Editorial: Frontiers in Wolbachia biology 2023

Wolbachia is likely the most successful endosymbiotic bacteria associated with insects and other arthropods, as well as nematodes (Werren et al., 2008). Over the past several decades, its widespread presence across the vast range of arthropod in the terrestrial ecosystem, as well as its various biological attributes, have led to an explosive development in Wolbachia research. These include the induction of striking reproductive phenotypes, namely cytoplasmic incompatibility (CI), male-killing (MK), feminization (Fem) and parthenogenesis induction (PI) (Kaur et al., 2021); obligatory and conditional beneficial fitness consequences such as nutrient provisioning and resistance to parasites, pathogens and viruses (Hamilton and Perlman, 2013; Pimental et al., 2021); essentiality for host growth, development and survival (Zug and Hammerstein, 2015; Taylor et al., 2005); and others. The biology of Wolbachia is not only a captivating area of basic research that covers host-microbe interactions ranging from cell-biology and physiology to ecology and evolution (Serbus et al., 2008; Sanaei et al., 2021), but it is also an important applied research field that contributes to Wolbachia-mediated control of vector-borne infectious diseases and pests (Iturbe-Ormaetxe et al., 2011; Ross et al., 2019), and Wolbachia-targeted prevention and remedy of filariasis (Taylor et al., 2005; Johnson et al., 2021).

Since the 1st Wolbachia Conference held at Crete, Greece, in 2000, Wolbachia Conferences have biennially provided a very active and important forum for world's researchers working on Wolbachia and other microbial symbionts of arthropods, nematodes etc. The 11th Wolbachia Conference was initially planned to be held on 5th-10th July 2020, but because of the COVID pandemic, it was finally postponed to 11th-16th June 2023 and held so successfully. In collaboration with the 11th Wolbachia Conference, the Research Topic “Frontiers in Wolbachia Biology 2023” was launched to provide a forum to overview achievements recently emerging in this research field. In total, 12 original research articles and two review articles are compiled in the Research Topic, which cover a variety of topics regarding the Wolbachia biology.

The reproductive manipulations induced by Wolbachia infection are among the focal Research Topics of the Wolbachia biology. While several (putative) effector proteins responsible for CI, MK and PI phenotypes have been uncovered for several insect systems including fruit flies, mosquitoes, moths and wasps (Beckmann et al., 2017; LePage et al., 2017; Perlmutter et al., 2019; Katsuma et al., 2022; Arai et al., 2023; Fricke and Lindsay, 2024), considering the diversity of the effector molecules (Cifs, Oscar, Wmk, Pifs, etc.), more extensive survey is needed for understanding the diversity and commonality of the Wolbachia-induced reproductive phenotypes. Pramono et al. reported a CI-inducing Wolbachia genome from a leaf-mining pest fly Liriomyza trifolii. Grève et al. reported the genomes of three feminizing Wolbachia strains from the pill bug Armadillidium vulgare. Beckmann et al. attempted a modeling approach using an evolutionary algorithm as to how CI-inducing and rescuing proteins evolve. These studies provide basic information toward our better understanding of the biology of Wolbachia and its intricate reproductive phenotypes.

Wolbachia-mediated control measures against mosquito-borne diseases comprise the recent focal topic in this research field, which are mainly based on Wolbachia-mediated suppression of pathogen infections and CI-driven spread of Wolbachia infection into host populations (Iturbe-Ormaetxe et al., 2011; Ross et al., 2019). Tafesh-Edwards et al. investigated innate immune responses of the fruit fly Drosophila melanogaster after Zika virus infection. They found that some immune-related genes such as drosocin and puckered are upregulated in a female specific manner, whereas the activity of RNA interference and Toll signaling remain unaffected. Minwuyelet et al. reviewed previous studies on utilization of Wolbachia for reducing the transmission of mosquito-borne diseases. Guo et al. also overviewed the Wolbachia-mediated technologies for suppressing the transmission of mosquito-borne pathogens, and argued application of the technologies to control of rice pest planthoppers.

Wolbachia is difficult to culture ex vivo. Thus far, several Wolbachia culture systems have been developed, where the fastidious endosymbiotic bacteria can be maintained only when co-cultured with insect cell lines (Masson and Lemaitre, 2020). On the other hand, it was reported that Wolbachia purified from insect cells could be maintained in cell-free culture media for at least 1 week without loss of viability or infectivity (Rasgon et al., 2006). Here Behrmann et al. monitored the proliferation of a Wolbachia isolate from the mosquito Aedes albopictus in a host cell-free in vitro culture system by quantitative PCR. By supplementing a mosquito cell membrane fraction and fetal bovine serum, extracellular Wolbachia replication for up to 12 days was detected. Notably, even after the ex vivo maintenance for 12 days, the Wolbachia cells could establish infection to a fresh mosquito cell line, suggesting the possibility that Wolbachia might be amenable to some experimental or genetic manipulations using such cell-free culture systems.

