Arsenophonus insect symbionts are commonly infected with APSE , a bacteriophage involved in protective symbiosis

Insects commonly have intimate associations with maternally inherited bacterial symbionts. While many inherited symbionts are not essential for host survival, they often act as conditional mutualists, conferring protection against certain environmental stresses. The defensive symbiont Hamiltonella defensa which protects aphids against attacks by parasitoid wasps is one of these conditional mutualists. The protection afforded by Hamiltonella depends on the presence of a lysogenic bacteriophage, called APSE , encoding homologs of toxins that are suspected to target wasp cells. In this study, an important diversity of APSE variants is reported from another heritable symbiont, Arsenophonus , which is exceptionally widespread in insects. APSE was found in association with two‐thirds of the Arsenophonus strains examined and from a variety of insect groups such as aphids, white flies, parasitoid wasps, triatomine bugs, louse flies, and bat flies. No APSE was, however, found from Arsenophonus relatives such as the recently described Aschnera chinzeii and ALO ‐3 endosymbionts. Phylogenetic investigations revealed that APSE has a long evolutionary history in heritable symbionts, being secondarily acquired by Hamiltonella through lateral transfer from Arsenophonus . Overall, this highlights the role of lateral transfer as a major evolutionary process shaping the emergence of defensive symbiosis in heritable bacteria.