Nationwide genomic atlas of soil-dwelling Listeria reveals effects of selection and population ecology on pangenome evolution

A population genomic analysis of 1,854 Listeria soil isolates collected across the contiguous United States identifies geographically prevalent phylogroups with increased pangenome openness and recombination, as a result of adaptation to variable environments. Natural bacterial populations can display enormous genomic diversity, primarily in the form of gene content variation caused by the frequent exchange of DNA with the local environment. However, the ecological drivers of genomic variability and the role of selection remain controversial. Here, we address this gap by developing a nationwide atlas of 1,854 Listeria isolates, collected systematically from soils across the contiguous United States. We found that Listeria was present across a wide range of environmental parameters, being mainly controlled by soil moisture, molybdenum and salinity concentrations. Whole-genome data from 594 representative strains allowed us to decompose Listeria diversity into 12 phylogroups, each with large differences in habitat breadth and endemism. 'Cosmopolitan' phylogroups, prevalent across many different habitats, had more open pangenomes and displayed weaker linkage disequilibrium, reflecting higher rates of gene gain and loss, and allele exchange than phylogroups with narrow habitat ranges. Cosmopolitan phylogroups also had a large fraction of genes affected by positive selection. The effect of positive selection was more pronounced in the phylogroup-specific core genome, suggesting that lineage-specific core genes are important drivers of adaptation. These results indicate that genome flexibility and recombination are the consequence of selection to survive in variable environments.

Automated summary

The study of 1,854 Listeria isolates from soil in the contiguous United States revealed geographically prevalent phylogroups influenced by environmental factors such as soil moisture, molybdenum, and salinity concentrations. Analysis of whole-genome data showed that cosmopolitan phylogroups with broad habitat ranges had more open pangenomes, weaker linkage disequilibrium, and a large fraction of genes affected by positive selection, indicating that genome flexibility and recombination are driven by adaptation to survive in variable environments.