Abstract
Bacterial endosymbioses are very common in insects and can range from obligate to facultative as well as from mutualistic to pathogenic associations. Several recent studies provide new insight into how endosymbionts manage to establish chronic infections of their hosts without being eliminated by the host immune system. Endosymbiont tolerance may be achieved either by specific bacterial adaptations or by host measurements shielding bacteria from innate defense mechanisms. Nevertheless, insect hosts also need to sustain control mechanisms to prevent endosymbionts from unregulated proliferation. Emerging evidence indicates that in some cases the mutual adaptations of the two organisms may have led to the integration of the endosymbionts as a part of the host immune system. In fact, endosymbionts may provide protective traits against pathogens and predators and may even be required for the proper development of the host immune system during host ontogeny. This review gives an overview of current knowledge of molecular mechanisms ensuring maintenance of chronic infections with mutualistic endosymbionts and the impact of endosymbionts on host immune competence.
Highlights
Insects are the most diverse and successful animal group on earth, which is reflected in the variety of habitats they live in, their abundance and species richness
It seems surprising that bacterial endosymbionts are not eradicated by the host immune system, as they are often very closely related to pathogenic bacteria [26], which are recognized and fought by the insect immune system. This raises the question, which mechanisms allow insect hosts to maintain a chronic infection with bacterial symbionts
Wolbachia are a group of obligate intracellular and maternally transmitted bacteria, which are extremely widespread in arthropods and are presumably found in up to 66% of all insects [35]
Summary
Insects are the most diverse and successful animal group on earth, which is reflected in the variety of habitats they live in, their abundance and species richness. Primary endosymbionts have an obligate and ancient association (up to estimated 250 million years) with their insect host [13] Their transmission to the host progeny occurs mainly vertically via the germ line. The humoral immune response includes the production and release of antimicrobial peptides (AMPs), as well as melanization and clotting processes [17,18] These defense reactions are triggered by the recognition of microbe-associated molecular patterns (MAMPs). It seems surprising that bacterial endosymbionts are not eradicated by the host immune system, as they are often very closely related to pathogenic bacteria [26], which are recognized and fought by the insect immune system This raises the question, which mechanisms allow insect hosts to maintain a chronic infection with bacterial symbionts. We will give an overview of current knowledge of the mechanisms leading to endosymbiont tolerance and control in insects
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