We experimentally and numerically investigate the evolutionary dynamics of four competing strains of E. coli with differing expansion velocities in radially expanding colonies. We compare experimental measurements of the average fraction, correlation functions between strains, and the relative rates of genetic domain wall annihilations and coalescences to simulations modeling the population as a one-dimensional ring of annihilating and coalescing random walkers with deterministic biases due to selection. The simulations reveal that the evolutionary dynamics can be collapsed onto master curves governed by three essential parameters: (1) an expansion length beyond which selection dominates over genetic drift; (2) a characteristic angular correlation describing the size of genetic domains; and (3) a dimensionless constant quantifying the interplay between a colony’s curvature at the frontier and its selection length scale. We measure these parameters with a new technique that precisely measures small selective differences between spatially competing strains and show that our simulations accurately predict the dynamics without additional fitting. Our results suggest that the random walk model can act as a useful predictive tool for describing the evolutionary dynamics of range expansions composed of an arbitrary number of genotypes with different fitnesses.
Two Strains: eCFP vs. eYFP inoculated in equal fractions
eCFP and eYFP inoculated in equal fractions. Imaged on the 8th day of growth. See the README for more info.
q2_eCFP_eYFP.zip
Three Strains: eCFP, eYFP, and black inoculated in equal fractions
eCFP, eYFP, and black inoculated in equal fractions. Imaged on the 8th day of growth. See the README for more info.
q3_eCFP_eYFP_black.zip
Two Strains: black vs. mCherry inoculated in equal fractions
black and mCherry inoculated in equal fractions. Imaged on the 8th day of growth. See the README for more info.
q2_black_mCherry.zip
Three Strains: eCFP, eYFP, and mCherry inoculated in equal fractions
eCFP, eYFP, and mCherry inoculated in equal fractions. Imaged on the 8th day of growth. See the README for more info.
q3_eCFP_eYFP_mCherry.zip
Four Strains: eCFP, eYFP, black, and mCherry inoculated in equal fractions
eCFP, eYFP, black, and mCherry inoculated in equal fractions. Imaged on the 8th day of growth. See the README for more info.
q4_eCFP_eYFP_black_mCherry.zip
Three Strains: 10% eCFP, 10% eYFP, and 80% mCherry
eCFP, eYFP, and mCherry inoculated in fractions of 10%, 10%, and 80% respectively. Imaged on the 8th day of growth. See the README for more info.
q3_10percent_eCFP_eYFP_80percent_mCherry.zip
Well-mixed fitness of each strain: FACS measurements
See the "Comparing well-mixed fitness to fitness from expansion velocities" section of our paper. The attached excel file contains the abundance of each strain competing against mCherry (BW003) in well-mixed culture vs. time; we obtained this data from a FACS machine. We used this data to determine the selective advantage of every strain relative to mCherry in well-mixed culture.
well_mixed_competition_FACS_data.xls
Calculating the Wall Velocity: Single Sectors of each Strain Sweeping through mCherry
See the "Measuring the domain wall velocities v_w^{ij}" section of our paper. We inoculated 10% of a more fit strain (eCFP, eYFP, or black) and 90% of mCherry so that single sectors of the more fit strain would sweep through mCherry (i.e. we wanted to avoid the collision of domain walls). We used the data contained in this package to calculate the wall velocity v_w^{iR} of each strain sweeping through mCherry. See the attached README for more info.
single_sectors_sweeping_through_mcherry.zip
Independent Radial Expansion Velocities of Each Strain
See the "Measuring radial expansion velocities u_i" section of our paper. This dataset contains radius vs. time data of single-strain colonies (one colony per plate) growing on two two separate batches of agar plates. We used this data to determine the average radial expansion velocity of each strain. The third and final batch of plates used to calculate the average radial expansion velocity is in the "Multiple Strains per Plate" repository (it was a control for that experiment, so we included it there). See the README for more information.
independent_radial_expansion_velocities_of_each_strain.zip
Multiple Strains per Plate: testing if independent radial expansion velocity can predict wall velocity
See Appendix S2. Contains data used to argue that even when controlling for colony expansion velocity's plate-to-plate variability, the wall velocity of one strain sweeping through another is vastly overestimated by standard "geometric optics" predictions. The data in this repository was used to generate all Figures and Tables in Appendix S2. See the README for additional information.
multiple_strains_per_plate_radial_expansion_velocities_and_wall_velocities.zip