Supergroup F Wolbachia with extremely reduced genome: transition to obligate insect symbionts.

BACKGROUND: Wolbachia belong to highly abundant bacteria which are frequently found in invertebrate microbiomes and manifest by a broad spectrum of lifestyles from parasitism to mutualism. Wolbachia supergroup F is a particularly interesting clade as it gave rise to symbionts of both arthropods and nematodes, and some of its members are obligate mutualists. Investigations on evolutionary transitions among the different symbiotic stages have been hampered by a lack of the known diversity and genomic data for the supergroup F members. RESULTS: Based on amplicon screening, short- and long-read WGS approaches, and laser confocal microscopy, we characterize five new supergroup F Wolbachia strains from four chewing lice species. These strains reached different evolutionary stages and represent two remarkably different types of symbiont genomes. Three of the genomes resemble other known members of Wolbachia F supergroup, while the other two show typical signs of ongoing gene inactivation and removal (genome size, coding density, low number of pseudogenes). Particularly, wMeur1, a symbiont fixed in microbiomes of Menacanthus eurysternus across four continents, possesses a highly reduced genome of 733,850 bp. The horizontally acquired capacity for pantothenate synthesis and localization in specialized bacteriocytes suggest its obligate nutritional role. CONCLUSIONS: The genome of wMeur1 strain, from the M. eurysternus microbiome, represents the smallest currently known Wolbachia genome and the first example of Wolbachia which has completed genomic streamlining as known from the typical obligate symbionts. This points out that despite the large amount and great diversity of the known Wolbachia strains, evolutionary potential of these bacteria still remains underexplored. The diversity of the four chewing lice microbiomes indicates that this vast parasitic group may provide suitable models for further investigations. Video Abstract.

A combined transcriptomic approach to identify candidates for an anti-tick vaccine blocking B. afzelii transmission.

Ixodes ricinus is the vector for Borrelia afzelii, the predominant cause of Lyme borreliosis in Europe, whereas Ixodes scapularis is the vector for Borrelia burgdorferi in the USA. Transcription of several I. scapularis genes changes in the presence of B. burgdorferi and contributes to successful infection. To what extend B. afzelii influences gene expression in I. ricinus salivary glands is largely unknown. Therefore, we measured expression of uninfected vs. infected tick salivary gland genes during tick feeding using Massive Analysis of cDNA Ends (MACE) and RNAseq, quantifying 26.179 unique transcripts. While tick feeding was the main differentiator, B. afzelii infection significantly affected expression of hundreds of transcripts, including 465 transcripts after 24 h of tick feeding. Validation of the top-20 B. afzelii-upregulated transcripts at 24 h of tick feeding in ten biological genetic distinct replicates showed that expression varied extensively. Three transcripts could be validated, a basic tail protein, a lipocalin and an ixodegrin, and might be involved in B. afzelii transmission. However, vaccination with recombinant forms of these proteins only marginally altered B. afzelii infection in I. ricinus-challenged mice for one of the proteins. Collectively, our data show that identification of tick salivary genes upregulated in the presence of pathogens could serve to identify potential pathogen-blocking vaccine candidates.

Identification of Tick Ixodes ricinus Midgut Genes Differentially Expressed During the Transmission of Borrelia afzelii Spirochetes Using a Transcriptomic Approach.

Lyme borreliosis is an emerging tick-borne disease caused by spirochetes Borrelia burgdorferi sensu lato. In Europe, Lyme borreliosis is predominantly caused by Borrelia afzelii and transmitted by Ixodes ricinus. Although Borrelia behavior throughout tick development is quite well documented, specific molecular interactions between Borrelia and the tick have not been satisfactorily examined. Here, we present the first transcriptomic study focused on the expression of tick midgut genes regulated by Borrelia. By using massive analysis of cDNA ends (MACE), we searched for tick transcripts expressed differentially in the midgut of unfed, 24h-fed, and fully fed I. ricinus nymphs infected with B. afzelii. In total, we identified 553 upregulated and 530 downregulated tick genes and demonstrated that B. afzelii interacts intensively with the tick. Technical and biological validations confirmed the accuracy of the transcriptome. The expression of five validated tick genes was silenced by RNA interference. Silencing of the uncharacterized protein (GXP_Contig_30818) delayed the infection progress and decreased infection prevalence in the target mice tissues. Silencing of other genes did not significantly affect tick feeding nor the transmission of B. afzelii, suggesting a possible role of these genes rather in Borrelia acquisition or persistence in ticks. Identification of genes and proteins exploited by Borrelia during transmission and establishment in a tick could help the development of novel preventive strategies for Lyme borreliosis.