Genome sequencing and comparative analysis of Wolbachia strain wAlbA reveals Wolbachia-associated plasmids are common

Julien Martinez; Thomas H. Ant; Shivan M. Murdochy; Lily Tong; Ana da Silva Filipe; Steven P. Sinkins

doi:10.1371/journal.pgen.1010406

Abstract

Wolbachia are widespread maternally-transmitted bacteria of arthropods that often spread by manipulating their host’s reproduction through cytoplasmic incompatibility (CI). Their invasive potential is currently being harnessed in field trials aiming to control mosquito-borne diseases. Wolbachia genomes commonly harbour prophage regions encoding the cif genes which confer their ability to induce CI. Recently, a plasmid-like element was discovered in wPip, a Wolbachia strain infecting Culex mosquitoes; however, it is unclear how common such extra-chromosomal elements are in Wolbachia. Here we sequenced the complete genome of wAlbA, a strain of the symbiont found in Aedes albopictus, after eliminating the co-infecting and higher density wAlbB strain that previously made sequencing of wAlbA challenging. We show that wAlbA is associated with two new plasmids and identified additional Wolbachia plasmids and related chromosomal islands in over 20% of publicly available Wolbachia genome datasets. These plasmids encode a variety of accessory genes, including several phage-like DNA packaging genes as well as genes potentially contributing to host-symbiont interactions. In particular, we recovered divergent homologues of the cif genes in both Wolbachia- and Rickettsia-associated plasmids. Our results indicate that plasmids are common in Wolbachia and raise fundamental questions around their role in symbiosis. In addition, our comparative analysis provides useful information for the future development of genetic tools to manipulate and study Wolbachia symbionts.

Author summary

Wolbachia is the most common bacterial symbiont of arthropods, being found in about half of terrestrial species around the globe. It is transmitted from mother to offspring, can spread rapidly by inducing various forms of reproductive parasitism and often provides protection against viral pathogens. These properties are being harnessed by disease control interventions introducing Wolbachia into wild populations of virus-transmitting mosquitoes. Here we sequenced the genome of wAlbA, a strain of the symbiont naturally found in Aedes albopictus mosquitoes and demonstrate that it is associated with two extrachromosomal plasmid elements. We then reanalysed publicly available sequencing data and found that plasmids are much more common in Wolbachia than previously thought. Some of them carry genes potentially important for symbiosis and reproductive manipulation as well as phage-like genes that may allow them to move between symbiont strains. Our findings provide a new framework for studying Wolbachia and will help the future development of genetic tools for manipulating symbiont genomes.

Citation: Martinez J, Ant TH, Murdochy SM, Tong L, da Silva Filipe A, Sinkins SP (2022) Genome sequencing and comparative analysis of Wolbachia strain wAlbA reveals Wolbachia-associated plasmids are common. PLoS Genet 18(9): e1010406. https://doi.org/10.1371/journal.pgen.1010406

Editor: Xavier Didelot, University of Warwick, UNITED KINGDOM

Received: July 4, 2022; Accepted: September 2, 2022; Published: September 19, 2022

Copyright: © 2022 Martinez et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: The raw sequencing reads (accessions: SRR21224253, SRR21224254) and the assembled wAlbA genome and plasmids (accessions: CP101657, CP101658, CP101659) are available from the NCBI Genbank database under Bioproject PRJNA852408. Other plasmid fasta sequences, CI crosses, fecundity, Wolbachia tissue tropism and plasmid copy number data are available from the Enlighten repository at http://dx.doi.org/10.5525/gla.researchdata.1342.

Funding: The study was undertaken using funding from the Wellcome Trust, awards 108508, 202888 to SPS. https://wellcome.org. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Wolbachia is the most abundant heritable bacterium in arthropods, present in around half of the species worldwide [1], as well as being an obligate symbiont of filarial nematodes [2]. It is primarily inherited through the female germline and has evolved various ways to spread through host populations by manipulating arthropod reproduction. Reproductive alterations include several forms of sex-ratio distortions and cytoplasmic incompatibility (CI), a type of selective sterility providing a reproductive advantage to female hosts carrying the symbiont [3]. Many Wolbachia strains are also capable of inhibiting viral replication [4–7] and this phenotype, combined with the self-spreading mechanism of CI, allowed the development of novel strategies for controlling mosquito-borne diseases [8–10].

Wolbachia is found exclusively within the host cell environment, and this has hampered the use of genetic tools to manipulate and study its genome at the mechanistic level. Nevertheless, genome research has led to considerable progress in understanding Wolbachia biology [11–15]. Wolbachia is commonly associated with prophage WO, a temperate bacteriophage integrated as a prophage region into the symbiont’s chromosome. Prophage WO has an important role in Wolbachia‘s evolution by allowing the transfer of genetic material between symbiont genomes [16]. Prophage WO also carry an accessory region called the Eukaryotic Association Module (EAM) which encodes a variety of genes that are eukaryotic-like in length, origin and/or predicted function such as ankyrin domain-containing proteins. Moreover, the EAM often carries the two syntenic genes cifA and cifB responsible for the CI phenotype [17–19]. While the mobility of phage WO has contributed to horizontal gene transfer over long evolutionary periods, evidence of active phage replication and production of virus particles remain limited to a few examples [19–22]. In addition, many prophage regions display signs of degradation through pseudogenization of core phage genes and chromosomal rearrangements [16].

