Green, R.; Wright, E.; Lowry Palmer, R.; Tank, R. K. G.; Tonge, E.; Scaife, J.; Marquiegui-Alvaro, A.; Knight, C. G.; Krasovec, R.

Eco-evolutionary feedbacks are typically thought to require an ecological change driving Darwinian selection. However, ecology can drive other evolutionary mechanisms too. In many microbial species mutation rates are elevated at low population density, mediated by nutrient availability and reactive oxygen species (ROS). These findings from monoculture populations led us to hypothesise that the presence and density of a cohabiting species will also affect mutation rates. Using Escherichia coli as a focal species we explore the mutational effects on both parties of co-culture with either Enterococcus faecalis or Pseudomonas aeruginosa. These species co-occur in multiple clinical settings and have different abilities to degrade ROS. In both co-culture systems we find evidence that the presence of a stronger ROS degrader reduces mutation rates in the weaker ROS degrading species. At the same time increasing population density of the weaker ROS degrader increases mutation rates in the stronger ROS degrading species. Simultaneous mutation rate estimation in co-cultured species raises multiple experimental challenges addressed here. Despite these challenges, we find evidence for a novel form of eco-evo feedback in which the environment shapes the supply of mutational variation to the population. This in turn feeds back to alter the chance of evolutionary rescue for one species or another during a subsequent environmental change. Such effects potentially matter not only for our fundamental understanding of the interplay between ecology and evolution, but for the treatment of polymicrobial infections in which rates of resistance evolution may be altered by the cross-species mutational effects of cohabiting species.