Virus-host interactions shape viral dispersal giving rise to distinct classes of travelling waves in spatial expansions

Abstract

Reaction-diffusion waves have long been used to describe the growth and spread of populations undergoing a spatial range expansion. Such waves are generally classed as either pulled, where the dynamics are driven by the very tip of the front and stochastic fluctuations are high, or pushed, where cooperation in growth or dispersal results in a bulk-driven wave in which fluctuations are suppressed. These concepts have been well studied experimentally in populations where the cooperation leads to a density-dependent growth rate. By contrast, relatively little is known about experimental populations that exhibit a density-dependent dispersal rate. Using bacteriophage T7 as a test organism, we present novel experimental measurements that demonstrate that the diffusion of phage T7, in a lawn of host E. coli, is hindered by the physical presence of the host bacteria cells. The coupling between host density, phage dispersal and cell lysis caused by viral infection results in an effective density-dependent diffusion rate akin to cooperative behavior. Using a system of reaction-diffusion equations, we show that this effect can result in a transition from a pulled to pushed expansion. Moreover, we find that a second independent density-dependent effect on phage dispersal spontaneously emerges as a result of the viral incubation period, during which phage is trapped inside the host unable to disperse. Our results indicate both that bacteriophage can be used as a controllable laboratory population to investigate the impact of density-dependent dispersal on evolution, and that the genetic diversity and adaptability of expanding viral populations could be much greater than is currently assumed.

Shifts from pulled to pushed range expansions caused by reduction of landscape connectivity

“…4), confirming our main prediction. Although it was not presented in connectivity terms, a similar result was recently found using a model inspired by phage–bacteria interactions (Hunter et al 2020), where reduced connectivity was de facto achieved using increased densities of phage–resistant bacteria as a barrier. In our simulations, these pushed expansions are characterized by a mean dispersal rate independent of population density; this means they would likely converge to a pulled wave as K tends to infinity and the effects of discreteness and stochasticity become negligible.…”

Landscape connectivity alters the evolution of density-dependent dispersal during pushed range expansions

“…In the current context of habitat loss and fragmentation, several studies have set to explore how habitat connectivity can affect range expansion speeds and/or the evolution of dispersal and other traits during range expansions (Gralka & Hallatschek, 2019; Hunter et al, 2021; Lutscher & Musgrave, 2017; Pachepsky & Levine, 2011; Urquhart & Williams, 2021; Williams, Snyder, et al, 2016; Williams, Kendall, et al, 2016; Williams & Levine, 2018). For instance, using experimental expansions, Williams et al (2016) showed that evolution had stronger effects on range expansion speeds in patchier landscapes where connectivity was lower (or, conversely, that evolution dampened the negative effects of low connectivity on speed).…”

Watching a Virus Expand

“…An experimental platform for studying viral expansion must be able to probe the complex virus-host interactions. A team led by Diana Fusco of the University of Cambridge in the UK has now developed such a platform and applied it to study the waves of viral expansion in an infected cell population [1].…”