An anaerobic Legionellales symbiont in Anaeramoeba pumila
This study characterizes *Candidatus* Centrionella anaeramoebae, a rare anaerobic Legionellales symbiont of *Anaeramoeba pumila* that has undergone genome reduction, acquired host-derived GTPase genes via horizontal transfer, and lost the symbiosome structure, thereby offering a unique model for the evolutionary transition from aerobic pathogenesis to anaerobic mutualism.