Abstract
Arthropods often have obligate relationships with symbiotic microbes, and recent investigations have demonstrated that such host-microbe relationships could be exploited to suppress natural populations of vector carrying mosquitos. Strategies that target the interplay between agricultural pests and their symbionts could decrease the burden caused by agricultural pests; however, the lack of comprehensive genomic insights into naturally occurring microbial symbionts presents a significant bottleneck. Here we employed amplicon surveys, genome-resolved metagenomics, and scanning electron microscopy to investigate symbionts of the wheat stem sawfly (Cephus cinctus), a major pest that causes an estimated $350 million dollars or more in wheat yield losses in the northwestern United States annually. Through 16S rRNA gene sequencing of two major haplotypes and life stages of wheat stem sawfly, we show a novel Spiroplasma species is ever-present and predominant, with phylogenomic analyses placing it as a member of the ixodetis clade of mollicutes. Using state-of-the-art metagenomic assembly and binning strategies we were able to reconstruct a 714 Kb, 72.7%-complete Spiroplasma genome, which represents just the second draft genome from the ixodetis clade of mollicutes. Functional annotation of the Spiroplasma genome indicated carbohydrate-metabolism involved PTS-mediated import of glucose and fructose followed by glycolysis to lactate, acetate, and propionoate. The bacterium also encoded biosynthetic pathways for essential vitamins B2, B3, and B9. We identified putative Spiroplasma virulence genes: cardiolipin and chitinase. These results identify a previously undescribed symbiosis between wheat stem sawfly and a novel Spiroplasma sp., availing insight into their molecular relationship, and may yield new opportunities for microbially-mediated pest control strategies.
Highlights
The wheat stem sawfly (Cephus cinctus) is a hymenopteran insect native to the Western U.S and Canada
We identified and downloaded all Spiroplasma genomes from NCBI using the program ncbi-genome-download using flags ‘‘–assembly-level complete bacteria –genus Spiroplasma’’ (Genome accessions and information is listed in Table S1), and stored the information about these genomes that is reported by ncbi-genomedownload into a text file
An α-helical bacterium consistent with Spiroplasma is observable in larval lysates via Scanning Electron Microscopy (SEM) imaging
Summary
The wheat stem sawfly (Cephus cinctus) is a hymenopteran insect native to the Western U.S and Canada. Wheat stem sawflies (further referred to as WSS) are some of the worst pests of wheat production (Shanower & Hoelmer, 2004). Current methods used by growers to reduce crop losses due to WSS include operational control measures (e.g., crop rotation, tillage, and swathing), infrequent use of insecticides, and biological control agents (e.g., parasitic wasps) (Shanower & Hoelmer, 2004; Beres et al, 2011). Most research efforts aimed at curbing this problem are to develop WSS-resistant wheat cultivars, including solid-stemmed varieties that provide less suitable habitat for WSS reproduction, but many of these wheat varieties have lower yield potentials and inconsistent pith expression, so the identification of alternative controls and other host plant resistance is desirable (Beres, Cárcamo & Byers, 2007; Beres et al, 2011; Knodel et al, 2009; Portman et al, 2018)
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