Multi-locus sequence typing of Ixodes ricinus and its symbiont Candidatus Midichloria mitochondrii across Europe reveals evidence of local co-cladogenesis in Scotland

Ticks have relatively complex microbiomes, but only a small proportion of the bacterial symbionts recorded fromticks are vertically transmitted. Moreover, co-cladogenesis between ticks and their symbionts, indicating anintimate relationship over evolutionary history driven by a mutualistic association, is the exception rather thanthe rule. One of the most widespread tick symbionts is Candidatus Midichloria, which has been detected in all ofthe major tick genera of medical and veterinary importance. In some species of Ixodes, such as the sheep tickIxodes ricinus (infected with Candidatus Midichloria mitochondrii), the symbiont is fixed in wild adult femaleticks, suggesting an obligate mutualism. However, almost no information is available on genetic variation inCandidatus M. mitochondrii or possible co-cladogenesis with its host across its geographic range. Here, we reportthe first survey of Candidatus M. mitochondrii in I. ricinus in Great Britain and a multi-locus sequence typing(MLST) analysis of tick and symbiont between British ticks and those collected in continental Europe. We showthat while the prevalence of the symbiont in nymphs collected in England is similar to that reported from thecontinent, a higher prevalence in nymphs and adult males is apparent in Wales. In general, Candidatus M.mitochondrii exhibits very low levels of sequence diversity, although a consistent signal of host-symbiont coe-volution was apparent in Scotland. Moreover, the tick MLST scheme revealed that Scottish specimens form aclade that is partially separated from other British ticks, with almost no contribution of continental sequencetypes in this north-westerly border of the tick’ s natural range. The low diversity of Candidatus M. mitochondrii,in contrast with previously reported high rates of polymorphism in I. ricinus mitogenomes, suggests that thesymbiont may have swept across Europe recently via a horizontal, rather than vertical, transmission route