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Applied and Environmental Microbiology, June 2007, p. 3605-3611, Vol. 73, No. 11
0099-2240/07/$08.00+0 doi:10.1128/AEM.00696-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

yna Topolska,4
Wolfgang Ritter,5
Hermann Pechhacker,6 and
Norbert Nowotny1,7
Zoonoses and Emerging Infections Group, Clinical Virology, Clinical Department of Diagnostic Imaging, Infectious Diseases and Clinical Pathology, University of Veterinary Medicine, Vienna, A-1210 Vienna, Austria,1 Department of Microbiology and Infectious Diseases, Faculty of Veterinary Science, Szent István University, H-1143 Budapest, Hungary,2 Institute for Apiculture, Agricultural Inspection Service and Research Centre Vienna, Austrian Agency for Health and Food Safety and Federal Office for Food Safety, A-1226 Vienna, Austria,3 Division of Bee Diseases, Department of Clinical Sciences, Faculty of Veterinary Medicine, Warsaw Agricultural University, PL-02-786 Warsaw, Poland,4 Department of Bee Pathology, CVUA-Animal Health, D-79108 Freiburg, Germany,5 Institute for Apiculture, Austrian Agency for Health and Food Safety and Federal Office for Food Safety, A-3293 Lunz, Austria,6 Department of Medical Microbiology, Faculty of Medicine and Health Sciences, United Arab Emirates University, Al Ain, United Arab Emirates7
Received 27 March 2007/ Accepted 2 April 2007
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DWV contains a positive, single-stranded, polyadenylated and monocistronic RNA genome comprising 10,144 nucleotides (nt) (23). The monopartite genome consists of one large, uninterrupted open reading frame encoding the viral polypeptide precursor, which is posttranslationally processed by proteases into active proteins. The N-terminal end of the polypeptide starts with a leader peptide (L protein), followed by the structural proteins VP2, putative VP4, VP1, and VP3. The C-terminal part of the polypeptide contains the nonstructural proteins; conserved motifs of the RNA helicase, the putative VPg protein, the C protease, and the RNA-dependent RNA polymerase (RdRp) were predicted in the deduced amino acid sequence (23) (Fig. 1A). DWV belongs to the unassigned genus Iflavirus of the insect picorna-like viruses, and serologically it is distantly related to Egypt bee virus (1, 4, 27).
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FIG. 1. Localization of the investigated genome regions of DWV. (A) Schematic depiction of the DWV genome (modified from reference 23 with permission). The shaded box represents the open reading frame, and vertical lines indicate predicted protease cleavage sites in the corresponding polypeptide. Products corresponding to the coding regions are indicated. L, coding region for the L protein; *, coding region for the putative VP4 protein; #, coding region for the putative VPg protein; 3C, coding region for the C protease. (B) Localization of the amplification products obtained by primer pairs listed in Table 1. Shaded boxes indicate amplification products used for the comprehensive phylogenetic analysis of 34 DWV genotypes. (C) DWV sequences available in the GenBank database. Shaded boxes represent the sequences obtained in this study. The geographic origins of the sequences are indicated on the left, and the GenBank accession numbers are shown on the right. **, 34 DWV strains from nine countries and three continents.
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Isolation of RNA.
The bees were homogenized in sterile ceramic mortars with sterile sand; thereafter, diethyl pyrocarbonate-treated water was added. The homogenates were centrifuged at 20,000 x g for 1 min, and 140 µl of the supernatant was used for RNA extraction by employing the QIAamp viral RNA mini kit according to the instructions of the manufacturer (QIAGEN, Hilden, Germany).
Primer design.
