Midichloria, the symbiont of ticks transmitted during the blood meal. What is the role of horizontal transmission?

The order Rickettsiales encompasses a variety of bacterial species, many of which are etiological agents of important diseases in humans and animals, while some are important symbionts for arthropods and nematodes. The Midichloriaceae family has been recently proposed as a new clade within this order (Montagna et al., 2013, Appl Environ Microbiol 79:3241-3248). In ecological terms, this new family is possibly even more complex than the sister groups Rickettsiaceae and Anaplasmataceae, and its hosts range from amoebae to parasitic arthropods (ticks, fleas, bedbugs). The type-representative species and best known member of Midichloriaceae is Midichloria mitochondrii, symbiont of Ixodes ricinus, with a prevalence of 100% in females and 50% in males. Most M. mitochondrii bacteria are localized in the cells of the ovary, either in the cytoplasm or within the outer membrane of the mitochondria. The bacterium is vertically transmitted from the mother to the offspring; nevertheless, there are some molecular (e.g. in dogs, horses, sheep) and serological evidences (e.g. in dogs, humans) suggesting that horizontal transmission to the vertebrate host can occur after the tick blood meal (Mariconti et al., 2012, Pathog Glob Health 106:391-96; Bazzocchi et al., 2013, Parasit Vectors 6:3503). Several additional hard and soft tick species have been found to harbor Midichloria-related bacteria (now ascribed to the genus Midichloria), showing different prevalence levels and variable bacterial loads, suggesting various roles in their arthropod hosts (Duron et al., 2017, Mol Ecol doi: 10.1111/mec.14094; Cafiso et al., 2016, Ticks Tick Borne Dis 7:1186-1192; Epis et al, 2008, Parasitol 135:485-94). It seems that no-cladogenesis has occurred between Midichloria bacteria and their tick hosts. As observed in Cafiso et al. (2016, Ticks Tick Borne Dis 7:1186-1192) for example, Midichloria bacteria associated to different species of the Ixodes genus do not form a monophyletic group, but are scattered in various clusters, sometimes grouping with sequences of Midichloria associated to other tick genera or even other tick families. These evidences suggest that horizontal transmission through the vertebrate host could allow Midichloria bacteria to spread between distant tick species.