Figure 2.

Morphological diversity of symbiotic structures. In organisms such as insects, high densities of endosymbionts are housed in specialized host cells, called bacteriocytes, that sometimes form a dedicated organ, the bacteriome (A). In the pea aphid A. pisum, giant bacteriocytes (diameter > 100 µm in adults) are grouped in two abdominal chains (B) [14]. They house B. aphidicola, which provides most essential amino acids to its sap-feeding host [63]. In the tsetse fly Glossina morsitans, bacteriocytes form a bacteriome around the gut (C) [67]. Its endosymbiont Wigglesworthia glossinidia synthesizes B vitamins that are absent in the fly’s hematophagous diet [68]. In the tube worm Riftia pachyptila, bacteriocytes form a trophosome that is directly linked to the circulatory system (D). This tubeworm species usually lives near hydrothermal vents. Carbon dioxide and hydrogen sulfide are internalized by the plume gills and transferred through the circulatory system to the trophosome, where chemosynthetic endosymbionts are able to metabolize those dissolved gases and to provide organic matter to their host [16].