Recently, a mobile genetic element was discovered in wPip, a strain of Wolbachia infecting Culex pipiens mosquitoes [23]. The 9,228 bp extrachromosomal and circular element bears the hallmarks of a candidate plasmid and was therefore named pWCP for “plasmid of Wolbachia endosymbiont in C. pipiens”. Since this discovery, no other Wolbachia-associated plasmids has been described and it is unclear how common these elements are and whether they play an important role in Wolbachia-host interactions.

There is abundant literature highlighting the contribution of plasmids in the horizontal transfer of adaptive traits such as antibiotic resistance and virulence in free-living bacteria [24–26]. Plasmids have also been described in arthropod symbionts and they often carry genes potentially involved in symbiosis. For instance, ankyrin repeat-containing and toxin-like genes are commonly found in the plasmids of Cardinium [27,28], Spiroplasma [29,30] and Rickettsia symbionts [31]. In Spiroplasma strain MSRO infecting Drosophila melanogaster, a male-killing phenotype is mediated by the spaid toxin encoded on plasmid pSMSRO [32]. Homologues of the Wolbachia cif genes have also been identified on plasmid pLbaR in Rickettsia felis; the presence of R. felis in Liposcelis bostrichophila correlates with parthenogenesis [33,34].

The invasive ‘Asian tiger’ mosquito Aedes albopictus, a vector of arboviruses such as dengue, chikungunya and Zika, harbours two co-infecting strains of Wolbachia called wAlbA and wAlbB. wAlbA has a lower overall density and in females is largely restricted to the ovaries, while wAlbB has a somewhat wider tissue distribution [35,36], a pattern that is also observed in other doubly-infected systems [37,38]. Following transfer into the naturally Wolbachia-free Ae. aegypti, much wider tissue distribution occurs for both strains in this novel host, but their relative density was reversed; however, despite its lower density wAlbB was a much more efficient inhibitor of arbovirus transmission in Ae. aegypti than wAlbA [6]. Subsequently wAlbB has been deployed in Malaysia for dengue control [8]. Several genomes of wAlbB have been sequenced [15,39,40], but due to the technical difficulties associated with its lower density and co-presence of wAlbB, no wAlbA genome has been reported to date.

Here we report the complete genome of Wolbachia strain wAlbA and two new associated plasmids, together with a number of similar plasmids and chromosomal islands in existing Wolbachia genome assemblies.

Results

wAlbA harbours prophage regions and multiple pairs of cif genes

In order to sequence the wAlbA genome, we generated a singly-infected Ae. albopictus line by eliminating the co-infecting strain wAlbB (see methods). We found that wAlbA is most abundant in the ovaries and its tissue distribution is not affected by the presence of wAlbB (S1A Fig). In addition, wAlbA does not affect the fecundity of female mosquitoes and is able to induce CI (S1B and S1C Fig). However, wAlbA is unable to rescue CI in crosses with doubly-infected males, suggesting that wAlbA and wAlbB carry incompatible pairs of cif genes (S1C Fig).

We assembled a complete 1,190,930 bp Wolbachia genome from wAlbA-infected ovaries (circularity determined by the Unicycler pipeline, see methods, Fig 1A) which is similar in size and gene number to other complete Wolbachia genomes (S1 Table). wAlbA belongs to the arthropod-specific Wolbachia supergroup A (Fig 2) and harbours a large WO prophage region (47,475 bp, Fig 1A) comprising a complete set of structural and non-structural gene modules thought to be essential for the production of phage particles (S2 Table). However, this region displays a sequencing depth similar to the rest of the chromosome, suggesting no active replication of WO phage (Fig 1A). We also found smaller WO phage islands containing core phage genes, often with signs of pseudogenization, and accessory genes commonly found in the phage eukaryotic association module (Fig 1A and S2 Table). This includes three nearly identical pairs of the cytoplasmic incompatibility genes cifA and cifB, all harbouring a cifB deubiquitinase domain that is characteristic of Type I cif homologues [33].

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Fig 1. Genome map of the wAlbA chromosome and plasmids.

(A) wAlbA chromosome. Two outer circles: protein-coding genes on forward and reverse strands (black). Third circle: WO prophage regions (yellow: phage core genes, red & blue: phage eukaryotic modules). Inner grey circle: Illumina sequencing depth per 2,000 bp window from a non-WGA mosquito sample (see methods). (B) wAlbA plasmids and predicted coding sequences.

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Fig 2. Wolbachia strain phylogeny and distribution of plasmid-like elements.

Maximum Likelihood phylogeny based on the concatenated nucleotide alignment of 36 Wolbachia core genes using the GTR GAMMA substitution model. Branch support was assessed with 1,000 bootstrap replicates. Capital letters indicate Wolbachia supergroups. Circles indicate strains in which plasmid-like elements were identified.

