Deformed Wing Virus in Two Widespread Invasive Ants: Geographical Distribution, Prevalence, and Phylogeny

Spillover of honey bee viruses have posed a significant threat to pollination services, triggering substantial effort in determining the host range of the viruses as an attempt to understand the transmission dynamics. Previous studies have reported infection of honey bee viruses in ants, raising the concern of ants serving as a reservoir host. Most of these studies, however, are restricted to a single, local ant population. We assessed the status (geographical distribution/prevalence/viral replication) and phylogenetic relationships of honey bee viruses in ants across the Asia–Pacific region, using deformed wing virus (DWV) and two widespread invasive ants, Paratrechina longicornis and Anoplolepis gracilipes, as the study system. DWV was detected in both ant species, with differential geographical distribution patterns and prevenance levels between them. These metrics, however, are consistent across the geographical range of the same ant species. Active replication was only evident in P. longicornis. We also showed that ant-associated DWV is genetically similar to that isolated from Asian populations of honey bees, suggesting that local acquisition of DWV by the invasive ants may have been common at least in some of our sampled regions. Transmission efficiency of DWV to local arthropods mediated by ant, however, may vary across ant species.

Genetic Strain Diversity of Multi-Host RNA Viruses that Infect a Wide Range of Pollinators and Associates is Shaped by Geographic Origins

Emerging viruses have caused concerns about pollinator population declines, as multi-host RNA viruses may pose a health threat to pollinators and associated arthropods. In order to understand the ecology and impact these viruses have, we studied their host range and determined to what extent host and spatial variation affect strain diversity. Firstly, we used RT-PCR to screen pollinators and associates, including honey bees (Apis mellifera) and invasive Argentine ants (Linepithema humile), for virus presence and replication. We tested for the black queen cell virus (BQCV), deformed wing virus (DWV), and Kashmir bee virus (KBV) that were initially detected in bees, and the two recently discovered Linepithema humile bunya-like virus 1 (LhuBLV1) and Moku virus (MKV). DWV, KBV, and MKV were detected and replicated in a wide range of hosts and commonly co-infected hymenopterans. Secondly, we placed KBV and DWV in a global phylogeny with sequences from various countries and hosts to determine the association of geographic origin and host with shared ancestry. Both phylogenies showed strong geographic rather than host-specific clustering, suggesting frequent inter-species virus transmission. Transmission routes between hosts are largely unknown. Nonetheless, avoiding the introduction of non-native species and diseased pollinators appears important to limit spill overs and disease emergence.

Foodborne Transmission and Clinical Symptoms of Honey Bee Viruses in Ants Lasius spp

Emerging infectious diseases are often the products of host shifts, where a pathogen jumps from its original host to a novel species. Viruses in particular cross species barriers frequently. Acute bee paralysis virus (ABPV) and deformed wing virus (DWV) are viruses described in honey bees (Apis mellifera) with broad host ranges. Ants scavenging on dead honey bees may get infected with these viruses via foodborne transmission. However, the role of black garden ants, Lasius niger and Lasius platythorax, as alternative hosts of ABPV and DWV is not known and potential impacts of these viruses have not been addressed yet. In a laboratory feeding experiment, we show that L. niger can carry DWV and ABPV. However, negative-sense strand RNA, a token of virus replication, was only detected for ABPV. Therefore, additional L. niger colonies were tested for clinical symptoms of ABPV infections. Symptoms were detected at colony (fewer emerging workers) and individual level (impaired locomotion and movement speed). In a field survey, all L. platythorax samples carried ABPV, DWV-A and -B, as well as the negative-sense strand RNA of ABPV. These results show that L. niger and L. platythorax are alternative hosts of ABPV, possibly acting as a biological vector of ABPV and as a mechanical one for DWV. This is the first study showing the impact of honey bee viruses on ants. The common virus infections of ants in the field support possible negative consequences for ecosystem functioning due to host shifts.

