Figure 1. Evolution of insect-bacterial mutualisms under different scenarios.

In these schematic phylogenies, taxa 1–10 refer to distinct insect species and their associated endosymbionts. Biological examples of each scenario are detailed in the text. (A) Strict cospeciation: Cospeciation between insect hosts (black) and primary mutualists (dark grey) generates a classic pattern of identical host and symbiont phylogenies (i.e., phylogenetic congruence). The grey circle marks the lineage along which the symbiosis originated. Cospeciation, typical of the obligate nutritional mutualisms discussed in this review, is often characterized by close host-symbiont genomic integration. (B) Symbiont loss and host-switching: By contrast, facultative symbionts (light grey) may be lost along host lineages and experience horizontal transmission among host species. These processes disrupt the congruence between host and symbiont phylogenies. (C) Teaming up: In a more complex evolutionary scenario, a formerly facultative endosymbiont may join an existing primary mutualist to form a stable, dual mutualism with the host (as illustrated for taxa 1–5). The light grey circle marks the transition from facultative to obligate mutualism for the second bacterial partner. This process might account for the dual bacterial mutualisms in some sap-feeding insects. (D) Symbiont replacement: Facultative symbionts may ultimately replace existing primary mutualists in some host lineages, as suggested in weevils. In this schematic example, the acquisition of a second, stable symbiont generated a dual partnership in the lineage leading to taxa 1 and 2, but led to symbiont replacement in taxa 3–5.