The symbiosis between two living organisms requires a co-evolution of the two associated partners in a mutual benefit that eventually results in significant changes in their genome. Intracellular symbiosis is a particular form of symbiosis in which a living organism, such as a bacterium, survives and multiplies within the cells of its host. This type of symbiosis is widespread among invertebrates and highly contributes to insects' adaptation to their environment, including lack of some nutrients in the case of very specific diet. However, adjusting the number of endosymbiotes to a beneficial level and maintaining immune homeostasis in an organism chronically infected with mutualist bacteria is a major challenge for the host.

Indeed, when an organism is infected by a pathogenic bacterium, the widely expressed immune receptors recognizing bacterial peptidoglycan or lipopolysaccharide induce an immediate inflammatory response to destroy the invaders. This response can be harmful to the body in the long term (which is also true for humans). In the case of endosymbiosis where an organism is chronically infected with mutualist bacteria, what are the mechanisms of bacteria tolerance?

Cereal weevil (Sitophilus zeamais) 

A set of Coleoptera insect pests that mainly attack cereals is called weevil. Some varieties also attack peas, hazelnuts, bananas, palm trees etc. It is thanks to its bacterioma that it can survive despite a poorly diversified diet.

Researchers at IRIG, specialists in the immune response in fruit fly, participated in a study of INRAE and INSA (Lyon) to understand the role of an evolutionary conserved peptidoglycan recognition protein (PGRP-LB) in the association between the cereal weevil (Sitophilus zeamais) and its bacterial endosymbiota (Sodalis pierantonius). This work shows that three different isoforms of the protein are differentially expressed depending on the context. An isoform is secreted and expressed in the insect's tissues only upon infection by pathogenic bacteria, while the two other isoforms (cytosolic and transmembrane) are permanently produced within the organ that hosts the endosymbiote: the bacterioma. In this bacterioma, PGRP-LB isoforms destroy peptidoglycans released by endosymbiotes, preventing chronic activation of the immune response and allowing endosymbiotes to survive while preserving the insect's health.