A dual endosymbiosis supports nutritional adaptation to hematophagy in the invasive tick Hyalomma marginatum

Abstract

Many animals are dependent on microbial partners that provide essential nutrients lacking from their diet. Ticks, whose diet consists exclusively on vertebrate blood, rely on maternally inherited bacterial symbionts to supply B vitamins. While previously studied tick species consistently harbor a single lineage of those nutritional symbionts, we evidence here that the invasive tick Hyalomma marginatum harbors a unique dual-partner nutritional system between an ancestral symbiont, Francisella, and a more recently acquired symbiont, Midichloria. Using metagenomics, we show that Francisella exhibits extensive genome erosion that endangers the nutritional symbiotic interactions. Its genome includes folate and riboflavin biosynthesis pathways but deprived functional biotin biosynthesis on account of massive pseudogenization. Co-symbiosis compensates this deficiency since the Midichloria genome encompasses an intact biotin operon, which was primarily acquired via lateral gene transfer from unrelated intracellular bacteria commonly infecting arthropods. Thus, in H. marginatum, a mosaic of co-evolved symbionts incorporating gene combinations of distant phylogenetic origins emerged to prevent the collapse of an ancestral nutritional symbiosis. Such dual endosymbiosis was never reported in other blood feeders but was recently documented in agricultural pests feeding on plant sap, suggesting that it may be a key mechanism for advanced adaptation of arthropods to specialized diets.

The origin and evolution of mitochondrial tropism inMidichloriabacteria

“…We find no evidence of full co-cladogenesis between hosts and symbionts, as Midichloria symbionts of Hyalomma and Ixodes ticks do not cluster separately. Considering the monophyly of the symbionts of Hyalomma , the previously postulated recent acquisition of Midichloria by this clade (Buysse et al 2021), and that the phylogeny of the symbionts of Ixodes ticks appears to be congruent with that of the hosts (D’Amico et al 2018), the more likely explanation of the presented topology is horizontal transmission from an Ixodes species to Hyalomma , consistent with previous results (Buysse et al 2021). The most parsimonious scenario is that of a single transfer event in the ancestor of the H. scupense - H. marginatum clade, but the possibility that two separate events occurred cannot be excluded.…”

“…The symbionts of I. frontalis and H. marginatum were observed in the cytosol and inside mitochondria (Figure 1A and 1B). Previous work showed that in H. marginatum cells, in addition to Midichloria, Francisella symbionts can be present at high concentrations (Buysse et al 2021). The intramitochondrial bacteria herein observed could thus, in theory, be Francisella .…”

“…The Midichloria of I. ricinus, I. aulacodi and Hyalomma species were quantified by qPCR assays targeting cal and gyrB (Sassera et al . 2008, Buysse et al 2021), while the Midichloria of I. holocyclus were subjected to qPCR assay targeting the 16S rRNA gene (Beninati et al 2009).…”

“…In order to select samples for genome sequencing, the presence and abundance of Midichloria were determined by qualitative and quantitative PCR (qPCR). The Midichloria of I. ricinus, I. aulacodi and Hyalomma species were quantified by qPCR assays targeting cal and gyrB (Sassera et al 2008, Buysse et al 2021, while the Midichloria of I. holocyclus were subjected to qPCR assay targeting the 16S rRNA gene (Beninati et al 2009).…”

Old zoonotic agents and novel variants of tick-borne microorganisms from Benguela (Angola), July 2017

“…Coxiella -like, Francisella -like) provide essential nutrients for ticks [ 21 ]. Although virulence genes identified in their pathogenic related species, C. burnetii and Francisella tularensis , could be absent or non-functional in symbionts, Coxiella -like has been considered a pathogen [ 21 – 23 ]. Herein, Coxiella -like was detected in all but H. truncatum pools, and potential novel Coxiella genotypes were detected in R. duttoni and Rhipicephalus sp.…”