Torix group Rickettsia are widespread in Culicoides biting midges (Diptera: Ceratopogonidae), reach high frequency and carry unique genomic features

Abstract

SummaryThere is increasing interest in the heritable bacteria of invertebrate vectors of disease as they present novel targets for control initiatives. Previous studies on biting midges (Culicoides spp.), known to transmit several RNA viruses of veterinary importance, have revealed infections with the endosymbiotic bacteria, Wolbachia and Cardinium. However, rickettsial symbionts in these vectors are underexplored. Here, we present the genome of a previously uncharacterized Rickettsia endosymbiont from Culicoides newsteadi (RiCNE). This genome presents unique features potentially associated with host invasion and adaptation, including genes for the complete non‐oxidative phase of the pentose phosphate pathway, and others predicted to mediate lipopolysaccharides and cell wall modification. Screening of 414 Culicoides individuals from 29 Palearctic or Afrotropical species revealed that Rickettsia represent a widespread but previously overlooked association, reaching high frequencies in midge populations and present in 38% of the species tested. Sequence typing clusters the Rickettsia within the Torix group of the genus, a group known to infect several aquatic and hematophagous taxa. FISH analysis indicated the presence of Rickettsia bacteria in ovary tissue, indicating their maternal inheritance. Given the importance of biting midges as vectors, a key area of future research is to establish the impact of this endosymbiont on vector competence.

A newly recorded Rickettsia of the Torix group is a recent intruder and an endosymbiont in the whitefly Bemisia tabaci

“…The draft genome of RiTBt has a length of 1,485, 811 bp (Table ), which is slightly larger than that of Torix group member, named RiCNE (Pilgrim et al ., ), previously sequenced from Culicoides newsteadi (1,456, 172 bp), and Rickettsia _bellii_MEAM1 from the whitefly MEAM1 (1,378, 618 bp). The draft genome has a 33.3% GC content and comprises 69 scaffolds with an N50 size of approximately 50, 029 bp (Table ).…”

“…The parameters for filtering redundant reads/contigs followed the approach described by Pilgrim et al . (). The final assembly was subjected to a final decontamination step to remove the contigs that had high similarity with the other endosymbionts found in B. tabaci .…”

“…The draft genome of RiTBt has a length of 1,485, 811 bp (Table 1), which is slightly larger than that of Torix group member, named RiCNE (Pilgrim et al, 2017), previously sequenced from Culicoides newsteadi (1,456, 172 bp), and Rickettsia_bellii_MEAM1 from the whitefly MEAM1 Portiera-specific probe (red) conjugated to cy3 and RiTBt-specific probe (green) conjugated to cy5 were used, and nuclei were stained with DAPI (blue). FISH images: A1-A4, bacteriocyte; B1-B3, midgut; C1-C3, salivary gland; D1-D4, ovary; and E1-E4, testis.…”

“…Up to now, over 60 Rickettsia genomes have been published from a variety of host systems. Most of these belong to Bellii, Canadensis, Spotted Fever, Typhus, and Transitional group, with only one to the Torix group (Pilgrim et al, 2017).…”

“…A notable characterization of Rickettsia is that some species are pathogenic to vertebrates (Weinert et al, 2009), such as R. prowazekii, R. rickettsii, and R. conorii. Rickettsia have also been detected in a wide variety of invertebrates and have profound effects on the biology of the hosts and vector-pathogen interactions (Pilgrim et al, 2017). As for B. tabaci, only Rickettsia_bellii_MEAM1 has been studied in some detail.…”