Towards the understanding of the evolution of intramitochondrial tropism in ticks symbionts
Résumé
Despite their wide distribution, Midichloria symbionts of ticks are understudied, and their role in the host physiology remains unknown. The most striking feature of some Midichloria is their unique capability of residing inside the mitochondria – a characteristic referred to as intramitochondrial tropism (IMT) – in the host oocytes. The exact distribution of IMT in the Midichloria genus is unknown: of the previously investigated species, two exhibit IMT, while Midichloria in the tick Ixodes holocyclus are only located in the cytoplasm. To further investigate IMT and its distribution, we applied a multidisciplinary approach: 999 ticks (47 species) were sampled from three continents and screened for the presence of Midichloria bacteria. The tropism of Midichloria species was observed in tick ovaries using transmission electron microscopy and Midichloria strains were classified as capable of IMT (IMT Midichloria) and incapable of such tropism (no-IMT). 15 representative samples of both IMT and no-IMT Midichloria were selected for metagenome sequencing, and metagenome-assembled genomes (MAGs) of Midichloria endosymbionts were obtained through the bioinformatic pipeline Blobology. MAGs allowed to obtain a robust phylogenetic reconstruction of the genus, and to perform genomic comparisons aimed at the reconstruction of the Midichloria core-genome and at the detection of genes involved in intramitochondrial tropism. The analyses on these genes and the reconstruction of the gene content of the Midichloria ancestor through birth-death models are being performed, with the goal of understanding the origin and the evolution of this fascinating tropism. Preliminary phylogenetic results suggest that IMT is not restricted to a monophyletic clade, indicating either an homoplasic multiple acquisition of the capability or a single ancestral apomorphic acquisition in the Midichloria ancestor, followed by the loss of such capability in at least a member of the Midichloria clade