Dissection of the mutation accumulation process during bacterial range expansions

We investigated the dynamics of fitness change during range expansion by analyzing the accumulation of mutations over time in 10 E. coli lines. A specific aim was to determine if there are changes in the effect of the mutations during the progression of the experiment. We showed that mutations were accumulating at a constant rate over time, but we found that there was a higher number of beneficial mutations appearing in the first quarter of the experiment. The effects of the mutations were more shifted towards negative values as the experiment was progressing. We used elastic net regression to estimate the positive or negative effect of mutations on colony growth. Beneficial mutations were enriched in flagella genes, genes encoding transport proteins, and genes coding for the membrane structure, whereas deleterious mutations show no enrichment for any biological process. This experiment shows that the interaction between genetic drift and the availability or depletion of beneficial mutations determine the change in fitness of bacterial populations during range expansion.