Different bacterial and viral pathogens trigger distinct immune responses in a globally invasive ant

Invasive species populations periodically collapse from high to low abundance, sometimes even to extinction. Pathogens and the burden they place on invader immune systems have been hypothesised as a mechanism for these collapses. We examined the association of the bacterial pathogen (Pseudomonas spp.) and the viral community with immune gene expression in the globally invasive Argentine ant (Linepithema humile (Mayr)). RNA-seq analysis found evidence for 17 different viruses in Argentine ants from New Zealand, including three bacteriophages with one (Pseudomonas phage PS-1) likely to be attacking the bacterial host. Pathogen loads and prevalence varied immensely. Transcriptomic data showed that immune gene expression was consistent with respect to the viral classification of negative-sense, positive-sense and double-stranded RNA viruses. Genes that were the most strongly associated with the positive-sense RNA viruses such as the Linepithema humile virus 1 (LHUV-1) and the Deformed wing virus (DWV) were peptide recognition proteins assigned to the Toll and Imd pathways. We then used principal components analysis and regression modelling to determine how RT-qPCR derived immune gene expression levels were associated with viral and bacterial loads. Argentine ants mounted a substantial immune response to both Pseudomonas and LHUV-1 infections, involving almost all immune pathways. Other viruses including DWV and the Kashmir bee virus appeared to have much less immunological influence. Different pathogens were associated with varying immunological responses, which we hypothesize to interact with and influence the invasion dynamics of this species.

Prospecting for viral natural enemies of the fire ant Solenopsis invicta in Argentina

Metagenomics and next generation sequencing were employed to discover new virus natural enemies of the fire ant, Solenopsis invicta Buren in its native range (i.e., Formosa, Argentina) with the ultimate goal of testing and releasing new viral pathogens into U.S. S. invicta populations to provide natural, sustainable control of this ant. RNA was purified from worker ants from 182 S. invicta colonies, which was pooled into 4 groups according to location. A library was created from each group and sequenced using Illumina Miseq technology. After a series of winnowing methods to remove S. invicta genes, known S. invicta virus genes, and all other non-virus gene sequences, 61,944 unique singletons were identified with virus identity. These were assembled de novo yielding 171 contiguous sequences with significant identity to non-plant virus genes. Fifteen contiguous sequences exhibited very high expression rates and were detected in all four gene libraries. One contig (Contig_29) exhibited the highest expression level overall and across all four gene libraries. Random amplification of cDNA ends analyses expanded this contiguous sequence yielding a complete virus genome, which we have provisionally named Solenopsis invicta virus 5 (SINV-5). SINV-5 is a positive-sense, single-stranded RNA virus with genome characteristics consistent with insect-infecting viruses from the family Dicistroviridae. Moreover, the replicative genome strand of SINV-5 was detected in worker ants indicating that S. invicta serves as host for the virus. Many additional sequences were identified that are likely of viral origin. These sequences await further investigation to determine their origins and relationship with S. invicta. This study expands knowledge of the RNA virome diversity found within S. invicta populations.

Single-stranded RNA viruses infecting the invasive Argentine ant, Linepithema humile

Social insects host a diversity of viruses. We examined New Zealand populations of the globally widely distributed invasive Argentine ant (Linepithema humile) for RNA viruses. We used metatranscriptomic analysis, which identified six potential novel viruses in the Dicistroviridae family. Of these, three contigs were confirmed by Sanger sequencing as Linepithema humile virus-1 (LHUV-1), a novel strain of Kashmir bee virus (KBV) and Black queen cell virus (BQCV), while the others were chimeric or misassembled sequences. We extended the known sequence of LHUV-1 to confirm its placement in the Dicistroviridae and categorised its relationship to closest relatives, which were all viruses infecting Hymenoptera. We examined further for known viruses by mapping our metatranscriptomic sequences to all viral genomes, and confirmed KBV, BQCV, LHUV-1 and Deformed wing virus (DWV) presence using qRT-PCR. Viral replication was confirmed for DWV, KBV and LHUV-1. Viral titers in ants were higher in the presence of honey bee hives. Argentine ants appear to host a range of' honey bee' pathogens in addition to a virus currently described only from this invasive ant. The role of these viruses in the population dynamics of the ant remain to be determined, but offer potential targets for biocontrol approaches.

Isolation and characterization of Nylanderia fulva virus 1, a positive-sense, single-stranded RNA virus infecting the tawny crazy ant, Nylanderia fulva

We report the discovery of Nylanderia fulva virus 1 (NfV-1), the first virus identified and characterized from the ant, Nylanderia fulva. The NfV-1 genome (GenBank accession KX024775) is 10,881 nucleotides in length, encoding one large open reading frame (ORF). Helicase, protease, RNA-dependent RNA polymerase, and jelly-roll capsid protein domains were recognized within the polyprotein. Phylogenetic analysis placed NfV-1 in an unclassified clade of viruses. Electron microscopic examination of negatively stained samples revealed particles with icosahedral symmetry with a diameter of 28.7±1.1nm. The virus was detected by RT-PCR in larval, pupal, worker and queen developmental stages. However, the replicative strand of NfV-1 was only detected in larvae. Vertical transmission did not appear to occur, but horizontal transmission was facile. The inter-colonial field prevalence of NfV-1 was 52±35% with some local infections reaching 100%. NfV-1 was not detected in limited samples of other Nylanderia species or closely related ant species.

