Another way to die for bacteriocytes in the aphid/Buchnera symbiotic system

Symbiosis is a key source of ecological and evolutionary diversification of eukaryotic organisms throughout the animal and plant kingdoms. In insects that are obligatorily dependent on intracellular bacterial symbionts, novel host cells, the bacteriocytes, have evolved for harboring beneficial microbial partners. One of the most fundamental questions concerning these enigmatic cells is how organismal homeostasis controls their numbers and elimination.Here we report that, in the pea aphid, Acyrthosiphon pisum, bacteriocytes degenerate through a hitherto unknown process distinct from evolutionarily conserved pathways, including apoptosis- or autophagy-dependent cell deaths. By combining electron microscopy, immunohistochemistry, and molecular analyses, we demonstrated that the initial event of bacteriocyte cell death is the cytoplasmic accumulation of non-autophagic vacuoles derived from the endoplasmic reticulum, followed by a sequence of cellular stress responses including the formation of autophagosomes in intervacuolar spaces, activation of reactive oxygen species, and Buchnera endosymbiont degradation by the lysosomal system (1). This new cell death process does not seem to affect the other tissues of the insect and is therefore specific to the bacteriocyte cell. It is not activated by caspases, the enzymes involved in triggering and executing apoptosis, whose pathway is incomplete in the pea aphid (2).Our findings reveal novel mechanisms by which bacteriocyte cell and symbiotic bacterial numbers are controlled by organismal homeostasis and shed light on previously unknown aspects of bacteriocyte biological functioning. The discovery of a non-apoptotic and non- autophagic cell-death process in an insect model contributes more broadly to the understanding of metazoan cell-death diversity.(1) Simonet P., Gaget K., Balmand S., Ribeiro-Lopes M., Parisot N., Buhler K., Duport G., Vulsteke V., Febvay G., Heddi A., Charles H., Callaerts P., Calevro F. (2018) Bacteriocyte cell death in the aphid/Buchnera symbiotic system. www.pnas.org/cgi/doi/10.1073/pnas.1720237115. (2) Ribeiro-Lopes M., Simonet P., Gaget K., Duport G., Charles H., Heddi A., Callaerts P., Parisot N., Calevro F. (2017) Effecteurs et inhibiteurs de l’apoptose chez le puceron du pois : annotation et implication dans la mort cellulaire bactériocytaire. 10ème Réunion du Réseau Français de Biologie Adaptative des Pucerons et Organismes Associés (BAPOA), novembre 2017, Colmar, France.