Figure 1.

Barriers to persistence of microbial symbionts across hemimetabolous and holometabolous insect life cycles. (a) Cockroaches possess obligate endosymbionts within fat body cells that are transmitted through eggs and which contribute to host nutrition [19] and also hindgut microbiota that are acquired through a faecal–oral route [20]. The gut structure and bacteriomes remain largely unchanged during development, enabling consistent habitat for both symbiont types. (b) In Sitophilus grain weevils, amino acid-provisioning intracellular symbionts migrate during metamorphosis, from bacteriocytes at the foregut/midgut junction in larvae to cells located on midgut caecae in adults, and then to ovarioles where they are transmitted to eggs [21]. Both bacteriomes and gut structure undergo restructuring, requiring symbionts to shift locations and adjust to novel habitats. Letters indicate barriers to symbiont transmission: (A) transmission from female to egg at oviposition, followed by maintenance of symbionts on or in the egg; (B) incorporation into the embryo during development or acquisition at hatching by ingestion; (C) transmission across larval moults including shedding of cuticle and lining of foregut and hindgut; (D,E) migration to new locations during organ remodelling during metamorphosis. Gut morphologies corresponding to different stages are shown in the centre. Red = location of intracellular, vertically transmitted endosymbionts. (Online version in colour.)