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

In this paper we investigate the interplay between two fundamental mechanisms of microbial population dynamics and evolution, namely 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, Méléard and Tran, 2021; Blath and Tóbiás 2020). Here, we investigate 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, which in our case means that they can turn trait 1 individuals into trait 2 ones, at a rate depending on the density of individuals. In the large-population limit, we examine the fate of (i) a single trait 2 individual (called 'mutant') arriving in a trait 1 resident population living in equilibrium, and (ii) a trait 1 individual ('mutant') arriving in a trait 2 resident population. We analyse the invasion dynamics in all cases where the resident population is individually fit and the behaviour of the mutant population is initially non-critical. This leads to the identification of 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). One of our key findings is that horizontal transfer can lead to stable coexistence even if trait 2 is unfit on its own. In the case of founder control, the limiting dynamical system also exhibits a coexistence equilibrium, which, however, is unstable, and with overwhelming probability none of the mutant sub-populations is able to invade. In all cases, we observe the classical (up to three) phases of invasion dynamics à la Champagnat (2006).