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wAlbA is associated with two plasmid-like elements

In addition to the wAlbA genome, we assembled two circular extrachromosomal elements, 18,803 and 16,844 bp in size, that show strong similarities to pWCP, the Wolbachia-associated plasmid found in C. pipiens (Fig 1B). Based on the comparative analysis below, we named these plasmids pWALBA1 and pWALBA2 (for plasmids of Wolbachia endosymbiont wAlbA 1 and 2). Sequencing depth was evenly distributed along the two plasmid genomes with only three low-frequency single nucleotide polymorphisms detected, indicating near complete plasmid clonality within our wAlbA-infected mosquito line (S2 Fig). Using specific PCR primers, we confirmed the association of the two plasmids with wAlbA, whereas no amplification was observed in the Ae. albopictus Aa23 cell line infected with wAlbB only (S3 Fig). pWALBA1 showed a higher copy number per Wolbachia cell than pWALBA2, although pWALBA2 copies tended to increase in older female mosquitoes (S4A Fig). Importantly, the relative amounts of the two plasmids per mosquito were strongly correlated with wAlbA density, further supporting their association with the symbiont strain (S4B Fig).

Similar to pWCP, both wAlbA plasmids encode a ParA-like plasmid partitioning gene (HHpred probability > 99%), a hypothetical protein with strong similarity to a ParG-like plasmid partition gene and other DNA-binding proteins found on extra-chromosomal elements (e.g. phage transcriptional Arc repressor, HHpred p > 99%), a hypothetical protein (HP1), a RelB/E toxin-antitoxin addiction module (HHpred p > 98%) as well as one or two transposases (Fig 1B, S3 Table). pWALBA1 shares additional genes with pWCP, namely a DnaB-like helicase (HHpred p = 99.96%) and a TerL-like phage terminase large subunit (HHpred p = 100%), but contrary to pWCP, these two genes do not show sign of pseudogenization. pWALBA1 encodes other phage-like proteins, namely a portal protein (HHpred p = 100%), a putative phage stabilization protein (HHpred p = 99.44%) and a hypothetical protein with weak homologies to a phage coat protein (HHpred p = 64.73%, S3 Table). Other pWALBA1 genes include a putative QueT-like queuosine transporter (HHpred p = 95.17%), a RuvC-like resolvase (HHpred p = 98.17%) and several hypothetical proteins absent in pWCP. We did not find phage-like genes in pWALBA2 but instead genes encoding a Fic/DOC family protein (HHpred p = 100%), a RuvB-like helicase (HHpred p = 99.02%) and other proteins with no predicted functions, some of which carry ankyrin or tetratricopeptide (TPR) repeat domains (Fig 1B and S3 Table).

Plasmids and related chromosomal islands are widespread in Wolbachia

In order to investigate how common plasmid-like elements are in Wolbachia, we conducted TBLASTN searches of pWALBA1 and pWALBA2 coding sequences in publicly available Wolbachia genomes. We found plasmid-like regions with similar gene organization to wAlbA plasmids in 47 out of 189 Wolbachia assemblies (~20% of the strains, several assemblies for some strains) and none were found in strains from nematodes (S4 Table). Their distribution is patchy across the Wolbachia phylogeny, suggesting that plasmids are often horizontally-transferred between symbiont strains and tend to be lost over long evolutionary timeframes (Fig 2). However, it is possible that plasmid sequences are missing from some Wolbachia assemblies. Indeed, filtering out contigs from final assemblies based on a lack of homology with known Wolbachia sequences or due to large differences in sequencing depth is common practice. For instance, not all draft assemblies of the Wolbachia infecting Diabrotica virigifera virgifera contained plasmid-like sequences (S4 Table). Moreover, we found a plasmid-like contig in several isolates of strain wDi [41] that was absent in other published wDi assemblies [42]. However, by re-assembling the raw reads from the latter study, we retrieved the corresponding circular plasmid genome that had not been reported (isolate wDi_KPSwDI05P26). We circularized additional plasmid-like elements from the wCon strain (SRA accession: SRR7213553), from a Wolbachia-infected sample of the Diabrotica virgifera virgifera beetle (SRA accession: SRR1106544) and from a sample labelled Insecta_WOLB1166 (mixture of Wolbachia-infected insect species, SRA accession: SRR748268). All circular plasmids show higher sequencing depth relative to their respective Wolbachia genome, ranging from ~4 to 10x, suggesting that they are maintained as multiple copies within bacterial cells (Fig 3). In some cases, copy number could not be estimated due to raw sequencing data being unavailable (wDacA) or to the presence of several Wolbachia strains in the same sample as indicated by multiple peaks in the sequencing depth distribution (Insecta_WOLB1166, Dactylopius coccus WOLB1009 and Megaselia abdita WOLB1013).

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Fig 3. Relative sequencing depth of Wolbachia-associated plasmid-like elements.

Each dot indicates the mean sequencing depth in 500 bp windows divided by the mean chromosome depth.

