RNAseq of the olive fly gastric caeca

The olive fruit fly, Bactrocera oleae, the most serious pest of olives, requires the endosymbiotic bacteria Erwinia dacicola in order to complete its development in unripe green olives. Hence a better understanding of the symbiosis of E. daciola and its insect host may lead to new strategies for B. oleae control. The relative abundance of bacteria during the fly life cycle comparing black and green olives was estimated by real time quantitative PCR revealing significant fluctuations during development in black olives and a peak of the bacteria in the second instar larvae. By microscopy analysis of larvae we show that the bacteria reside extracellularly in the gastric caeca. During the transition to late third instar larvae the bacteria were discharged into the midgut concomitant with a change in caeca size and morphology due to the contraction of the muscles surrounding the caecas. A similar alteration was also observed in a laboratory strain devoid of bacteria. To further investigate the symbiotic interaction and the change in caeca morphology a comparative transcriptomics analysis was undertaken. Samples of dissected caeca from second and third instar larvae collected from the field, both with Erwinia symbionts, as well as second instar larvae from a laboratory strain devoid of Erwinia symbionts revealed significant changes in gene expression under these conditions.