Invasive ants carry novel viruses in their new range and form reservoirs for a honeybee pathogen

When exotic animal species invade new environments they also bring an often unknown microbial diversity, including pathogens. We describe a novel and widely distributed virus in one of the most globally widespread, abundant and damaging invasive ants (Argentine ants, Linepithema humile). The Linepithema humile virus 1 is a dicistrovirus, a viral family including species known to cause widespread arthropod disease. It was detected in samples from Argentina, Australia and New Zealand. Argentine ants in New Zealand were also infected with a strain of Deformed wing virus common to local hymenopteran species, which is a major pathogen widely associated with honeybee mortality. Evidence for active replication of viral RNA was apparent for both viruses. Our results suggest co-introduction and exchange of pathogens within local hymenopteran communities. These viral species may contribute to the collapse of Argentine ant populations and offer new options for the control of a globally widespread invader.

Solenopsis invicta virus 3: pathogenesis and stage specificity in red imported fire ants

Solenopsis invicta colonies were exposed to purified preparations of Solenopsis invicta virus 3 (SINV-3) to investigate virus pathogenesis at the colony level. Time course experiments revealed an infection exhibiting specificity for the adult stage (workers). SINV-3 genome and a capsid protein were increasingly present in worker ants with time. Northern blot analysis revealed two bands in RNA preparations from worker ants infected with SINV-3 corresponding to the genomic and sub-genomic species. Conversely, larval RNA preparations from SINV-3-infected colonies showed a near-complete absence of SINV-3 genome or sub-genome. The data confirm that SINV-3 is the etiological agent causing mortality among S. invicta colonies in the laboratory. We propose that SINV-3 infection somehow alters worker ant behavior, which may prevent them from acquiring and/or distributing solid food to the larvae. Consequently, larval mortality and impaired queen health occur as a result of starvation or neglect by the worker caste.

Solenopsis invicta virus 3: mapping of structural proteins, ribosomal frameshifting, and similarities to Acyrthosiphon pisum virus and Kelp fly virus

Solenopsis invicta virus 3 (SINV-3) is a positive-sense single-stranded RNA virus that infects the red imported fire ant, Solenopsis invicta. We show that the second open reading frame (ORF) of the dicistronic genome is expressed via a frameshifting mechanism and that the sequences encoding the structural proteins map to both ORF2 and the 3' end of ORF1, downstream of the sequence that encodes the RNA-dependent RNA polymerase. The genome organization and structural protein expression strategy resemble those of Acyrthosiphon pisum virus (APV), an aphid virus. The capsid protein that is encoded by the 3' end of ORF1 in SINV-3 and APV is predicted to have a jelly-roll fold similar to the capsid proteins of picornaviruses and caliciviruses. The capsid-extension protein that is produced by frameshifting, includes the jelly-roll fold domain encoded by ORF1 as its N-terminus, while the C-terminus encoded by the 5' half of ORF2 has no clear homology with other viral structural proteins. A third protein, encoded by the 3' half of ORF2, is associated with purified virions at sub-stoichiometric ratios. Although the structural proteins can be translated from the genomic RNA, we show that SINV-3 also produces a subgenomic RNA encoding the structural proteins. Circumstantial evidence suggests that APV may also produce such a subgenomic RNA. Both SINV-3 and APV are unclassified picorna-like viruses distantly related to members of the order Picornavirales and the family Caliciviridae. Within this grouping, features of the genome organization and capsid domain structure of SINV-3 and APV appear more similar to caliciviruses, perhaps suggesting the basis for a "Calicivirales" order.

Interspecific competition between Solenopsis invicta and two native ant species, Pheidole fervens and Monomorium chinense

This study was designed to understand the effects of the interspecific competition between red imported fire ant, Solenopsis invicta Buren and two native ant species, Pheidole fervens Smith and Monomorium chinense Santschi, by conducting colony interference and individual confrontation tests under laboratory conditions. The colony interference test showed that both native ant species, owing to their numerical advantage, killed the Solenopsis invicta virus-1 (SINV-1)-infected or healthy queens of S. invicta. Significantly less time was required for M. chinense to kill all SINV-1-infected S. invicta compared with the time required to kill the healthy S. invicta. Compared with healthy S. invicta, SINV-1-infected S. invicta spent a longer time eliminating the P. fervens colonies. In confrontation tests, M. chinense killed a significantly higher number of infected S. invicta minors than they did healthy minors, but the number of S. invicta majors (either infected or healthy) killed was substantially less. This study found that the viral infection weakened the competitive ability of S. invicta and made them prone to be eliminated by M. chinense but not by P. fervens.

