Evidence for low-titre infections in insect symbiosis:Wolbachiain the bark beetlePityogenes chalcographus(Coleoptera, Scolytinae)

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SummaryWolbachiaare obligatory endosymbiotic α-proteobacteria found in many insect species. They are maternally transmitted and often exhibit reproductive phenotypes like cytoplasmic incompatibility.Pityogenes chalcographusis a bark beetle causing severe damage in spruce stands. Its European populations are divided into several mitochondrial clades separated by partial crossing barriers. In this study, we tested a large sample set covering the natural range of the beetle in Europe for the presence ofWolbachiaand associations between infection pattern and mitotypes using a highly sensitive nested PCR technique. 35.5% of the individuals were infected with the endosymbiont and two distinct strains were identified. Both strains occur in low titre not accessible by conventional detection methods. The infections are present all over Europe, unlikely to cause the partial crossing barriers in this host and uncoupled from mitochondrial clades. This pattern is indicative for populations evolving towards endosymbiont loss and for repeated intraspecific horizontal transfer ofWolbachia. Alternatively, the low-titre infections found inP. chalcographusare yet another example forWolbachiathat can persist in host species at low densities and frequencies.

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