Population dynamics of Wolbachia within host cells and tissues is important for understanding phenotypic consequences of the symbiont infection such as fitness effects, intensity of reproductive manipulation, level of reproductive performance, and others (López-Madrigal and Duarte, 2019). Sharmin et al. attempted to elucidate the molecular and cellular mechanisms involved in regulation of intra-host Wolbachia titer by adopting chemical and genetic approaches using Drosophila fruit flies. In total, 37 chemical inhibitors targeting 14 host cellular/molecular processes, which were reported to affect intracellular bacterial abundance in previous studies, were administrated to D. melanogaster and D. simulans, and examined for their effects on Wolbachia titers. Finally, 5 compounds were identified to significantly increase the intra-host Wolbachia titers, which were associated with host Imd signaling, Calcium signaling, Ras/mTOR signaling, and Wnt signaling functions, suggesting that these host mechanisms may negatively regulate the Wolbachia titers. By making use of ample molecular genetic tools available for D. melanogaster, genetic disruption assays confirmed that disruption of Wnt and mTOR pathways upregulates the Wolbachia titers, uncovering that interactions of Wnt and mTOR pathways with autophagy may underlie the negative regulation over Wolbachia population. Poulain et al. reported detailed population dynamics of the Wolbachia strain wCle associated with the common bedbug Cimex lecturalius. In the bedbug, the Wolbachia cells densely and endocellularly populate the well-developed symbiotic organs, called the bacteriomes, where the specific Wolbachia strain synthesizes B vitamins that are deficient in host's blood meal (Hosokawa et al., 2010; Nikoh et al., 2014). In this context, the population dynamics data of wCle in the bedbug will provide insight into how the exceptional Wolbachia strain that turned into an obligatory nutritional mutualist contributes to survival and proliferation in the life cycle of the blood-sucking insect pest that is recently re-emerging worldwide (Doggett and Lee, 2023). Giordano et al. investigated reproductive performance of the soybean aphid Aphis glycines infected with or without the facultative symbionts Arsenophonus and/or Wolbachia under different aphid and soybean genotypes. These studies highlight how Wolbachia population is controlled and integrated into the endosymbiotic system in cellular, physiological and ecological contexts.

Since Wolbachia is indispensable for survival and reproduction of filarial nematodes, Wolbachia-targeting drugs are regarded as promising for medical prevention and remedy of filariasis (Johnson et al., 2021). Hegde et al. reported that an azaquinazoline anti-Wolbachia agent, AWZ1066S, facilitates the sterilizing and curing effects of benzimidazole in an experimental model of filariasis. Wangwiwatsin et al. reported in silico screening of co-evolving protein sequences between host filarial nematodes and their Wolbachia endosymbionts, which identified candidate “hub” genes that may connect multiple host-symbiont interactions and thus may provide potential drug targets via disruption of host-Wolbachia interactions.

For genomic and transcriptomic studies, the preparation of Wolbachia-derived DNA/RNA usually suffers heavy contamination of host-derived DNA/RNA due to the endosymbiotic nature of Wolbachia. Cantin, Dunning Hotopp et al. reported an improved procedure for metagenomic assembly of Wolbachia genome through selective enrichment of bacterial DNA from nematode host DNA using ATAC-seq technique. Cantin, Gregory et al. also reported an improved procedure for dual RNA-seq in filarial nematodes and Wolbachia endosymbionts using RNase H based ribosomal RNA depletion. These technical improvements may be useful for Wolbachia studies of not only nematodes but also insects and other arthropods.

In conclusion, the Research Topic provides a valuable overview of the recent research progress in the field of Wolbachia biology. We hope that this Research Topic shows future directions of this research field, which will be manifested in the forthcoming 12th Wolbachia Conference to be held at Okinawa, Japan, from 13th to 19th April 2025 (https://web.tuat.ac.jp/~insect/wolbachia2025/).

Funding Statement

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. TF was funded by the Japan Science and Technology Agency (JST) ERATO grant no. JPMJER1902.

Author contributions

TF: Writing – original draft, Writing – review & editing. YG: Writing – review & editing. GT: Writing – review & editing. EM: Writing – review & editing. SP: Writing – review & editing. DB: Writing – review & editing. KJ: Writing – review & editing. MT: Writing – review & editing.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

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