Multiplex detection of Solenopsis invicta viruses -1, -2, and -3

Multiplex reverse transcription and polymerase chain reaction (PCR) methods were developed to detect Solenopsis invicta viruses -1, -2, and -3 simultaneously in their host, the red imported fire ant, S. invicta. cDNA synthesis was conducted in a single reaction containing an oligonucleotide primer specific for each virus. Multiplex PCR was subsequently conducted with oligonucleotide primer pairs specific for each virus. The method was specific and sensitive, capable of detecting as few as 500 copies of the viral genomes consistently. Specificity was verified by PCR and amplicon sequencing. The method was evaluated against field-collected samples of ant workers from colonies in Argentina (n=135 ant colonies) and the United States (n=172 ant colonies). The prevalence of each virus in fire ant colonies varied considerably from site to site. A number of colonies exhibited multiple virus infections. However, the multiple SINV infection rate was lower than for single infections. Comparison of viral infection prevalence between S. invicta colonies in Argentina and the U.S. showed no statistical differences, regardless of infection category. This method is anticipated to facilitate epidemiological and related studies concerning the S. invicta viruses in fire ants.

Phenology, distribution, and host specificity of Solenopsis invicta virus-1

Studies were conducted to examine the phenology, geographic distribution, and host specificity of the Solenopsis invicta virus-1 (SINV-1). Two genotypes examined, SINV-1 and -1A, exhibited similar seasonal prevalence patterns. Infection rates among colonies of S. invicta in Gainesville, Florida, were lowest from early winter (December) to early spring (April) increasing rapidly in late spring (May) and remaining high through August before declining again in the fall (September/October). Correlation analysis revealed a significant relationship between mean monthly temperature and SINV-1 (p<0.0005, r=0.82) and SINV-1A (p<0.0001, r=0.86) infection rates in S. invicta colonies. SINV-1 was widely distributed among S. invicta populations. The virus was detected in S. invicta from Argentina and from all U.S. states examined, with the exception of New Mexico. SINV-1 and -1A were also detected in other Solenopsis species. SINV-1 was detected in Solenopsis richteri and the S. invicta/richteri hybrid collected from northern Alabama and Solenopsis geminata from Florida. SINV-1A was detected in S. geminata and Solenopsis carolinensis in Florida and the S. invicta/richteri hybrid in Alabama. Of the 1989 arthropods collected from 6 pitfall trap experiments from Gainesville and Williston, Florida, none except S. invicta tested positive for SINV-1 or SINV-1A. SINV-1 did not appear to infect or replicate within Sf9 or Dm-2 cells in vitro. The number of SINV-1 genome copies did not significantly increase over the course of the experiment, nor were any cytopathic effects observed. Phylogenetic analyses of SINV-1/-1A nucleotide sequences indicated significant divergence between viruses collected from Argentina and the U.S.

A new positive-strand RNA virus with unique genome characteristics from the red imported fire ant, Solenopsis invicta

We report the discovery of a new virus with unique genome characteristics from the red imported fire ant, Solenopsis invicta. This virus represents the second identified from this ant species. It is provisionally named Solenopsis invicta virus 2 (SINV-2). The SINV-2 genome was constructed by compiling sequences from successive 5' RACE reactions, a 3' RACE reaction, and expressed sequence tag, c246 (accession number EH413675), from a fire ant expression library. The SINV-2 genome structure was monopartite, polycistronic and RNA-based. The genome consensus sequence (EF428566) was 11,303 nucleotides in length, excluding the poly(A) tail present on the 3' end. Analysis of the genome revealed 4 major open reading frames (ORFs; comprised of > or =100 codons) and 5 minor ORFs (comprised of 50-99 codons) in the sense orientation. No large ORFs were found in the inverse orientation suggesting that the SINV-2 genome was from a positive-strand RNA virus. Further evidence for this conclusion includes abolished RT-PCR amplification by RNase treatment of SINV-2 nucleic acid template, and failure to amplify without first conducting cDNA synthesis. Blastp analysis indicated that ORF 4 contained conserved domains of an RNA-dependent RNA polymerase, helicase, and protease, characteristic of positive-strand RNA viruses. However, the protease domain and putative structural proteins (ORFs 1, 2, and 3) were less well conserved. Phylogenetic analysis of the RdRp, helicase, and ORF 1 indicate unique placement of SINV-2 exclusive from the Dicistroviridae, iflaviruses, Picornaviridae, and plant small RNA viruses.

