Social conflict drives the evolutionary divergence of quorum sensing

Abstract

In microbial "quorum sensing" (QS) communication systems, microbes produce and respond to a signaling molecule, enabling a cooperative response at high cell densities. Many species of bacteria show fast, intraspecific, evolutionary divergence of their QS pathway specificity--signaling molecules activate cognate receptors in the same strain but fail to activate, and sometimes inhibit, those of other strains. Despite many molecular studies, it has remained unclear how a signaling molecule and receptor can coevolve, what maintains diversity, and what drives the evolution of cross-inhibition. Here I use mathematical analysis to show that when QS controls the production of extracellular enzymes--"public goods"--diversification can readily evolve. Coevolution is positively selected by cycles of alternating "cheating" receptor mutations and "cheating immunity" signaling mutations. The maintenance of diversity and the evolution of cross-inhibition between strains are facilitated by facultative cheating between the competing strains. My results suggest a role for complex social strategies in the long-term evolution of QS systems. More generally, my model of QS divergence suggests a form of kin recognition where different kin types coexist in unstructured populations.

Fig. 1.

A model for the divergence of QS systems. (A) I assume QS to control the production of public goods. A signaling molecule is secreted out of the cell and accumulates in the environment. A cellular receptor is activated by the signaling molecule at high cell densities and leads to the production of public goods: an exo-enzyme that metabolizes a complex nutrient into a usable form. Enzyme production carries a growth cost to the producing cell, but usable nutrient brings benefit to the whole community. See Materials and Methods and SI Text, section 2 for further discussion and model equations. (B) For simplicity, I assume that both receptor and signal have two alleles with specific and orthogonal interaction (i.e., each receptor is completely specific to its cognate signal) and that a single mutation allows transition between corresponding alleles. In SI Text, section 7 I show how diversification can evolve in the presence of null alleles of receptor, signal, and public goods enzyme or if receptor and signal alleles are not fully orthogonal.

Fig. 2.

Coevolution of receptor and signaling molecule during divergence is positively selected. (A) The original QS system R₁S₁ (‘Naive’) can evolve into the novel QS system R₂S₂ (‘Immune’) through one of two intermediates. I find that the evolutionary trajectory through the receptor-modified intermediate (R₂S₁, ‘cheater’) is positively selected at both steps but the one through the signal-modified intermediate (R₁S₂, ‘lame’) is not (SI Text, section 5). (B) Scheme of contribution to public goods and communication during the two evolutionary steps (compare circled numbers to those in A). Shown are the two competing strains, the signaling relations between them, and the contribution to public goods production (red) and benefit (green). (Upper) Competition between the original QS system (R₁S₁) and the intermediate cheater strain (R₂S₁). Only R₁S₁ produces public goods. (Lower) Competition between the intermediate cheater strain (R₂S₁) and the novel QS system (R₂S₂). R₂S₂ signal induces public goods production by both itself and the intermediate strain R₂S₁ and is therefore immune to cheating by R₂S₁. (C and D) Results of invasion simulations of R₂S₁ into R₁S₁ (black line), R₂S₂ into R₂S₁ (gray solid line), and R₁S₁ into R₂S₁ (gray dashed line) in well-mixed conditions. Invading strain initial frequency is 2%. (C) Frequency of invading strain as a function of time. (D) Total cell density as a function of time. I note that the immune cooperator (R₂S₂) frequency remains constant but the production of public goods leads to a higher total cell density compared with a pure cheater (R₂S₁) population. Inset in D is as in C. (E) The immune cooperator outcompetes its ancestor in a structured population. Shown are the asymptotic frequencies as a function of bottleneck size of naive (R₁S₁, orange) and diverged immune (R₂S₂, cyan) QS strains when separately competed with the intermediate cheater strain (R₂S₁). Each pair of strains undergoes cycles of growth and population bottlenecks, as explained in Materials and Methods and SI Text, section 6. Whereas the naive cooperator frequency is quickly reduced when bottleneck size increases, the immune cooperator’s level remains maximal for any bottleneck size.

Fig. 3.

Maintenance of diversity and evolution of cross-inhibition. (A) Divergent QS systems act as facultative cheaters. A minority strain (cyan) activates its quorum response later than a majority strain and therefore facultatively exploits the majority strain. Shown are cell densities of each strain (Upper) and the level of public goods produced by single cells from each strain (Lower). The facultative cheating domain is colored pink and cooperation (arbitrarily defined at half-maximum production rate) is green. (Right) Graphs show convergence of strain frequencies to 50% at later times. (Inset) A scheme of public goods production and communication in the competition between divergent strains. (B–E) The evolution of cross-inhibition. (B) The QS diversification model can be extended by assuming that one of the receptors (R₁ⁱⁿ, orange) can mutate to a novel form (R₁ⁱⁿ, deep orange) that is inhibited by a divergent signal (cyan). (C) A scheme of signaling and public goods contribution in the three-way competition between the two divergent strains and the one with cross-inhibited receptor. (D) Cross-inhibition is beneficial for the inhibited strain. A strain with a receptor that is inhibited by a divergent strain can invade into its parental population that is fully orthogonal to the divergent strain. Initial conditions of the divergent strains are their steady-state level when mixed without the cross-inhibited strain. The cross-inhibited strain’s initial density is 1% of the other strains. (E) Cross-inhibition is selected in structured populations. Shown are the frequencies of the three strains described in C as a function of bottleneck size for a three-way competition analysis in a population going through cycles of growth and bottleneck phases, similar to Fig. 2E. The strain with the cross-inhibited receptor (dashed deep orange line) is selected over its ancestral orthogonal strain (solid orange line). The nonmonotonicity of the frequencies at low bottleneck sizes is discussed in SI Text, section 8.