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Some linear contigs could not be circularized but show a similar sequencing depth relative to other Wolbachia contigs and could be single-copy plasmids or be part of the bacterial chromosome (e.g. in Wolbachia of Diachasma alloeum WOLB1001, Fig 3). In support of the latter hypothesis, we found several genomic regions similar to pWALBA1 integrated on the Wolbachia chromosome (Fig 4). These chromosomal islands include a twelve gene region previously named “Dozen island” present in Wolbachia strains from Drosophila teissieri, santomea and yakuba that encompasses the variable region of pWALBA1-like plasmid genomes [43]. In most cases, these islands are flanked by transposable elements and show varying degrees of chromosomal rearrangements and degradation with genes carrying premature stop codons or being truncated by transposase sequences (Fig 4). An interesting observation is that plasmid-like islands in strains wOegib-WalA and wOegib-WalB found in O. gibbosus spiders are located next to WO prophage regions (Fig 4).

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Fig 4. Gene organisation and comparison of pWALBA1-like sequences.

Similarity is indicated by gene colours (TBLASTN) and by the grey areas between sequences (BLASTN) where darker grey means more similar.

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Other circular plasmid sequences, including pWALBA2, were found to share the same set of core genes with the plasmids described above (ParA-like, ParG-like, HP1, RelB/E) while encoding different genes in their accessory region (Fig 5). These include a Fic/DOC family protein and other hypothetical proteins carrying ankyrin, PD-(D/E)XK nuclease domains and RDD (repeated D domain). PD-(D/E)XK nucleases are a large and diverse family of proteins including restriction endonucleases, Holliday junction resolvases, transposases, and DNA repair enzymes [44]. The two plasmid-encoded nucleases in D. virgifera virgifera are unrelated to cifB genes which also carry PD-(D/E)XK domains, instead they are homologues of nucleases found in some WO phage head modules, the function of which is unknown [45]. In addition, the plasmid found in the Insecta_WOLB1166 sample encodes a cifA/B gene pair and the one from D. virgifera virgifera a cifB homologue lacking the 5’ end of the gene (Fig 5). In both cases, cifB harbours an ankyrin-repeat region as well as a C-terminal latrotoxin domain which was only found previously in divergent Type V cifB homologues (Fig 5) [33].

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Fig 5. Gene organisation and comparison of other plasmid-like sequences.

Similarity is indicated by gene colours (TBLASTN) and by the grey areas between sequences (BLASTN) where darker grey means more similar.

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Plasmid-like sequences were also identified in draft genome assemblies of several species of spider (Trichonephila clavata, T. clavipes, Araneus ventricosus, Nephila pilipes, Oedothorax gibbosus) and one in the parasitic wasp Cotesia glomerata (S4 Table and Figs 4 and 5). Most of these sequences are made of a short contig, with no eukaryotic-like genes and could be Wolbachia-associated plasmids or chromosomal islands that were not filtered out from the host genome assembly. For instance, O. gibbosus is known to host two Wolbachia strains [46]. Nevertheless, two large spider contigs (A. ventricosus contig BGPR01000016.1, T. clavipes contig BMAU01021280.1) only showed homologies to Wolbachia in the region overlapping the plasmid-like sequences and could be cases of Wolbachia-to-spider horizontal transfers (S5 Fig). However, we observed large variation in sequencing depth in the regions flanking the two putative plasmid inserts and low numbers of long Nanopore reads supporting these insertions (S5 Fig). Therefore, we conclude that these are most likely chimeric contigs generated during the genome assembly of a Wolbachia-infected sample.

Pervasive gene flow between Wolbachia and plasmid genomes

Phylogenetic analysis of the genes found in Wolbachia plasmids and chromosomal islands revealed the dynamic nature of lateral gene transfer and recombination between plasmids and the bacterial genome. Genes located on chromosomal islands do not form a monophyletic group which indicates that plasmid sequences have integrated into Wolbachia genomes more than once (S6–S9 Figs). The phylogenetic relationships of genes in the Dozen island found in the wTei-wYak-wSan clade and other closely-related Wolbachia strains suggest, however, a single origin of this island that has most likely codiverged with the bacterial chromosome following its integration. Genes of the Dozen island are on average more similar to homologues found in pWALBA1, while genes located on other islands such as in wLug and wStri tend to be more closely-related to the wCon plasmid (S6–S8 Figs). The opposite pattern was found for the putative coat protein where the wCon homologue is more similar to its counterparts located within the Dozen island suggesting a possible recombination event. Another case of a probable recombination is observed between pWALBA1 and pWALBA2 since they carry divergent ParA-like plasmid partition genes while their genes encoding HP1 are closest relatives (S6 Fig).

Several plasmid genes also have closely-related homologues on the bacterial chromosome located outside of plasmid-like islands, although we cannot rule out that they are island remnants. In the case of ParA-like homologues, chromosomal versions of the gene form a separate clade and, interestingly, they are all fused with the downstream ParG-like gene, as opposed to plasmid-encoded homologues (S6 Fig). For other genes, there is evidence of multiple horizontal transfers between plasmids and Wolbachia genomes, including with WO prophage regions. RelB/E addiction modules are widespread in Wolbachia genomes [47] and their phylogenetic distribution does not suggest single origins for chromosomal, WO phage and plasmid homologues but rather multiple transfers between the different genomic entities (S10 Fig). Similarly, the two PD-(D/E)XK nucleases located on the Wolbachia plasmid in D. virgifera virgifera are related to different lineages of WO phage nucleases (S9 Fig). Finally, the plasmid Fic/DOC family proteins of pWALBA2 and D. virgifera virgifera group with two different clades of chromosomal Fic proteins commonly found in Wolbachia genomes, again suggesting multiple gene transfers between plasmids and Wolbachia genomes [47] (S9 Fig).

