The origin and evolution of mitochondrial tropism inMidichloriabacteria

Abstract

Midichloria are intracellular bacterial symbionts of ticks. Some representatives of this genus have the unique capability to colonize mitochondria in the cells of their hosts. Hypotheses on the nature of this interaction have proven difficult to test, partly due to a lack of data. Indeed, until now, mitochondrial tropism information and genomes were available only for symbionts of three and two tick host species, respectively. Here we analyzed the mitochondrial tropism of three additional Midichloria and sequenced nine novel genomes, showing that the tropism is pnon-monophyletic, either due to losses of the trait or multiple parallel acquisitions. Comparative genome analyses support the first hypothesis, as the genomes of non-mitochondrial symbionts appear to be reduced subsets of those capable of colonizing the organelles. We detect genomic signatures of mitochondrial tropism, showing a set of candidate genes characteristic of the strains capable of mitochondrial colonization. These include the type IV secretion system and the flagellum, which could allow the secretion of unique effectors, direct interaction with, or invasion of the mitochondria. Other genes, including putative adhesion molecules, proteins possibly involved in actin polymerization, cell wall and outer membrane proteins, are only present in mitochondrial symbionts. The bacteria could use these to manipulate host structures, including mitochondrial membranes, in order to fuse with the organelles or manipulate the mitochondrial network.

Host association and intracellularity evolved multiple times independently in theRickettsiales

“…In Rickettsiales , the hallmark apparatus is type IV secretion system (Supplementary figure 3), representing a probable ancestral horizontal acquisition 29 , and a possible prerequisite for establishing interactions with host cells 10 . Only few genomes among Rickettsiales are devoid of this apparatus 17,30 , and few others display an incomplete gene set, possibly indicative of ongoing loss, in particular Bandiella and the Midichloriaceae symbiont of Reticulomyxa (Supplementary figure 3). Type VI secretion system is very rare, being found only in two Rickettsiaceae (Supplementary figure 3), in both cases encoded on plasmids, a probable indication of horizontal acquisition.…”

“…Within families Rickettsiaceae and Midichloriaceae , they are extremely rare in terrestrial representatives (the only cases being Midichloria mitochondrii and the Rickettsiaceae symbiont of Amblyomma Ac37b), but quite common in aquatic ones, in particular basal Rickettsiaceae and Midichloriaceae in general, thereby also confirming the few experimental observations of flagella 43–45 . The poor correlation of the presence of flagellar genes with Rickettsiales phylogeny (including differential cases within the same genus, such as Megaira and Midichloria 30 ) would be indicative of multiple independent reduction/loss events (Supplementary figure 5). Similar considerations may hold for chemotaxis, which possibly works in conjunction with flagella for host targeting 46 , and is present only in basal Rickettsiales and in the basal members of the family Rickettsiaceae (Supplementary figure 5).…”

“…Interestingly, the correlation with other traits allows the inference of some functional links, such as flagella and chemotaxis in aquatic environments, likely involved in non host-associated stages such as horizontal transmission 21 , pili for extracellular attachment to host cells in Deianiraea 8 and possibly other members of the DDG clade. At the same time, alternative/additional functions for these apparatuses due to their homologies with secretion systems 59,60 could be possible, and may account for the exceptions to such correlation patterns 30,42 .…”

Novel evolutionary insights on the interactions of the Holosporales (Alphaproteobacteria) with eukaryotic hosts from comparative genomics

“…Alternatively, it could be that flagellar gene expression is conditional, and triggered by conditions different from those experienced in the previously examined laboratory settings (Castelli, Nardi, et al, 2022). Specifically, as proposed for other symbionts, in Bealeia and other Holosporales , flagella may be involved in extracellular motility during horizontal transmission (Collingro et al, 2017; Sassera et al, 2010; Schulz et al, 2016), adhesion to/invasion of host cells (Floriano et al, 2022; Siewert et al, 2022), or even in delivery of effector molecules, due to their type III secretion component (Abby & Rocha, 2012). Further experimental analyses will be necessary to elucidate this.…”