Abstract
Population mixing, and transmission modes strongly influence social interactions. However, the impact of repeated mixing on the evolutionary dynamics of microbial predator-prey interactions remains underexplored1,2. Hence, we conducted a laboratory evolution experiment with bacterial predator-prey communities under two transfer regimens: repeated mixing (horizontal transfer) versus no mixing (vertical transfer). For this, Myxococcus xanthus served as the generalist predator3,4 and Escherichia coli as prey. We show that prey populations from vertical regimen were less resistant to predation than the ones from horizontal regimen. This was because prey isolates in the vertical treatment showed varying resistance levels relative to ancestors, while those in the horizontal treatment demonstrated less variation. Moreover, analysis of prey populations over evolutionary time revealed that the populations from horizontal treatment showed increasing levels of resistance to predation over time, whereas the ones from vertical treatment did not show similar trend. The differences in the outcomes of the two treatments was because the variants better at intraspecies competition, can only be maintained in the vertical treatment, whereas in horizontal treatment the benefits of superior intraspecies competitive fitness are nullified because of population mixing, as predicted by mathematical modeling approach. These predictions were empirically confirmed. Moreover, contrary to our expectations, predatory efficiency of evolved M. xanthus isolates was either the same or lesser than the ancestor. Together, we demonstrate that mixing affects the evolution of prey bacteria but has little effect on the hunting ability of the social predator M. xanthus.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Population mixing can significantly influence the interaction dynamics, and thereby the evolutionary trajectories of microbial communities. Since microbial populations are frequently subjected to mixing, brought about by several biotic and abiotic agents, it is important to study the influence of mixing on microbial populations' evolution and interaction dynamics. In the previous version of the manuscript, we demonstrated that repeated mixing events (Horizontal transfer regimen) in laboratory-evolved communities of the bacterial predator Myxococcus xanthus and its prey E. coli resulted in the evolution of higher levels of resistance to predation in the prey population over evolutionary time. We further demonstrated that in the absence of such mixing events (Vertical transfer regimen), the prey population evolved to have two adaptive strategies. First, a fraction of strains that exhibited significant adaptive fitness relative to their ancestors but did not evolve resistance to predation. Second, a second fraction evolved higher levels of resistance to predation. Following are the major updates in the manuscript: Analysis of the E. coli population growth dynamics in the presence of predators revealed two distinct phases. During the first phase, E. coli populations grew without significant predation pressure. During the second phase, predators grew by killing prey populations, thus imposing higher levels of predation pressure (Supplementary figure S6). The two phases of growth were implemented in the mathematical model. Using a mathematical model, we show that the variants better at the intraspecies competition can only be maintained in the vertical treatment, whereas in the horizontal treatment, the benefits of superior intraspecies competitive fitness are nullified. The results of the simulations are mentioned in the new figure, Figure 4 in the updated manuscript. The mathematical model predicted that the variants that evolve to be better at intraspecies competition would be lost if the vertical populations were mixed and propagated as the horizontal populations. By performing additional evolution experiments, we show that this prediction is indeed true (Figure 6).