Table 1.
Examples of compartments divided into four main types according to their how hosts enclose their partners. These are: (i) strict compartmentalization, where symbionts, often single genotypes, are fully enveloped by well-defined, (semi) permanent boundaries; (ii) fluid compartmentalization, where the boundaries of the compartment are less discrete or permanent; (iii) partial compartmentalization, where only part of the symbiont—such as the nutrient exchange structure—is compartmentalized; and (iv) compartmentalization of non-microbial partners, whereby the enclosed mutualist is itself a large macro-organism. (?), tests of these mechanisms are not yet known.
| host | symbiont | name of compartment | containment to prevent spread within host tissue | control reproduction | mediate discrimination | control resource allocation | reduce within-host conflict | references |
|---|---|---|---|---|---|---|---|---|
| 1. host–microbe mutualisms—strict compartmentalization | ||||||||
| cicadas, mealy bugs | Sulcia, Hodgkinia, Ophiocordyceps, Moranella endobia, Tremblaya princeps | bacteriocytes within bacteriomes | yes | yes | ? | ? | yes | [32,34] |
| aphids | Buchnera, Serratia | bacteriocyte | yes | yes | no (?) | ? | yes | [35,36] |
| tortoise leaf beetles (Cassida) | Stammera | extracellular bacterium housed in specialized organs connected to the foregut | yes | yes | ? | ? | no (?) | [37] |
| sepiolid squid | Vibrio bacteria | crypts | yes | ? | yes | ? | yes | [38,39] |
| legumes, Parasponia | Rhizobia | symbiosomes within nodules (legumes) intracellular fixation threads in nodule cells (Parasponia) |
yes | yes, in some legume lineages | yes | yes | yes | [40–42] |
| scleratinian corals | Symbionidium | symbiosome (host vacuole) | yes | yes | ? | yes | no | [43,44] |
| Paracatenula flatworm | Alphaproteobacteria | trophosome | yes | yes | no (?) | yes | no (?) | [45,46] |
| Trichoplax sp. H2 | Grellia incantans and Ruthmannia eludens | rough endoplasmic reticulum of the host's internal fibre cells [G. incantans]; ventral epithelial cells [Ruthmannia eludens] | probably | ? | ? | ? | probably | [47] |
| 2. host–microbe mutualisms—fluid compartmentalization | ||||||||
| shield bug | Gammaproteobacteria | symbiont capsules, jelly secretions, faecal droplets | no | yes | no (?) | no (?) | no (?) | [24,48] |
| humans | skin microbiome | defined niches, such as hair follicles | yes | ? | no (?) | no (?) | no (?) | [49] |
| bean bug | Burkholderia | gut crypts | yes (?) | no | no (?) | ? | yes, by reducing physical contact | [50] |
| lichenicolous fungi (i.e. lichens) | algae, yeast, bacterial biome | thallus | yes | yes | no (?) | ? | probably | [51] |
| vertebrates | gut microbiome | lamina propria, Peyer's patches | yes | ? | no (?) | ? | ? | [52] |
| honeybee, termite | gut microbiome | midgut, ileum, rectum | yes | ? | no (?) | ? | ? | [53,54] |
| attine ants | Pseudonocardia bacteria | specialized cuticular structures, including crypts and tubercles | no | yes | no (?) | no (?) | yes (?) | [55,56] |
| 3. host–microbe mutualisms—partial compartmentalization | ||||||||
| land plants | arbuscular mycorrhizal fungi | root cell containing arbuscule | no | no | yes | yes | yes | [57] |
| 4. compartmentalization in non-microbial mutualisms | ||||||||
| Yucca | yucca moth | entire flower | no | yes | yes | — | ? | [58] |
| Ficus | fig-wasp | entire inflorescence (fig) | no | yes | yes | — | ? | [59,60] |
| Glochidion | Epicephala moths | entire flower | no | yes | yes | — | ? | [61] |