Midichloria mitochondriiis widespread in hard ticks (Ixodidae) and resides in the mitochondria of phylogenetically diverse species

Abstract

SUMMARYThe hard tickIxodes ricinus(Ixodidae) is the sole animal thus far shown to harbour an intra-mitochondrial bacterium, which has recently been namedMidichloria mitochondrii. The objectives of this work were (i) to screen ixodid ticks forMidichloria-related bacteria and (ii) to determine whether these bacteria exploit the intra-mitochondrial niche in other tick species. Our main goal was to discover further models of this peculiar form of symbiosis. We have thus performed a PCR screening forMidichloria-related bacteria in samples of ixodid ticks collected in Italy, North America and Iceland. A total of 7 newly examined species from 5 genera were found positive for bacteria closely related toM. mitochondrii. Samples of the tick speciesRhipicephalus bursa, found positive in the PCR screening, were analysed with transmission electron microscopy, which revealed the presence of bacteria both in the cytoplasm and in the mitochondria of the oocytes. There is thus evidence that bacteria invade mitochondria in at least 2 tick species. Phylogenetic analysis on the bacterial 16S rRNA gene sequences generated from positive specimens revealed that the bacteria form a monophyletic group within the order Rickettsiales. The phylogeny ofMidichloriasymbionts and related bacteria does not appear completely congruent with the phylogeny of the hosts.