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Abstract :

Ixodes ricinus is the most common tick in Western Europe and the main vector of Borrelia burgdorferi s.l., the agent of Lyme disease. In addition to pathogens, ticks, like many animals, harbour numerous non-pathogenic microorganisms,
whose effect on their host extends along a continuum from mutualism to parasitism. Within this microbial community associated with I. ricinus, the Alphaproteobacterium (Rickettsiales) Midichloria mitochondrii is the most widespread in the different tick populations. This endosymbiont, capable of residing within the mitochondria of tick cells, is particularly abundant in the ovaries of adult females where the bacterium is systematically transmitted to the offspring and is suspected to be an obligate mutualistic symbiont. However, the very existence of tick lines without the symbiont calls into question the obligatory nature of this symbiosis. Thus, the study of the M. mitochondrii-I. ricinus interaction is of major interest in order to establish whether the bacterium is essential to tick fitness, which would constitute a potential lever for vector control based on antisymbiotic control. Firstly, the dynamics of M. mitochondrii between male and female nymphs of I. ricinus were studied. In a second step, an tetracycline-based antibiotic treatment aimed at removing the bacterium from its host as well as the hypothesis of a B vitamins provision by M. mitochondrii to the tick were tested without making it possible to draw any conclusions. Thirdly, the analysis of the transcriptome of ticks harbouring or not M. mitochondrii made it possible to identify differentially expressed genes that could be involved in the tick-bacteria interaction. The nature of the M. mitochondrii–I. ricinus relationship thus appears even more complex than that observed in other tick–endosymbiont pairs.

Key words :

Ticks, symbiosis, mutualism, transcriptomics, antibiotics