Are plasmids vectors of cif genes across the Rickettsiales?

The cif genes are most of the time associated with WO prophage regions in Wolbachia. Previous studies have delimited multiple cif clades denominated as Type I to IV and a proposed Type V comprising homologues carrying additional domains on the C-terminal part of the cifB protein. cif homologues have also been reported in other Rickettsiales, some of them carried by plasmids [33,34,48,49]. We investigated the evolutionary origin of the cif genes located on Wolbachia-associated plasmid sequences and found that they group with Type V homologues (Fig 6), which is consistent with the length and predicted domains of their cifB protein (Fig 5). Unlike Type I-IV homologues, Type V cif genes are composed of several divergent clades with both Wolbachia and non-Wolbachia homologues and may not form a single monophyletic group (Fig 6). While the exact position of the tree root is currently unknown, we found evidence for several horizontal transfers of Type V cif genes between Wolbachia and other arthropod symbionts (Fig 6). Interestingly, the cif genes found in Wolbachia- and Rickettsia-associated plasmids do not cluster together suggesting independent acquisitions of cif genes by plasmids. In addition, we often found genes of the conjugation machinery in the regions flanking non-Wolbachia cif genes. In the chromosome of Orientia tsutsugamushi and the Rickettsia symbiont Oegib-Wal of the spider O. gibbosus, the cif genes are located next to an Integrative Conjugal Element (ICE) named Rickettsiales amplified genetic element (RAGE), a type of mobile genetic element commonly found in Rickettsia genomes and plasmids [31,50] (Figs 6 and S11). Other non-Wolbachia cif genes were located on short contigs but we often found genes of the conjugative machinery in the flanking regions suggesting that they are also associated with a chromosomal ICE or a conjugative plasmid (Fig 6).

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Fig 6. Cif gene phylogeny.

Maximum Likelihood phylogeny based on the nucleotide alignment of cifA (left) and their cognate cifB genes (right) using the GTR GAMMA substitution model. The tree is midpoint rooted. Previously defined cif Types I-IV are labelled and colour-shaded (blue/yellow/green/purple). Orange shading indicate Type V-like homologues found outside Wolbachia symbionts. Wolbachia homologue labels are in bold. Bootstrap values were estimated from 100 replicates.

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Discussion

Until the recent discovery of the pWCP element [23], it was assumed that plasmids were either absent or extremely rare in Wolbachia. Baião et al. (2021) later speculated that a putative plasmid was present in wCon and that the Dozen island might stem from an integrated plasmid. Our study shows that these elements have been largely overlooked, likely as a consequence of them being discarded as contaminants or assumed to be part of the bacterial chromosome in Wolbachia sequencing projects. Our characterization of plasmids associated with wAlbA and other Wolbachia strains will facilitate the identification of related elements in future studies and should encourage the reanalysis of raw sequencing data available from public repositories. In particular, the new sequences we identified could be used in more in-depth analyses aiming to detect plasmids by recruiting unassembled sequencing reads from genome databases.

In wAlbA, pWALBA1 and pWALBA2 were maintained as multiple copies per Wolbachia cell, however, pWALBA1 copy number was relatively stable while pWALBA2 copy number increased with the age of female mosquitoes. Sequencing depth analysis indicated that other Wolbachia-associated plasmids also tend to be maintained as multi-copy plasmids, similar to what was observed in [23]. We found large variation in size among the complete circular plasmid genomes, ranging from ~9,000 to 21,000 bp, which is primarily driven by differences in the presence/absence and pseudogenization of accessory genes. Nevertheless, all plasmids appear to share a set of core genes likely to be essential to their maintenance. The ParA-like and downstream ParG-like genes have strong structural similarities with proteins involved in plasmid partition systems that ensure plasmid inheritance during cell division [51]. ParA family proteins are ATPases that drive the segregation of newly replicated plasmids into daughter cells [52] while ParG is known to act as a centromere-binding protein modulating the action of its cognate ATPase [53]. All plasmids also carry one or several RelB/E addiction modules which are Type II toxin-antitoxin (TA) systems that can promote plasmid maintenance through post-segregational killing of plasmid-free daughter cells [54]. RelB/E modules are also widespread in bacterial chromosomes, including in Wolbachia within and outside of prophage regions [47,55]. The role of chromosomal TA systems is still being debated but they have been implicated in various functions such as stress-response [56], anti-phage defences [57] and may also prevent the loss of large genomic regions under fluctuating selection regimes [58]. RelB/E modules may also accumulate as selfish DNA in bacterial chromosomes without providing fitness benefits due to their addictive nature [55].

