New insights into the interaction of the symbiotic bacterium Candidatus Erwinia dacicola with the adult olive fly Bactrocera oleae combining microscopy and proteomics analyses

The olive fruit fly Bactrocera oleae is the major pest of olive production. The fly depends on the symbiotic bacterium Candidatus Erwinia dacicola for the survival of the larvae in unripe olives, and in the adult stage it enhances fecundity. Here, we provide a detailed description of the three organs in the adult where the bacterium resides: the esophageal bulb (EB), the midgut and the ovipositor. Microscopy analysis of the EB reveals that the bacteria were located in the lumen which is contained by a single layer of epithelial cells. In the ovipositor the symbionts were present in blind diverticula, each consisting of one cup-shaped cell. In all three organs the bacteria were tightly associated to each other. In the EB dividing bacteria were commonly seen. Proteomic analyses were carried out to further understand these organs revealing that the EB when compared to the midgut overexpresses proteins involved in transport. Proteomic analyses of the three different populations of bacteria revealed that each had distinct properties. In the EB the symbiont expressed enzymes involved in nitrogen assimilation consistent with the notion that the bacterium in the adult contributes to its nutrition. DNA replication was restricted to the EB, while cell division may take place also in the midgut. The symbiont was metabolically active in these two organs while in the ovipositor it was less metabolically active, and protein synthesis was reduced. Our study provides new insights into the symbiont life cycle within the olive fly and its interaction with the insect.