Contributors: Biologie Évolutive de la Cellule Microbienne - Evolutionary Biology of the Microbial Cell; Université Paris Cité (UPCité)-Microbiologie Intégrative et Moléculaire (UMR6047); Institut Pasteur Paris (IP)-Centre National de la Recherche Scientifique (CNRS)-Institut Pasteur Paris (IP)-Centre National de la Recherche Scientifique (CNRS); Hub d'analyse d'images - Image Analysis Hub (Platform) (IAH); Institut Pasteur Paris (IP)-Université Paris Cité (UPCité); Hub Bioinformatique et Biostatistique - Bioinformatics and Biostatistics HUB; Arizona State University Tempe (ASU); Plate-forme de bioimagerie ultrastructurale - Ultrastructural BioImaging Core Facility; Biologie, Epidémiologie et analyse de risque en Santé Animale (BIOEPAR); École nationale vétérinaire, agroalimentaire et de l'alimentation Nantes-Atlantique (ONIRIS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE); Università degli Studi di Pavia = University of Pavia (UNIPV); This research was financially supported by the Human Frontier Science Program (HFSP), Young Investigator Program grant RGY-0075.; We are grateful to Dr. Valérie Choumet and Dr. Elisabeth Ferquel (Institut Pasteur, France) for their contributions to tick handling, and Maarteen Voourdouw (University of Saskatchewan, Canada) for providing the Neuchâtel line of I. ricinus. Professor Simonetta Gribaldo, Professor Pascale Cossart, and Professor John McCutcheon are thanked for their valuable support, and Professor John Murray for proofreading.
نبذة مختصرة : International audience ; The hard tick, Ixodes ricinus , a main Lyme disease vector, harbors an intracellular bacterial endosymbiont. Midichloria mitochondrii is maternally inherited and resides in the mitochondria of I. ricinus oocytes, but the consequences of this endosymbiosis are not well understood. Here, we provide 3D images of wild-type and aposymbiotic I. ricinus oocytes generated with focused ion beam-scanning electron microscopy. Quantitative image analyses of endosymbionts and oocyte mitochondria at different maturation stages show that the populations of both mitochondrion-associated bacteria and bacterium-hosting mitochondria increase upon vitellogenisation, and that mitochondria can host multiple bacteria in later stages. Three-dimensional reconstructions show symbiosis-dependent morphologies of mitochondria and demonstrate complete M. mitochondrii inclusion inside a mitochondrion. Cytoplasmic endosymbiont located close to mitochondria are not oriented towards the mitochondria, suggesting that bacterial recolonization is unlikely. We further demonstrate individual globular-shaped mitochondria in the wild type oocytes, while aposymbiotic oocytes only contain a mitochondrial network. In summary, our study suggests that M. mitochondrii modulates mitochondrial fragmentation in oogenesis possibly affecting organelle function and ensuring its presence over generations.
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