A striking feature of some of the Wolbachia plasmids is the presence of several phage-like genes raising the possibility that they produce virus particles and could be transmitted horizontally via transduction. The two DNA packaging genes encoding the terminase large subunit and portal protein may allow plasmid genomes to be incorporated into viral capsids, while the presence of a phage stabilization and a putative coat protein indicate that plasmids might encode their own capsid structural proteins. We did not detect phage tail genes; however, such genes could be among the hypothetical proteins present on the plasmids or tail-less virus particles could be produced instead. Another hypothesis is that plasmids may highjack the tail proteins of WO phage, although this is perhaps unlikely since plasmid-encoded DNA packaging and structural phage-like proteins are not closely-related to WO homologues. It is however interesting to note that several plasmid genes have homologues in WO prophage regions (addiction modules, cif genes, non-cif PD-(D/E)XK nuclease) and that in two Wolbachia genomes, plasmid-like islands are located next to phage regions suggesting that plasmids may sometimes replicate and/or be packaged along with WO phage genomes. The existence of extrachromosomal phages, maintained as circular plasmids has been known for decades [59–61] and a recent computational analysis revealed that such phage-plasmids are widespread in bacteria and encompass genetically diverse groups of mobile genetic elements [62]. Whether Wolbachia-associated plasmids produce phage particles remains to be investigated.

While being genetically related, not all Wolbachia plasmids carry phage-like genes. This could mean that they rely solely on vertical transmission as we did not find that they encode their own conjugative machinery. There is evidence that some plasmids and phage-inducible chromosomal islands can undergo transduction by hijacking and even remodelling the capsid of coinfecting bacteriophages [63–65]. Thus, it is possible that plasmids not carrying phage genes behave like phage satellites by being packaged into virions produced by WO prophage regions or by another coinfecting plasmid such as pWALBA1.

Over long evolutionary times, some Wolbachia-associated plasmids become integrated into the bacterial chromosome, a process that could be facilitated by the presence of many transposase sequences in plasmids and Wolbachia genomes. It also appears that both plasmids and chromosomal islands rapidly go extinct as indicated by their patchy distribution among Wolbachia strains and the abundance of pseudogenes. Their long-term persistence may therefore depend on a positive balance between new acquisitions through horizontal transfers of plasmids and losses through intense pseudogenization prior to or following an integration into the bacterial chromosome. The presence of RelB/E addiction modules in plasmids and islands likely contributes to lower the speed at which they become lost by preventing large deletions and plasmid losses during cell division. Selection may also act to maintain some of the plasmid/island genes if they confer benefits to Wolbachia. In particular, it is of interest that some cif genes are found on plasmids in both Wolbachia and Rickettsia. Although the function of plasmid-encoded cif proteins remains to be tested experimentally, they may provide the ability to induce CI or other reproductive phenotypes such as parthenogenesis [34] and therefore contribute to the spread of the symbiont through host populations. There may even be an advantage for the symbiont in carrying cif genes on multi-copy plasmids rather than in the chromosome if an increase in the expression level of the cif proteins can modulate the CI phenotype and its rescue. Importantly, we showed that beyond phage WO, the cif genes can be associated with other mobile genetic elements like plasmids and integrative conjugal elements which likely contributed to their early evolution and spread across alphaproteobacterial symbionts.

In conclusion, our discovery of new plasmids associated with wAlbA and other Wolbachia strains provides a new framework for studying the role of these mobile genetic elements in Wolbachia. Importantly, our comparative analysis will guide future attempts aiming to develop a genetic tool kit for Wolbachia transformation. In particular, we expect that successful plasmid constructs will likely require some of the core genes we characterized such as those of the plasmid partition system and addiction modules.

Material and methods

Generation of a wAlbA-infected mosquito line

The Ae. albopictus wild type strain used in this study was collected from the Jalan Fletcher area of Kuala Lumpur (JF), Malaysia, in 2017. To generate a wAlbA-single infection, wild-type (wAlbA/wAlbB-carrying) larvae were reared under conditions of heat stress (diurnal cycle: 12hr at 37°C / 12 hr at 32°C). Twenty-two resulting adult females were mated en-masse to males of a previously tetracycline-treated Wolbachia-negative JF line. Eleven of these successfully blood-fed and were individualised for oviposition. Viable egg batches were obtained from 7 females, which were subsequently hatched independently. Resulting male pupae were screened for wAlbA and wAlbB by strain specific PCR primers (S5 Table). Two of the 7 sibling pools contained males with a positive signal for wAlbA and no signal for wAlbB, with the remaining 5 pools giving no signal for either strain. Female siblings from the wAlbA-positive pools were crossed to males of the Wolbachia-negative JF line, blood-fed and individualised for oviposition. An isofemale line carrying only wAlbA was isolated. Females from this line were subsequently outcrossed to Wolbachia-negative males from the JF line for five consecutive generations before a stable self-crossed colony was established.

Wolbachia density was measured in the dissected tissues of 5-day old adult females by quantitative PCR (qPCR) using wAlbA-specific primers (S5 Table) normalised to the host homothorax (HTH) gene using a SYBR Green qPCR master mix (Bimake), with the following conditions: 95°C for 5 mins, followed by 40 cycles of 95°C for 15 secs and 60°C for 60 secs, and subsequent melt curve analysis.

