Evidence of two lineages of the symbiont 'Candidatus Erwinia dacicola' in Italian populations of Bactrocera oleae (Rossi) based on 16S rRNA gene sequences

A close association between the olive fly Bactrocera oleae (Rossi) (Diptera: Tephritidae) and bacteria has been known for more than a century. Recently, the presence of an host specific, hereditary, unculturable symbiotic bacterium, designated as "Candidatus Erwinia dacicola", has been described inside a cephalic organ of the fly, called oesophageal bulb. In the present work we analyzed the 16s rRNA variability of "Ca. E. dacicola" within and among 26 Italian olive flies populations sampled across areas where olive trees occur in the wild, and areas where cultivated olive trees were introduced in historical times. The bacterial content of the oesophageal bulbs of 314 olive flies was analyzed and a minimum of 781bp of 16S rRNA gene was sequenced. The corresponding host fly genotype was assessed by sequencing a 776 bp portion of its mitochondrial genome. Two "Ca. E. dacicola" haplotypes were found (htA and htB), one being slightly more prevalent than the other (57%). The two haplotypes do not co-exist in the same fly individual, as confirmed by cloning. Interestingly the olive fly populations of the two main Italian islands, Sicily and Sardinia, appeared exclusively represented respectively by htB and htA while peninsular populations showed both bacterial haplotypes in different proportions. No significant correlation emerged between the two symbiont haplotypes and the 16 host fly haplotypes observed, suggesting evidences for a mixed model of vertical and horizontal transmission of the symbiont during the fly life cycle.