Figure 1.

Insect–microbe symbioses are studied from different angles. A, The microbiome perspective concerns microbial communities’ composition and function across individuals, populations or species. B, The symbiont's perspective concerns specific microbial clades, focusing on their distribution, transmission, genomic evolution or functions. A distinct level of investigation concerns cellular and molecular mechanisms of host–symbiont interaction. C, The host perspective addresses questions about how symbionts affect insect life-history traits and functions. D, The population perspective concerns symbiont effects on insect populations—performance, functions, genetic diversity and evolutionary potential. E, The community perspective addresses questions about symbiotic host's interactions with other species in the community, and thus symbionts' indirect effects on community processes, composition and functions. Levels D and E are the focus of this review.