effects across a wide parameter space, we developed an individual-based simulation platform, consisting of digital cooperator and cheater bacteria inhabiting a finite two-dimensional continuous toroidal surface. Our simulations, which closely mimic microbial microcolony growth, revealed that: (i) either reduced cell diffusion (which keeps cooperators together) or reduced public good diffusion (which keeps the public goods closer to the producer) is not only essential but also sufficient for cooperation to be promoted; (ii) the sign of selection for or against cooperation can change as a function of cell density and in interaction with diffusion parameters; and (iii) increased public goods durability has opposing effects on the evolution of cooperation depending on the level of cell and public good diffusion. Our work highlights that interactions between key parameters of public goods cooperation give rise to complex fitness landscapes, a finding that calls for multifactorial approaches when studying microbial cooperation in natural systems.

Division of Labor during Biofilm Matrix Production

“…To better understand the conditions required for genetic division of labor to evolve between EPS- and TasA-producing specialists, we used an individual-based modelling platform, specifically developed to simulate microbial interactions [41]. The platform consists of a two-dimensional toroidal surface, where bacterial cells are modeled as discs.…”

“…We performed individual-based simulations, using the platform developed previously [41]. Microbial simulations occur on a two-dimensional toroidal surface with connected edges (i.e.…”

“…One practical complication of this approach is that cells overlap with each other following diffusion. To cope with this issue, we applied an overlap correction after each time step following the procedure described in [41]. …”

Habitat structure and the evolution of diffusible siderophores in bacteria

“…; Dobay et al . ). Although caution is always required with correlational data, the above considerations suggest that habitat structure indeed represents a major selective force shaping siderophore diffusivity properties.…”

“…; Dobay et al . ), we argue that diffusible metabolites face two problems: (1) metabolites might get lost due to diffusion; and (2) metabolites could be exploited by cheating individuals that no longer contribute but still benefit from the metabolite produced by others. One possibility to control both (1) and (2) would be for natural selection to act on diffusivity properties of metabolites, thereby influencing their loss rate and shareability in the community.…”