Microbial evolution and transitions along the parasite-mutualist continuum

Symbiotic interactions can be neutral, harmful or have beneficial effects for host organisms. In this Review, Drew, Stevens and King discuss the evolutionary transitions of host-microorganism symbioses along the parasite-mutualist continuum, the mechanisms underlying evolutionary changes, the selective pressures involved and common empirical approaches for studying them. Virtually all plants and animals, including humans, are home to symbiotic microorganisms. Symbiotic interactions can be neutral, harmful or have beneficial effects on the host organism. However, growing evidence suggests that microbial symbionts can evolve rapidly, resulting in drastic transitions along the parasite-mutualist continuum. In this Review, we integrate theoretical and empirical findings to discuss the mechanisms underpinning these evolutionary shifts, as well as the ecological drivers and why some host-microorganism interactions may be stuck at the end of the continuum. In addition to having biomedical consequences, understanding the dynamic life of microorganisms reveals how symbioses can shape an organism's biology and the entire community, particularly in a changing world.