The interplay of dormancy and transfer in bacterial populations: Invasion, fixation and coexistence regimes

TL;DR

This paper models the combined effects of dormancy and horizontal gene transfer on microbial population dynamics, revealing conditions for invasion, fixation, coexistence, and founder control in a unified stochastic framework.

Contribution

It introduces a comprehensive model integrating dormancy and gene transfer, analyzing their joint impact on invasion and coexistence in microbial populations.

Findings

Stable coexistence can occur even if one trait is unfit alone.

Founder control can dominate despite unstable coexistence equilibria.

Classical invasion phases are observed in all scenarios.

Abstract

We investigate the interplay between two fundamental mechanisms of microbial population dynamics and evolution called dormancy and horizontal gene transfer. The corresponding traits come in many guises and are ubiquitous in microbial communities, affecting their dynamics in important ways. Recently, they have each moved (separately) into the focus of stochastic individual-based modelling (Billiard et al. 2016, 2018; Champagnat, M\'el\'eard and Tran, 2021; Blath and T\'obi\'as 2020). Here, we examine their combined effects in a unified model. Indeed, we consider the (idealized) scenario of two sub-populations, respectively carrying 'trait 1' and 'trait 2', where trait 1 individuals are able to switch (under competitive pressure) into a dormant state, and trait 2 individuals are able to execute horizontal gene transfer, turning trait 1 individuals into trait 2 ones, at a rate depending on the frequency of individuals. In the large-population limit, we examine the fate of a single trait i individual (a 'mutant') arriving in a trait j resident population living in equilibrium, for $i,j=1,2,i \neq j$. We provide a complete analysis of the invasion dynamics in all cases where the resident population is individually fit and the initial behaviour of the mutant population is non-critical. We identify parameter regimes for the invasion and fixation of the new trait, stable coexistence of the two traits, and 'founder control' (where the initial resident always dominates, irrespective of its trait). The most striking result is that stable coexistence occurs in certain scenarios even if trait 2 (which benefits from transfer at the cost of trait 1) would be unfit when being merely on its own. In the case of founder control, the limiting dynamical system has an unstable coexistence equilibrium. In all cases, we observe the classical (up to 3) phases of invasion dynamics \`a la Champagnat (2006).