Repeated Evolution in Diverse Strains of Pseudomonas aeruginosa

Abstract

Evolution can often repeat itself across different populations and species, but the reasons aren't fully understood. The likelihood of repeated evolution between populations is affected by both the similarity of their environments and their genetic backgrounds. Studying genetically different populations grown in the same environment could show the extent of genetic background on repeated evolution. We aimed to study this using genetically different strains of the Gram-negative bacteria Pseudomonas aeruginosa, an opportunistic pathogen known to grow in different environments around the world. Six independently evolved populations from each of 8 strains of P. aeruginosa were grown for 750 generations in M9 minimal salts media. For each strain, isolates taken from the initial ancestral populations (0th generation) were competed against their respective descendants (750th generation) in competitive fitness assays. We extracted DNA from each ancestor and descendant isolate, performed genome assembly, then compared detected mutations both within and between strains. Measures of genetic distance between strains were collected from P. aeruginosa phylogenetic data. We hypothesized that repeated evolution would be inversely correlated to the change in relative fitness and to the genetic distance between strains. We found an inverse correlation between repeated evolution and genetic distance, but no significant correlation between repeated evolution and relative fitness. This suggests that repeated evolution is more likely to occur in genetically similar populations.