Fecundity of females of the wild-type, wAlbA-only and Wolbachia-negative lines was measured by crossing females of each line to Wolbachia-negative males in pools of 10 males to 5 females, followed by blood-feeding and selection of engorged females and subsequent individualisation of gravid females for oviposition on a damp filter-paper substrate. Egg batch per female was counted manually using a dissection microscope.

Levels of cytoplasmic incompatibility induction and rescue were measured by crossing various combinations of males and females from the wild-type, wAlbA-only and Wolbachia-negative lines in pools of 10 males to 5 females, followed by flood-feeding and individualisation as described above. Female mating status post-oviposition was assessed by dissection of spermathecae and visual confirmation of sperm transfer using an inverted 20x compound microscope. The eggs of females lacking visible sperm in spermathecae were discarded from further analysis. Eggs on filter papers from individual females were counted and dried for 5-days, prior to subsequent hatching in water containing bovine liver powder. L1 larval numbers were counted on emergence.

Wolbachia purification and whole-genome amplification

For Illumina sequencing, wAlbA was purified by dissecting and pooling ovaries from 27 wAlbA-infected Ae. albopictus females. Pooled ovaries were rinsed in PBS three times, resuspended in Schneider’s media and homogenized with a sterile pestle. The homogenate was centrifuged at 2,000 g for 2 min to remove cellular debris and the supernatant was filtered through 5 and 2.7 μm sterile filters. The filtrate was centrifuged at 18,500 g for 15 min. The bacterial pellet was resuspended in Schneider’s media and treated with DNase I at 37°C for 30 min to remove host DNA. Following digestion, the DNase was inactivated at 75°C for 10 min and the sample centrifuged at 18,500 g to discard the supernatant. DNA was amplified directly from the bacterial pellet using the REPLI-G Midi kit (Qiagen). The whole-genome amplified DNA (WGA) was cleaned using the QIAamp DNA mini kit (Qiagen). For Oxford Nanopore Technology (ONT) sequencing, ovaries from 100 females were dissected and pooled for DNA extraction as above except that no Wolbachia purification or whole-genome amplification was conducted (non-WGA). Finally, DNA was extracted from another pool of 23 pairs of ovaries without WGA to conduct an additional run of Illumina sequencing. However, this sample did not allow us to complete the wAlbA genome. Therefore, the corresponding Illumina data was only used for analysing variation in sequencing depth (see below) since reads mapped onto the wAlbA genome and plasmids were more evenly distributed than for the WGA sample due to amplification bias.

Genome sequencing and de novo assembly

A DNA library was prepared from the WGA sample using the Kapa LTP Library Preparation Kit (KAPA Biosystems, Roche7961880001) and sequenced on the Illumina MiSeq platform with the MiSeq Reagent Kit v3 to generate 2×150 bp paired-end reads. The non-WGA sample was used to prepare an ONT library by shearing the DNA into ~8 kb fragments followed by purification and size-selection using AMPure XP beads (Beckman Coulter). The ONT library was then prepared with the Ligation Sequencing Kit (SQK-LSK109) and sequenced for 72 hours using a GridION (ONT) controlled by the MinKNOW software v20.06.9. Illumina and ONT adapters were removed with Trimmomatic v0.38.0 [66] and Porechop v0.2.4 [67] respectively. Mosquito reads were filtered out by mapping the Illumina and ONT reads against the Ae. albopictus reference assembly (Genbank accession: GCF_006496715.1) using Bowtie2 v2.4.2 [68] and Minimap2 v2.23 [69] respectively. Unmapped reads were then assembled using the Unicycler hybrid assembly pipeline which assesses the circularity of assembled replicons based on the presence/absence of an end in the assembly graph structure [70]. Circular genome maps were created in DNAPlotter [71].

Endpoint PCR and quantitative PCR

Genomic DNA was extracted by homogenizing individual female mosquitoes in 200 μL STE buffer with a sterile pestle, followed by a 30 min incubation at 65°C with 2 μL of Proteinase K (20 mg/mL) and a final incubation for 10 min at 95°C. The extracted DNA was diluted 1/5 in water and samples were centrifuged at 2,000 g for 2 min before PCR. For each PCR reaction, 2 μL of DNA template was amplified using the 2x Taq master mix (Vazyme) in a 25 μL reaction: 12.5 μL of master mix, 12.5 μL of water, 1 μL of each 10 μM primer (S5 Table) and the following PCR cycles: 95°C for 3 min, 35 cycles of 15s denaturation at 95°C, 15s for primer annealing (see temperatures in S5 Table), 1 min extension at 72°C and a 5 min final extension step at 72°C. Wolbachia density relative to host DNA and plasmid copy number were measured by qPCR using the QuantiNova SYBR Green PCR kit (Qiagen) in 10 μL reactions: 5 μL of master mix, 2 μL of water, 0.5 μL of each 5 μM primer (S5 Table) and the following cycles: 95°C for 15 min, 40× cycles of 95°C for 15 s and 60°C for 20 s, followed by a melt-curve analysis.

