Mutability of demographic noise in microbial range expansions

Qinqin Yu,Matti Gralka,Arbel Harpak,Marie-Cécilia Duvernoy,Oskar Hallatschek,Megan Sousa

doi:10.1038/s41396-021-00951-9

Abstract

Demographic noise, the change in the composition of a population due to random birth and death events, is an important driving force in evolution because it reduces the efficacy of natural selection. Demographic noise is typically thought to be set by the population size and the environment, but recent experiments with microbial range expansions have revealed substantial strain-level differences in demographic noise under the same growth conditions. Many genetic and phenotypic differences exist between strains; to what extent do single mutations change the strength of demographic noise? To investigate this question, we developed a high-throughput method for measuring demographic noise in colonies without the need for genetic manipulation. By applying this method to 191 randomly-selected single gene deletion strains from the E. coli Keio collection, we find that a typical single gene deletion mutation decreases demographic noise by 8% (maximal decrease: 81%). We find that the strength of demographic noise is an emergent trait at the population level that can be predicted by colony-level traits but not cell-level traits. The observed differences in demographic noise from single gene deletions can increase the establishment probability of beneficial mutations by almost an order of magnitude (compared to in the wild type). Our results show that single mutations can substantially alter adaptation through their effects on demographic noise and suggest that demographic noise can be an evolvable trait of a population.

Highlights

Demographic noise, referred to as “genetic drift”, “neutral drift”, or “drift”, is the change in the composition of a population due to random births and deaths
Recent work in microbial colonies has shown that different strains from the same species can exhibit different strengths of demographic noise under the same growth conditions [26, 31], and that the observed differences in demographic noise can have a substantial impact on the establishment probability of beneficial mutations [26, 32, 33]
As metabolism relates to growth rate, this suggests that growth rate plays a role in determining the strength of demographic noise, which is consistent with our finding described that colony area positively correlates with demographic noise

Summary

Introduction

Demographic noise, referred to as “genetic drift”, “neutral drift”, or “drift”, is the change in the composition of a population due to random births and deaths. Recent work in microbial colonies has shown that different strains from the same species can exhibit different strengths of demographic noise under the same growth conditions [26, 31], and that the observed differences in demographic noise can have a substantial impact on the establishment probability of beneficial mutations [26, 32, 33]. It is unknown how much single mutations can affect the strength of demographic noise and whether those changes would be sufficient to alter the efficacy of natural selection. We focus on loss of function mutations using single gene deletion mutant strains, as loss of function mutations are a common type of single step mutation in microbes

Methods

Results

Conclusion