Widespread Horizontal Gene Transfer from Circular Single-stranded DNA Viruses to Eukaryotic Genomes
© Liu et al; licensee BioMed Central Ltd. 2011
Received: 28 March 2011
Accepted: 26 September 2011
Published: 26 September 2011
In addition to vertical transmission, organisms can also acquire genes from other distantly related species or from their extra-chromosomal elements (plasmids and viruses) via horizontal gene transfer (HGT). It has been suggested that phages represent substantial forces in prokaryotic evolution. In eukaryotes, retroviruses, which can integrate into host genome as an obligate step in their replication strategy, comprise approximately 8% of the human genome. Unlike retroviruses, few members of other virus families are known to transfer genes to host genomes.
Here we performed a systematic search for sequences related to circular single-stranded DNA (ssDNA) viruses in publicly available eukaryotic genome databases followed by comprehensive phylogenetic analysis. We conclude that the replication initiation protein (Rep)-related sequences of geminiviruses, nanoviruses and circoviruses have been frequently transferred to a broad range of eukaryotic species, including plants, fungi, animals and protists. Some of the transferred viral genes were conserved and expressed, suggesting that these genes have been coopted to assume cellular functions in the host genomes. We also identified geminivirus-like and parvovirus-like transposable elements in genomes of fungi and lower animals, respectively, and thereby provide direct evidence that eukaryotic transposons could derive from ssDNA viruses.
Our discovery extends the host range of circular ssDNA viruses and sheds light on the origin and evolution of these viruses. It also suggests that ssDNA viruses act as an unforeseen source of genetic innovation in their hosts.
In addition to vertical transmission and gene acquisition from other distantly related species via horizontal gene transfer (HGT), organisms can also capture genetic material from extra-chromosomal elements (plasmids and viruses) during evolution. It is widely accepted that phages represent substantial forces in prokaryotic evolution, with the integrated phages (prophages) accounting for as much as 10-20% of some bacterial genomes [1, 2]. In eukaryotes, animal retroviruses, which can integrate into host genome as an obligate step in their replication strategy, comprise approximately 8% of the human genome in the form of inherited endogenous retroviruses . Moreover, the integrated retroviral genes have been demonstrated to play critical role in mammalian reproduction [4, 5]. Recent data reveal that several non-retroviral viruses have also contributed to the genetic makeup of many eukaryotic organisms [6–15]. Especially, genes derived from ancestral nudiviruses have been co-opted to facilitate a parasitic lifestyle in parasitoid wasps ; and a gene derived from partitiviruses was exapted to regulate the activities of the phytohormone auxin, indole-3-acetic acid (IAA) in Arabidopsis thaliana . Still, this type of transfer is thought to be rare in eukaryotes.
Viruses with circular single stranded DNA (ssDNA) genomes are the smallest viruses known to infect eukaryotes and are currently grouped into four families: Anelloviridae, Circoviridae, Geminiviridae and Nanoviridae (Virus Taxonomy: 2009, ICTV, http://www.ictvonline.org/virusTaxonomy.asp?version=2009). The members of the first two families infect vertebrates and of the last two families infect plants. Recently a virus distantly related to circoviruses carrying a covalently closed circular, partially double-stranded ssDNA genome has been found to infect the marine diatom Chaetoceros salsugineum . A similar virus was also discovered in C. debilis . Moreover, recent viral metagenomic studies have shown that small circular ssDNA viruses are more prevalent and diverse in the environment than previously recognized [18–22].
Small circular ssDNA viruses commonly replicate their genomes in the nuclei of infected cells via a rolling circle replication (RCR) mechanism initiated by virus-encoded replication initiation protein (Rep), and there are clear similarities among the sequences of these proteins [23, 24]. So far, no associated integrase activity has been identified for these viruses. However, Bejarano et al  reported multiple repeats of geminiviral Rep DNA that have been integrated into the nuclear genome of tobacco. In addition, Rep-like genes were also found in genomes of the parasitic protozoan Entamoeba histolytica and Giardia intestinalis . These discoveries suggest that the small circular ssDNA viruses could also contribute to the genetic heritage of eukaryotic organisms. Considering that the circular ssDNA viruses are widespread in nature, the role played by these viruses in eukaryotic evolution needs to be evaluated.