Search of plasmid sequences in publicly available sequencing data

The presence of plasmid sequences in publicly available Wolbachia genomes was searched with TBLASTN [72] using amino acid sequences of all pWALBA1 and pWALBA2 genes. Default parameters and an e-value threshold of 0.05 were used. TBLASTN hits were then visually inspected in Artemis [73]. When raw sequencing data was available from the Sequence Read Archive (https://www.ncbi.nlm.nih.gov/sra), we attempted to circularize plasmid-like sequences by conducting de novo assemblies using Unicycler. In the case of the wCon plasmid, circularization was achieved by visualizing the final assembly graph in Bandage [74] and removing an ambiguous path that linked the plasmid to a short multicopy transposase sequence also present in the Wolbachia genome (40x sequencing depth relative to the bacterial chromosome). Circular plasmid sequences were then manually rotated to start from the ParA-like plasmid partition gene. pWALBA1 and pWALBA2 amino acid sequences were also individually used as queries in the BLASTP online tool (https://blast.ncbi.nlm.nih.gov/Blast.cgi; last accessed January, 2022) to find additional plasmid-like sequences in genome assemblies not annotated as Wolbachia. Homologues of the cif genes in Wolbachia and other Rickettsiales genomes were also detected using TBLASTN and representative sequences of the five cif types as explained in [33].

In order to investigate gene function and synteny conservation between plasmid sequences, all sequences were reannotated using the RAST annotation pipeline [75] and putative pseudogenes were manually corrected in Artemis. The function of the predicted coding sequences was also inferred from BLASTP searches and using the HHpred webserver [76] against the following databases: SCOPe70 (v2.07), Pfam (v35), SMART (v6.0), and COG/KOG (v1.0). Annotated plasmid sequences were then plotted side-by-side along with BLASTN similarities using the R package GenoPlotR [77].

Finally, plasmid copy number relative to the Wolbachia genome was estimated by mapping the raw sequencing reads onto their respective genome assembly using Bowtie2 v2.4.2 [68] and extracting the sequencing depth values at each position with the Samtools depth command [78]. Sequencing depth per 500 bp windows was calculated and visualized in the R software with a custom script [79]. A more detailed analysis of sequencing depth and clonality of the wAlbA plasmids was also carried out using anvi’o v7.1 [80].

Phylogenetic analysis

The Wolbachia phylogeny was generated with RAxML v7.7.6 [81] based on the concatenated nucleotide alignment of 36 core genes selected from a set of orthologous single-copy genes described in [82]. For the plasmid and cif genes, nucleotide sequences were aligned with MAFFT using the codon-aware tool of the TranslatorX webserver [83] after manually removing premature stop codons from pseudogene sequences. Poorly aligned regions were then trimmed from the alignments using trimAl v1.3 with the–automated1 method [84]. PhyML v3.0 [85] was used to build gene phylogenies with the GTR GAMMA substitution model and 100 bootstrap replicates. Trees were annotated in the iTOL online tool [86] and the cophylo function from the phytools R package [87] for the plasmid and cif genes respectively.

Supporting information

S1 Fig. wAlbA tissue tropism and fitness effects.

(A) wAlbA relative density within tissues of individual females. (B) Number of eggs laid by individual females. (C) Egg hatch rates in the progeny of individual females. Blue, red and grey indicate doubly-infected, wAlbA-infected and Wolbachia-free mosquito lines respectively. NS: non-significant; *: p < 0.05; **: p < 0.01; ***: p < 0.001.

https://doi.org/10.1371/journal.pgen.1010406.s001

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S2 Fig. Sequencing depth and clonality analysis of wAlbA plasmid genomes.

https://doi.org/10.1371/journal.pgen.1010406.s002

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S3 Fig. Endpoint PCR targeting wAlbA and its associated plasmids.

DNA was extracted from individual female mosquitoes and from the Aa23 cell line (n = 4 per Wolbachia infection status). Nomenclature: host background-Wolbachia infection status. NC: PCR negative control. L: DNA ladder.

https://doi.org/10.1371/journal.pgen.1010406.s003

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S4 Fig. Quantitative PCR analysis of wAlbA plasmid copy numbers.

DNA was extracted from individual wAlbA-infected female mosquitoes at different timepoints. Females were blood-fed at 8 and 18 day-old. (A) Copy number of a plasmid-specific target (DnaB-like gene and Fic family protein for pWALBA1 and pWALBA2 respectively) relative to Wolbachia 16S rRNA copies. The p-values were calculated with a t test on paired log-transformed data. (B) Correlation between wAlbA densities and plasmid copy number per mosquito (blue: pWALBA1, red: pWALBA2). The dashed lines show predicted values from linear regressions and r is the Pearson’s correlation coefficient.

https://doi.org/10.1371/journal.pgen.1010406.s004

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S5 Fig. Putative chimeric spider contigs.

A. araneus (A) and T. clavipes (B) contigs were blasted against their respective plasmid-like region, the wMel (Supergroup A) and wAlbB (Supergroup B) reference genomes. BLASTN hits were visualized in Bandage. Sequencing depth was measured by mapping the Illumina and Nanopore reads from the corresponding sample onto the contig of interest.

https://doi.org/10.1371/journal.pgen.1010406.s005

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