Eight primer pairs were selected based on the complete DWV genomes deposited in the GenBank database under accession numbers NC_004830 and AY292384 (Fig. 1B) by employing a primer designer program (Scientific and Educational Software, version 4.1). At first the primers were tested with one Austrian DWV strain by amplifying different genomic regions of the virus. The amplification products were sequenced and compared with the previously mentioned complete genome records from GenBank. For a more comprehensive analysis, four primer pairs that were expected to produce overlapping amplification products corresponding to the partial VP2, putative VP4, and VP1 structural-protein genes and the partial RNA helicase enzyme coding genome region were selected (Fig. 1B). These primer pairs were applied in reverse transcription-PCR (RT-PCR) assays to the remaining 36 DWV samples. Since in the case of the first two primer pairs, the overlapping area was only 19 bases long, another primer pair corresponding to this region was designed in order to create a longer overlapping genome stretch. The primer sequences, orientations, and locations and the expected sizes of the amplified products are shown in Table 1.
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TABLE 1. Oligonucleotide primers employed for the amplification of selected DWV genome regions
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Nucleotide sequencing and phylogenetic analysis.
The PCR products were excised from the gel and extracted by employing the QIAquick gel extraction kit according to the instructions of the manufacturer (QIAGEN). Fluorescence-based direct sequencing of the amplicons in both directions was performed by using the ABI PRISM BigDye Terminator cycle-sequencing ready-reaction kit (21). The nucleotide sequences were identified by the basic local alignment search tool BLAST (2) and aligned by employing the Align Plus program (Scientific and Educational Software, version 4.1). Multiple alignments for phylogenetic analyses were created with the help of the ClustalX program (33). Phylogenetic analyses were conducted by using the PAUP*4.0 beta 10 version (29) with maximum-parsimony, distance (neighbor-joining), and maximum-likelihood criteria. Bootstrap analyses of 1,000 replicates of the parsimony and distance trees were performed. The same data sets were also analyzed by using the PHYLIP package 3.6 beta version (15) with maximum-parsimony, distance (neighbor-joining, fitch, and kitsch), and maximum-likelihood algorithms. Trees were drawn with the help of the TreeView 1.6.6. software.
Nucleotide sequence accession numbers.
The DWV sequences described in this paper were submitted to the GenBank database under accession numbers DQ224278 to DQ224311.
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All partial nucleotide sequences of DWV strains found in GenBank were also aligned with the complete genome records and, when applicable, with the sequences obtained in this study (Fig. 1C). A 4,700-nt-long sequence covering the complete nonstructural-protein coding region of a DWV strain from France (France-1) was previously deposited in GenBank under accession number AY224602. The sequence is 98% similar to those of the corresponding regions in the complete genome records of the Italy-1 and USA-1 strains (23) and 98 to 99% similar to the sequences obtained in this study and to other sequences deposited in GenBank. The 5' untranslated regions of the Italian, U.S., and Austria-1 sequences were aligned with a sequence from a DWV strain detected in Costa Rica (accession number DQ139981). These sequences exhibited 98 to 99% identity to one another. Another genome stretch of the same Costa Rican genotype in the putative VPg protein coding region is available under accession number DQ139982. This region is 98% similar to the corresponding sequences of the Italy-1, USA-1, and France-1 strains. Sequence information on the partial RdRp coding regions of 15 DWV strains was found in GenBank: besides those from the Italy-1, USA-1, and France-1 strains, sequences of the RdRp coding regions from two additional strains from France (accession numbers DQ139981 and DQ139982) (31), one strain from Uruguay (accession number DQ364631) (3), and nine strains from Spain (accession numbers DQ385499 to DQ385507) were available. The RdRp coding regions in these genotypes showed again 98 to 99% similarity to the corresponding regions of the reference sequences.
Phylogenetic analyses of DWV genotypes.
