Ancient endosymbiotic interactions between alphaproteobacteria and archaea/proto eukaryotes gave rise to the mitochondrion, on which all eukaryotic evolution is built. Intriguingly, a novel Alphaproteobacterium, Candidatus Midichloria mitochondrii, is the only known example of an organism that has evolved to colonise these organelles. This bacterium lives in the cellular cytoplasm and the intermembranal space (IMS) of the mitochondrion of its host, Ixodes ricinus (the European castor bean tick). and this is known as Intramitochondrial tropism, (IMT) The same species is also found in the Australian paralysis tick, Ixodes holocyclus. However, in the latter host, it is confined to the cytoplasm only. The functional consequences of IMT for the host, whether beneficial or pathogenic, are not known. It is not clear if or how the bacterium invades the mitochondria, nor whether or how it is able to repair the mitochondrial outer membrane upon invasion. In I. ricinus, M. mitochondrii appears to replicate within the IMS and degrade the mitochondrial matrix, yet this does not appear to result in cell death. Understanding this fascinating endobiotic interaction has intriguing implications for evolutionary and cellular biology. Beyond its relevance to tick biology, understanding IMT has the potential to greatly improve our understanding of mitochondrial targeting, membrane repair and degradation. These have broad implications in mitochondrial disorders, cancers and aging.
However, study of this system is inhibited by a number of substantial obstacles. In addition to the requirement for methods to cultivate these bacteria in vitro, or to functionally perturb them or their host cells, there are major, fundamental knowledge gaps in the basic biology of ticks. There is a need for greater understanding of their cell death systems, and for resources to study them, including reference genomes, transcriptomes or proteomes. Here, we described the development of a reference standard genome and transcriptome for I. holocyclus. We curated this resource to undertake proteomic analyses of adult male and female I. holocyclus, in both the presence and absence of blood-feeding. We then undertook detailed in silico analyses of I. holocyclus proteins to identify conserved orthologs of mitophagy and apoptotic proteins. This work underpins our efforts to explore the host cell changes associated with M. mitochondrii endobiosis. This will be a critical resource for comparison with I. ricinus laboratory colonies with or without M. mitochondrii endobiosis and IMT.