Fig. 1

Main endosymbiotic features of cereal weevils and tsetse flies. a Schematics of weevil (left) and tsetse (right) larvae. In both models, the obligate symbionts (Sodalis pierantonius in the weevil and Wigglesworthia glossinidia in tsetse) are present intracellularly in a bacteriome (red) located around the gut (light blue). In tsetse flies, obligate and facultative symbionts (Sodalis glossinidius) can be found extracellularly in the midgut lumen, both originating from maternal milk secretions provided as nourishment to developing intrauterine larvae. b Schematics of weevil (left) and tsetse (right) adults. In both models, the obligate symbionts are present intracellularly in bacteriomes (red). In the weevil, bacteriomes are present at the apex of midgut mesenteric caeca (light blue), as well as at the apex of female ovaries (light green), from which maternal transmission occurs. In tsetse flies, the obligate symbiont is also located intracellularly in a bacteriome (red) located around the midgut (blue) as well as extracellularly in the lumen of the milk glands (pink). In tsetse, the facultative symbiont is distributed intra- and extracellularly throughout the whole fly, including the lumen of the milk glands. Both obligate and facultative symbionts are maternally transmitted through milk feeding. c Schematics of weevil (left) and tsetse (right) bacteriocytes. S. pierantonius is an elongated bacterium that exhibit high size variability. W. glossinidia is a large rod-shaped bacterium. In both model, the obligate symbionts located in midgut bacteriocytes supports their host with nutrients that are used to build exoskeleton and for reproduction in the weevil and tsetse, respectively. d Obligate symbiont growth dynamics in the weevil (left, adapted with permission from [35]) and in tsetse (right, adapted with permission from [100]). In both models, bacterial load dynamics follow the main biological needs of the host. In the weevil, the obligate endosymbiont’s density increases considerably during exoskeleton synthesis. After cuticle completion, the endosymbionts located in mesenteric caeca are eliminated and recycled. Female weevils keep a stable load of endosymbiont in their ovaries for transmission. In adult male tsetse, W. glossinidia density increases dramatically immediately following eclosion (likely in response to the onset of mating activities), and then slowly declines thereafter. In adult females, W. glossinidia density constitutively increases, likely as a reflection of the metabolically costly process of nourishing intrauterine larvae