Because the aforementioned comparisons did not reveal regions of higher diversity than others in the investigated genome stretches of the Italian, U.S., and Austrian DWV strains, two regions were selected for further analyses on the basis of their putative functions. In viruses of vertebrates, frequently the structural proteins (antigens) are subjected to immunoselection, and therefore the corresponding genes often contain hypervariable regions. In contrast, nonstructural proteins (enzymes) are usually more conserved, because mutations may disturb their efficacy. Such tendencies in viruses of invertebrates have not been recorded so far, and therefore, we decided to compare the levels of variability of corresponding partial structural-protein coding regions and partial nonstructural-protein coding regions from DWV samples collected from all over the world: a genome stretch between nucleotide positions 1967 and 2731, located in the VP2, putative VP4, and VP1 structural-protein coding region, and a genome stretch between nucleotide positions 6053 and 6614, located in the putative RNA helicase coding region. RT-PCR assays of the remaining 36 honeybee samples were performed with primer pairs which result in overlapping PCR amplification products. The selected regions were amplified in 33 samples. Interestingly, all three samples originating from New Zealand proved to be negative for these regions in all RT-PCR assays employed. The amplicons from the positive samples were directly sequenced in both directions. The sequences were compiled and aligned by using the complete DWV sequences deposited in GenBank as references. After the completion of these investigations, continuous sequence information was available from every investigated DWV sample for a 764-nt fragment within the putative VP2-VP1 capsid protein gene region and a 562-nt fragment within the helicase gene region. Together, the two fragments cover 13% of the complete DWV genome. The sequences were 98 to 99% similar to those of the reference strains; nucleotide substitutions at 50 positions within the structural-protein coding fragment and at 49 positions within the helicase coding fragment were observed. A more detailed investigation of these positions revealed that at each position, the nucleotides in the studied DWV genotypes always varied between two alternatives. The distribution of the transitions did not show any tendencies: for instance, at position 6430 in 14 genotypes, cytosine was found, and in 23 genotypes, uracil was present; at position 6565, 17 genotypes contained adenine and 20 contained guanine; and at position 6604, 20 sequences exhibited uracil and 17 exhibited cytosine, regardless of the geographic origins of the strains. The putative amino acid sequences corresponding to the two investigated DWV genome stretches were also determined, and these amino acid sequences were aligned and compared. Amino acid substitutions at four positions within the capsid protein region and at seven positions in the helicase protein region were found (Table 2).
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TABLE 2. Substitutions in the deduced amino acid sequences corresponding to the investigated DWV genotypesa
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FIG. 2. Phylogram illustrating the genetic relationships among DWV genotypes, based on the partial structural-polypeptide coding region between nucleotide positions 1967 and 2731. The phylogram was constructed by PAUP* using maximum-parsimony criteria. VDV-1 was used as the outgroup, and the branch length of VDV-1 was reduced by 400% to increase resolution. Scale bars indicate the numbers of steps for parsimony analysis; the bootstrap support value is indicated only for the DWV/KGV-VDV-1 node. U.A.E., United Arab Emirates.
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FIG. 3. Phylogram illustrating the genetic relationships among DWV genotypes, based on the partial RNA helicase coding region between nucleotide positions 6053 and 6614. The phylogram was constructed by PAUP* using maximum-parsimony criteria. VDV-1 was used as the outgroup, and the branch length of VDV-1 was reduced by 400% to increase resolution. Scale bars indicate the numbers of steps for parsimony analysis; the bootstrap support value is indicated only for the DWV/KGV-VDV-1 node. U.A.E., United Arab Emirates.
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The genetic diversity of two other honeybee viruses, ABPV and SBV, was investigated previously by our research group by using the same methods applied in this study of DWV (5, 21). When ABPV strains from different continents (Europe and North America) were compared, nucleotide similarity levels varied between 89 and 91% (at the structural-polyprotein partial coding sequence) (5). In the case of SBV, the other bee-infecting member of the Iflavirus genus, isolates from Europe were 90 to 94% similar to Asian strains and 78 to 83% similar to a South African strain in different coding regions of the genome (21). Virus strains from the same continents or from the same countries showed higher levels of similarity, and phylogenetic analyses unambiguously indicated the genetic clustering of the strains according to their geographic origins. Practically, this means that these viruses have been present in the honeybee populations for a long time, that an exchange of viruses among the host populations has been infrequent, and that therefore, the viruses have evolved more or less independently.
