UNLABELLED: Wolbachia pipientis is an obligate intracellular bacterium, associated with several arthropods and filarial nematodes. Wolbachia establishes a variety of symbiotic relationships with its hosts, with consequent genomic rearrangements, variation in gene content, and loss of regulatory regions. Despite this, experimental studies show that Wolbachia gene expression is coordinated with host developmental stages, but the mechanism is still unknown. In this work, we analyzed published RNA-seq data of four Wolbachia strains, finding a correlation between gene nucleotide composition and gene expression. The strength and direction of this phenomenon changed with the expression of the S-adenosyl-methionine-dependent methyltransferase midA. Specifically, when midA is overexpressed, there is a negative relationship between gene adenine content and gene expression, while downregulation of midA reverses this trend. MidA is known to methylate protein arginine, with potential effect on protein affinity for substrates, including nucleic acids. To expand our understanding of this poorly characterized enzyme, we investigated its ability to methylate DNA expressing it in Escherichia coli. The experiment revealed that the Wolbachia MidA can methylate both adenine and cytosine. Lastly, we found upstream the midA gene, a conserved binding site for the Ccka/CtrA signaling transduction system, and we hypothesize that this mechanism could be involved in the communication between the host and the bacterium. Overall, these findings suggest a cascade mechanism in which the host activates the bacterium Ccka/CtrA signaling system, thus inducing the expression of the midA gene, with subsequent effect on the expression of several Wolbachia genes on the basis of their nucleotide composition. IMPORTANCE: Wolbachia pipientis is one of the most common intracellular bacteria in insects, and it is currently utilized as a tool for the control of vector-borne diseases. As for many other endosymbiont bacteria, Wolbachia experienced important genome rearrangements, gene content changes, and the loss of several regulatory sequences, affecting the integrity of operons and promoters. Nevertheless, experimental studies have shown that Wolbachia gene expression is coordinated with the host physiology (e.g., developmental stages), although the underlying mechanism remains unclear. In this work, based on in silico analyses and an experimental study on wOo methyltransferase, we propose that bacterial DNA methylation could be a key mechanism regulating Wolbachia gene expression. Additionally, we found evidence suggesting that the DNA methylation process in Wolbachia can be activated by the host.

• Keywords
• MidA, Wolbachia pipientis, endosymbionts, gene expression, regulation of gene expression,
• MeSH
• Bacterial Proteins * genetics   metabolism   MeSH
• Methyltransferases * genetics   metabolism   MeSH
• DNA Methylation MeSH
• Nucleotides * genetics   MeSH
• Gene Expression Regulation, Bacterial * MeSH
• Wolbachia * genetics   enzymology   metabolism   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins * MeSH
• Methyltransferases * MeSH
• Nucleotides * MeSH

The parvorder Rhynchophthirina with a single genus Haematomyzus is a small group of ectoparasites of unclear phylogenetic position, related to sucking and chewing lice. Previous screening based on the 16S rRNA gene indicated that Haematomyzus harbors a symbiotic bacterium whose DNA exhibits a strong shift in nucleotide composition typical of obligate mutualistic symbionts in insects. Within Phthiraptera, the smallest known genomes are found in the symbionts associated with sucking lice, which feed exclusively on mammal blood, compared to the generally larger genomes of the symbionts inhabiting chewing lice, which feed on skin derivatives. In this study, we investigate the genome characteristics of the symbiont associated with Haematomyzus elephantis. We sequenced and assembled the H. elephantis metagenome, extracted a genome draft of its symbiotic bacterium, and showed that the symbiont has a significantly reduced genome, which is with 0.39 Mbp the smallest genome among the symbionts known from Phthiraptera. Multigenic phylogenetic analysis places the symbiont into one of three clusters composed of long-branched symbionts from other insects. More specifically, it clusters together with symbionts from several other sucking lice and also with Wigglesworthia glossinidia, an obligate symbiont of tsetse flies. Consistent with the dramatic reduction of its genome, the H. elephantis symbiont lost many metabolic capacities. However, it retained functional pathways for four B vitamins, a trait typical for symbionts in blood-feeding insects. Considering genomic, metabolic, and phylogenetic characteristics, the new symbiont closely resembles those known from several sucking lice rather than chewing lice.IMPORTANCERhynchophthirina is a unique small group of permanent ectoparasites that is closely related to both sucking and chewing lice. These two groups of lice differ in their morphology, ecology, and feeding strategies. As a consequence of their different dietary sources, i.e., mammals' blood vs vertebrate skin derivatives, they also exhibit distinct patterns of symbiosis with obligate bacterial symbionts. While Rhynchophthirina shares certain traits with sucking and chewing lice, the nature of its obligate symbiotic bacterium and its metabolic role is not known. In this study, we assemble the genome of symbiotic bacterium from Haematomyzus elephantis (Rhynchophthirina), demonstrating its close similarity and phylogenetic proximity to several symbionts of sucking lice. The genome is highly reduced (representing the smallest genome among louse-associated symbionts) and exhibits a significant loss of metabolic pathways. However, similar to other sucking louse symbionts, it retains essential pathways for the synthesis of several B vitamins.

• Keywords
• endosymbionts, genomics, lice,
• MeSH
• Bacteria * genetics   classification   isolation & purification   MeSH
• Phylogeny MeSH
• Genome, Bacterial * MeSH
• Phthiraptera * microbiology   MeSH
• RNA, Ribosomal, 16S genetics   MeSH
• Symbiosis * MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• RNA, Ribosomal, 16S MeSH

Interactions between a host organism and its associated microbiota, including symbiotic bacteria, play a crucial role in host adaptation to changing environmental conditions. Antarctica provides a unique environment for the establishment and maintenance of symbiotic relationships. One of the most extensively studied symbiotic bacteria in invertebrates is Wolbachia pipientis, which is associated with a wide variety of invertebrates. Wolbachia is known for manipulating host reproduction and having obligate or facultative mutualistic relationships with various hosts. However, there is a lack of clear understanding of the prevalence of Wolbachia in terrestrial invertebrates in Antarctica. We present the outcomes of a literature search for information on the occurrence of Wolbachia in each of the major taxonomic groups of terrestrial invertebrates (Acari, Collembola, Diptera, Rotifera, Nematoda, Tardigrada). We also performed profiling of prokaryotes based on three marker genes and Kraken2 in available whole genome sequence data obtained from Antarctic invertebrate samples. We found no reports or molecular evidence of Wolbachia in these invertebrate groups in Antarctica. We discuss possible reasons underlying this apparent absence and suggest opportunities for more targeted future research to confirm bacteria's presence or absence.

• MeSH
• Invertebrates * microbiology   MeSH
• Phylogeny MeSH
• Symbiosis * MeSH
• Wolbachia * genetics   classification   isolation & purification   physiology   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Geographicals
• Antarctic Regions MeSH