Evidence for low-titre infections in insect symbiosis:Wolbachia in the bark beetle Pityogeneschalcographus (Coleoptera, Scolytinae)

摘要

Wolbachia are obligatory endosymbiotic a-proteobacteria found in many insect species. Theyare maternally transmitted and often exhibit repro-ductive phenotypes like cytoplasmic incompatibility.Pityogenes chalcographus is a bark beetle causingsevere damage in spruce stands. Its European popu-lations are divided into several mitochondria! cladesseparated by partial crossing barriers. In this study,we tested a large sample set covering the naturalrange of the beetle in Europe for the presenceof Wolbachia and associations between infectionpattern and mitotypes using a highly sensitive nestedPCR technique. 35.5% of the individuals were infectedwith the endosymbiont and two distinct strainswere identified. Both strains occur in low titre notaccessible by conventional detection methods. Theinfections are present all over Europe, unlikely tocause the partial crossing barriers in this host anduncoupled from mitochondria! clades. This pattern isindicative for populations evolving towards endosym-biont loss and for repeated intraspecific horizontaltransfer of Wolbachia. Alternatively, the low-titreinfections found in P. chalcographus are yet anotherexample for Wolbachia that can persist in hostspecies at low densities and frequencies.