Accordingly, we performed a systematic search for sequences related to known small circular ssDNA viruses in the publicly available eukaryotic genome databases. As our study was being prepared for publication, an independent group of investigators reported that sequences related to circoviruses were detected in the genomes of six vertebrate species . Here we report our more comprehensive and convincing results based on sufficiently critical data analysis, bench research and phylogenetic analysis. Our studies have not only corroborated the integration of circovirus-related sequences in these six species, but they have also revealed that numerous sequences related to circoviruses, geminiviruses and nanoviruses have been integrated into the germlines of diverse eukaryotes including plants, fungi, animals and protists. Furthermore, we have demonstrated some of these integrated genes were conserved and expressed in eukaryotic organisms. In addition, we also identified geminivirus-like and parvovirus-like transposable elements in the genomes of fungi and lower animals, respectively. The origin and evolution of small circular ssDNA viruses were also discussed.
Results and discussion
Identification of circular ssDNA viral Rep-related proteins in eukaryotic systems
Rep proteins are commonly encoded by mobile elements (most phages and eukaryotic ssDNA viruses, some plasmids of Gram-positive bacteria, eukaryotic helitron transposons, etc.) but without cellular homologs and therefore have been recognized as virus/plasmid-specific proteins (hallmark proteins) [28, 29]. The Rep proteins of eukaryotic ssDNA viruses contain RCR catalytic domain and a C-terminal NTPase/helicase domain [30, 31]. With such structure, the sequence of the Rep protein of geminiviruses readily detected those of the geminivirus Rep catalytic domain (Gemini_AL1), central domain (Gemini_AL1_M) and the RNA helicase domain (RNA_helicase) by using the NCBI Conserved Domain Database searches (http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi) (see Additional file 1: Figure S1). Likewise, the Rep of nanoviruses and circoviruses detected the putative viral replication protein domain (Viral_Rep) and the RNA helicase domain. On the other hand, the Rep of pLS1 family of prokaryotic plasmids  comprises only plasmid replication protein domain (Rep_2). Recently, the plasmid Rep containing an additional helicase domain has also been reported in a phytoplasma .
To investigate sequences closely related to Rep proteins of eukaryotic circular ssDNA viruses in other systems, we used the representative Rep proteins of geminiviruses and circoviruses to search against NCBI non-redundant (NR) protein database by PSI-BLAST . After removing the known eukaryotic circular ssDNA viruses and subsequent reverse BLAST comparisons, we obtained a final dataset containing 113 Rep-like protein sequences from plasmids, other eukaryotic viruses and cellular genomes respectively (see Additional file 2: Tabular data S1). Most of these belonged to bacterial plasmids (typically phytoplasmal plasmids) and bacterial genomes. Conserved Domain searches showed that, apart from the known Rep_2 plus RNA_helicase domains, Rep proteins from bacterial plasmids had two other types of domain arrangements: Gemini_AL1 plus RNA_helicase and Viral_Rep plus RNA_helicase (see Additional file 1: Figure S1 and Additional file 2: Tabular data S1). The domain organization of Rep-like proteins from bacterial genomes showed high similarity to those of plasmids suggesting that these cellular homologs may originate from their plasmid counterparts.
Our dataset also included the previously reported Rep-like genes in the genome of G. intestinalis isolate BRIS/92/HEPU/1541 and E. histolytica HM-1:IMSS . These genes have been shown to be present in G. intestinalis ATCC 50581 strain GS/M H7 but not present in the G. intestinalis ATCC 50803 WB genome . We found that two sequences of human gut viral metagenome from Genomic Survey Sequence (GSS) database share sequence similarities not only with the Rep-like genes of G. intestinalis but also with their neighboring genes (Figure 1B). This could provide evidence that these genes were originated from integrated proviruses. In addition to the known Rep-like genes in genomes of E. histolytica and G. intestinalis, we also identified 20 new Rep-like genes in genomes of other protozoan species as well as fungi, placozoans, and roundworms (see Additional file 2: Tabular data S1). Among these, the fungal Rep-like proteins contained the geminivirus-like domain, while the rest have domain similarities with Reps of nanoviruses or circoviruses. These findings suggest that the eukaryotic circular ssDNA viral genes may be of widespread occurrence in their host genomes but have yet to be discovered.