Detection and quantitation of Solenopsis invicta virus in fire ants by real-time PCR

A quantitative real-time PCR (QPCR) method was developed to detect and quantify the amount of Solenopsis invicta virus (SINV) infecting individual ants of S. invicta. The two-step method utilized a gene-specific oligonucleotide primer targeting the SINV RNA-dependent RNA polymerase (RdRp) for cDNA synthesis. Dithiothreitol used in the cDNA synthesis step was found to significantly decrease the detection sensitivity for SINV RdRp and was therefore omitted. SINV RdRp cDNA was then quantified by QPCR using SYBR Green dye and a standard curve generated from SINV RdRp plasmid clones. A standard curve was successfully constructed from clones of the SINV RdRp region. A strong linear relationship [r2=0.998; y=(-3.63+/-0.37)x+(39.19+/-1.33)] between C(T) and starting SINV RdRp copy number was observed within a dynamic range of 5-5 x 10(6) copies. SINV RdRp copy number was determined with the optimized QPCR method in individual S. invicta ants taken from an infected field colony. Worker ants exhibited the highest RdRp copy number (2.1 x 10(9) copies/worker ant) and pupae exhibited the lowest (4.2 x 10(2) copies/pupa). Mean RdRp copy number was lowest in early larvae and pupae. Overall, SINV RdRp copy number increased through larval development, sharply declined during pupation, then sharply increased in adults.

Solenopsis invicta virus-1A (SINV-1A): Distinct species or genotype of SINV-1?

We have cloned and sequenced a 2845 bp cDNA representing the 3'-end of either a new picorna-like virus species or genotype of Solenopsis invicta virus-1 (SINV-1). Analysis of the nucleotide sequence revealed 1 large open reading frame. The amino acid sequence of the translated open reading frame was most identical to structural proteins of SINV-1 (97%), followed by the Kashmir bee virus (KBV, 30%), and acute bee paralysis virus (ABPV, 29%). A PCR-based survey for SINV-1 and the new species or genotype (tentatively named S. invicta virus-1A, SINV-1A) using RNA extracts of S. invicta collected around Gainesville, Florida, revealed a mean colony infestation rate of 25% by SINV-1 and 55% by SINV-1A. Both SINV-1 and SINV-1A were found to co-infect 17.5% of the nests surveyed. Although the data preclude definitive species or genotype assignment, there is no doubt that SINV-1A is distinct from SINV-1, identifiable, and infects S. invicta. We provide a simple RT-PCR technique capable of discerning SINV-1 and SINV-1A infection of S. invicta.

A picorna-like virus from the red imported fire ant, Solenopsis invicta: initial discovery, genome sequence, and characterization

We report the first discovery and genome sequence of a virus infecting the red imported fire ant, Solenopsis invicta. The 8026 nucleotide, polyadenylated, RNA genome encoded two large open reading frames (ORF1 and ORF2), flanked and separated by 27, 223, and 171 nucleotide untranslated regions, respectively. The predicted amino acid sequence of the 5' proximal ORF1 (nucleotides 28 to 4218) exhibited significant identity and possessed consensus sequences characteristic of the helicase, cysteine protease, and RNA-dependent RNA polymerase sequence motifs from picornaviruses, picorna-like viruses, comoviruses, caliciviruses, and sequiviruses. The predicted amino acid sequence of the 3' proximal ORF2 (nucleotides 4390-7803) showed similarity to structural proteins in picorna-like viruses, especially the acute bee paralysis virus. Electron microscopic examination of negatively stained samples from virus-infected fire ants revealed isometric particles with a diameter of 31 nm, consistent with Picornaviridae. A survey for the fire ant virus from areas around Florida revealed a pattern of fairly widespread distribution. Among 168 nests surveyed, 22.9% were infected. The virus was found to infect all fire ant caste members and developmental stages, including eggs, early (1st-2nd) and late (3rd-4th) instars, worker pupae, workers, sexual pupae, alates ( male symbol and female symbol ), and queens. The virus, tentatively named S. invicta virus (SINV-1), appears to belong to the picorna-like viruses. We did not observe any perceptible symptoms among infected nests in the field. However, in every case where an SINV-1-infected colony was excavated from the field with an inseminated queen and held in the laboratory, all of the brood in these colonies died within 3 months.