Heritable, facultative symbionts are common in arthropods, often functioning in host defence. Despite moderately reduced genomes, facultative symbionts retain evolutionary potential through mobile genetic elements (MGEs). MGEs form the primary basis of strain‐level variation in genome content and architecture, and often correlate with variability in symbiont‐mediated phenotypes. In pea aphids ( Acyrthosiphon pisum ), strain‐le...

Insects often harbour heritable symbionts that provide defence against specialized natural enemies, yet little is known about symbiont protection when hosts face simultaneous threats. In pea aphids ( Acyrthosiphon pisum ), the facultative endosymbiont Hamiltonella defensa confers protection against the parasitoid, Aphidius ervi , and Regiella insecticola protects against aphid‐specific fungal pathogens, including Pandora neoap...