Horizontal transfer of Bactrocera oleae endosymbiont, Candidatus Erwinia dacicola: a strategic tool for future Sterile Insect Technique applications

Bactrocera oleae (Rossi) is a key pest of olive crops in the Mediterranean basin, causing economic damage to olive production worldwide. Besides to general research on B. oleae control methods, an important issue regarding B. oleae is its symbiosis with Candidatus Erwinia dacicola. This bacterium is considered essential for the olive fly. It is vertically transmitted through generations and it benefits both larvae and adults in field. On the contrary, it has been found rarely in lab colonies, probably because of the use of antibiotics and preservatives required for the preparation of artificial diets. Since its importance and its role for the olive fly fitness, the endosymbiont transfer from wild B. oleae populations to laboratory-reared ones would allow a better olive fly mass-rearing, producing more competitive flies for future Sterile Insect Technique (SIT) applications. So that, we tried a horizontal transfer among adults, exposing lab flies to different contamination sources of Ca. E. dacicola, such as ripe olives and gelled water contaminated by wild flies, wax domes containing eggs laid by wild females, cages dirtied by feces dropped by wild flies in cohabitation with wild adults. As a result, PCR-DGGE performed with the primer set 63FGC/518R highlighted that the horizontal transfer of the endosymbiont occurred in the case of cohabitation. Thus, our results indicate how cohabitation between wild flies and lab ones permits the horizontal transfer. Moreover, PCR-DGGE performed with the above mentioned primer set was proved to be a consistent method for Ca. E. dacicola screening, also showing the potential to distinguish between the two haplotypes (htA and htB). This study represents the first successful attempt of Ca. E. dacicola horizontal transfer and the first step in acquiring a better understanding of the endosymbiont physiology and its relationship with the olive fly. Hence, our research opens further possibilities to create and establish a permanent symbiotic olive fly lab-reared colony, representing a strategic tool for future Sterile Insect Technique applications.