The DWV genotypes investigated in this study, however, showed similarity levels of
98%, regardless of the genome stretch investigated and the geographic origins of the viruses. Although the genotypes are very similar to one another, nucleotide substitutionspredominantly transitionswere observed at certain loci from several viruses, indicating the divergence of these genotypes. Notwithstanding, the phylogenetic analyses could not confirm the shared derived character of these nucleotide substitutions. Due to the low level of diversity, the statistical analysis could not reveal significant clustering of the investigated DWV genotypes, except for their separation from those of KGV and VDV-1.
Although the comprehensive phylogenetic analysis of the 34 strains was applied to only two selected regions of the DWV genome, an analysis of the sequence data for other regions obtained by independent research groups indicated similarly low levels of sequence diversity among different virus strains from Europe and South America. The nucleotide sequences of seven amplification products from a DWV strain from Germany were identified by Genersch (18). The partial nucleotide sequences covered 31% of the DWV genome, spanning between the VP1 and RdRp coding regions. Levels of similarity to the published complete sequences varied between 98.9 and 99.7%. Because the sequences obtained in that study have not yet been deposited in gene bank databases, they could not be included in the present analysis, but the similarity values further support our findings.
DWV was first detected in honeybees from Japan in the early 1980s (4, 23). Since that time, DWV has usually been found in honeybee colonies infested with V. destructor. The natural host of this parasitic mite is Apis cerena, present in the Far East regions of Asia. The mite first emerged on Apis mellifera, also in Japan (where the European honeybee was introduced in 1876), at the beginning of the 20th century. Due to the international and transcontinental trade of varroa mite-infested honeybees, the mite has spread all around the world's honeybee population within the last 50 years (4). Quantitative analyses of the DWV loads in V. destructor indicate that the mite is an efficient host for the multiplication of the virus (28, 32, 34). Although the mite plays a central role in the transmission of DWV, previous investigations revealed alternative routes for the spread of the virus as well (9, 10, 11, 26, 34). Although DWV has been detected in varroa mite-free colonies, too, these findings were reported after the mite was introduced into the particular country where the investigations were made (3, 9, 11, 18, 22, 26, 28, 31, 34). The transient varroa mite-free status of apiaries could easily be achieved by employing acaricide treatments, but these obviously do not influence the previously acquired DWV-infected status of the colonies. Moreover, infected bees can transmit the virus to other colonies via the previously reported alternative routes. Reports on the presence of DWV in bee colonies unambiguously independent from V. destructor infestation were not found in the scientific literature.
The results of this study indicate that the investigated DWV genotypes belong to one close monophyletic cluster of the virus, despite the fact that the viruses originated from distant geographic regions. DWV was presumably introduced into the honeybee populations more recently than other common bee viruses (e.g., ABPV and SBV), and its evolutionary divergence is just at its beginning. Because the virus multiplies sufficiently in V. destructor, it is possible that an originally mite-infecting virus has adapted recently to A. mellifera or that the virus was transmitted from A. cerena with the help of the arthropod vector. The worldwide occurrence of the virus is most probably the result of the emergence and spread of V. destructor during recent decades. In this context, it is noteworthy that the three investigated honeybee samples from New Zealand were the only ones in our study which were DWV negative.
Further analyses involving DWV genotypes from A. cerena, or other bee species, as well as from varroa mites may reveal the origin of DWV infection in the honeybee.
Two viruses that are closely related to DWV but nonetheless distinct have been identified so far, i.e., KGV and VDV-1. While KGV was isolated in Japan from A. mellifera (16), VDV-1 was detected in Europe in V. destructor (27). Although these viruses are considered by some authors (23) to be biological and geographic variants of DWV, their segregation from the worldwide common DWV genogroup is indicated by the results of this study.
Published ahead of print on 13 April 2007. ![]()
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