Authors

Discussion

The sheep tick Ixodes ricinusis the most important tick of medical and veterinary importance Europe. It also harbours a vertically-transmitted bacterial endosymbiont, Midichloria mitochondrii (Rickettsiales: Midichloriaceae), which is found in 100% of adult female and ~50% of adult male I. ricinusin continental Europe. The nature of the relationship between M. mitochondrii and I. ricinusis unknown, but the lifecycle of the symbiont involves replication inside mitochondria and the bacterial density increases dramatically following a blood meal. In this study, we sought to estimate the prevalence of M. mitochondrii in English I. ricinusand the population genetic structure of the symbiont in relation to its host across Europe. We assayed a total of 500 nymphs from five sites across southern England, which revealed a prevalence of M. mitochondrii of 80%. However, the density of M. mitochondrii in English nymphs was apparently higher than that previously reported from the Czech Republic. A multi-locus sequence typing (MLST) for M. mitochondrii was developed and used alongside an established mitochondrial MLST scheme for I. ricinus. New data were compared with 318 British and 188 Latvian published I. ricinus mitochondrial sequences that exhibited 10% and 14% variability at the DNA and amino-acid level, respectively, allowing some differentiation between British and Latvian samples. Among French, German and Italian ticks, small clusters of Italian and German mitochondrial sequences were observed, while other sequences were distributed throughout the tree. The MLST scheme for M. mitochondrii indicated low levels of diversity, with some geographic clustering in continental Western Europe and partial congruence with the phylogeny of the tick host. Further co-evolutionary analyses of M. mitochondrii and its host will facilitate the characterisation of this symbiosis (parasitic, mutualist or commensal), as well as its origin and spread.