Host-symbiont specificity in insects: Underpinning mechanisms and evolution

How host organisms evolved and maintain specific mutualisms with microorganisms is a fundamental question that is subject to intensive research. In the large majority of insect mutualisms, the host-microbe specificity is maintained by a “partner fidelity” mechanism, mainly through direct symbiont transmission from mother to offspring. Such vertical symbiont transmission is remarkably diverse in insects, including ovarial transmission, milk-gland transmission, coprophagy, egg-smearing, and capsule transmission. In contrast to the insect-microbe symbioses, many animals and plants do not vertically transmit their symbionts but acquire symbionts from ambient environments every generation. Sophisticated “partner choice” mechanisms are at play to establish these mutualisms and maintain them over evolutionary timescales. This symbiont transmission mode, called horizontal transmission or environmental acquisition, is rarely found in insects that maintain specific associations, but recent studies have described this type of symbiosis in a few insect groups. The symbiosis between the bean bug Riptortus pedestris and its gut symbiont Burkholderia insecticola is one of the model systems that is intensively studied to understand how host-symbiont specificity and mutualistic interactions are maintained in insects with horizontal symbiont transmission. Phylogenetic analyses of symbionts in natural insect populations and bacterial inoculation tests in the laboratory revealed a high degree of specificity in this symbiosis while mutant screening of the symbiotic bacterium, genomics and transcriptomics, and histological observations have identified underpinning morphological, genetic and molecular bases. In this chapter, we focus on symbiont transmission modes and mechanisms observed in the amazing diversity of microbial symbioses in insects, highlighting the ways in which they have likely evolved.