Identification and validation of eukaryotic endogenous circular ssDNA virus-like sequences in germline genomes
Numbers of endogenous circular ssDNA virus-like sequences in eukaryotic genomes
No. of virus-related genes
Populus trichocarpa (black cottonwood)
Nicotiana tabacum (common tobacco)
Micromonas pusilla (green algae) CCMP1545
Aspergillus nidulans FGSC A4
Aspergillus fumigatus A1163
Aspergillus niger CBS 513.88
Trichoderma atroviride IMI 206040
Magnaporthe oryzae 70-15 (rice blast fungus)
Nectria haematococca mpVI 77-13-4
Tuber melanosporum Mel28 (Perigord truffle)
Laccaria bicolor S238N-H82 (Bicoloured deceiver)
Entamoeba invadens IP1
Entamoeba histolytica HM-1:IMSS
Entamoeba dispar SAW760
Blastocystis hominis Singapore isolate B (sub-type 7)
Giardia intestinalis ATCC 50581 strain GS/M H7
Giardia intestinalis isolate BRIS/92/HEPU/1541
Phaeodactylum tricornutum (diatom)
Canis lupus familiaris (dog) *
Monodelphis domestica (gray short-tailed opossum) *
Felis catus(domestic cat) *
Ailuropoda melanoleuca (giant panda) *
Choloepus hoffmanni (Hoffmann's two-fingered sloth) *
Aplysia californica (California sea hare)
Xenopus (Silurana) tropicalis(western clawed frog) *
Branchiostoma floridae (Florida lancelet) strain S238N-H82
Brugia malayi (agent of lymphatic filariasis)
Loa loa (African eyeworm)
Wuchereria bancrofti (agent of lymphatic filariasis)
Onchocerca volvulus (agent of onchocerciasis)
Lepeophtheirus salmonis (salmon louse) strain Pacific
mites & ticks
Varroa destructor (honeybee mite)strain Korean
Trichoplax adhaerens (placozoan) strain Grell-BS-1999
Hydra magnipapillata (hydrozoan) strain 105
Altogether, we discovered endogenous virus-like sequences in at least 35 species broadly distributed among nuclear genomes of plants, fungi, animals and protists. Remarkably, no anellovirus-like sequence was detected in any eukaryotic genome, although these viruses have been noted in various animal species .
Characteristics and phylogenies of endogenous circular ssDNA virus-like sequences
Unlike Rep-like sequence in tobacco that were acquired more recently from members of begomovirus , one genera in the family Geminiviridae, the geminiviral Rep-like sequence in Populus, was located at the base of the Geminiviridae clade in the phylogenetic tree (Figure 4), suggesting that it was derived from integration of a Geminiviridae ancestor. Indeed, this sequence was degenerate, containing three inframe stop codons and one frameshift, an indication that it has been inserted a million years ago. Alternatively, it represents a distantly related geminiviral lineage infecting Populus.
All the virus-like sequences from fungi clustered together and were most closely related to the Sclerotinia sclerotiorum hypovirulence associated DNA virus 1 (SsHADV-1) (Figure 4), a mycovirus recently reported by us , suggesting that these endogenous viral sequences originated from SsHADV-1 like mycoviruses. Moreover, the SsHADV-1-like Reps were prevalent in viral metagenomes of different samples, including freshwater, human gut, rice paddy soil, marine environments and mosquito (see Additional file 1: Figure S5).
Our phylogenetic analysis also suggests that the circular ssDNA viruses were likely to co-evolve with their hosts over a long evolutionary time frame. For example, the virus-like sequences from lower eukaryotes (such as protozoans) were generally present at the base of each clade while those in relatively higher eukaryotes were more closely related to the known circoviruses, geminiviruses and nanoviruses that were infecting higher eukaryotes (see Additional file 1: Figure S4). There were, however, several exceptions, possibly due to horizontal viral transfers over short periods of time.
In most cases, the endogenous virus-like sequences from one species clustered together (such as those in salmon louse, honeybee mite, Hydra and roundworm species) (Additional file 1: Figure S4). Sequence comparison showed that, in each genome, some endogenous viral copies may have resulted from segmental duplication within host genomes after a single original integration, as similar levels of identity are observed between them as well as between their flanking genomic regions. While others may have been derived from multiple independent integration events, as no similarity was observed among their flanking genomic sequences (see Additional file 1: Figure S6).
Generally, the copy numbers of integrated viral sequences are less than 10 copies per species; whereas near sixty copies were identified in genomes of salmon louse (Lepeophtheirus salmonis) and honeybee mite (Varroa destructor) (see Additional file 1: Figure S4). Comparison of the viral copies and their adjacent host sequences in these two genomes showed that for most viral copies, no similarity was observed among their flanking genomic sequences, suggesting that most were derived from multiple invasions of same or very similar viruses. However, considering that the Rep protein of eukaryotic ssDNA viruses has DNA binding, endonuclease and NTPase activity required for viral DNA replication [30, 31], the integrated genes encoding Rep-like proteins may catalyze their own single-strand excision and invasion, and therefore act as selfish genetic elements capable of parasite-like proliferation in the host genome. This scenario could be supported by the fact that the putative products of Helitrons , a eukaryotic rolling-circle transposon, shares motifs with the Reps of RCR plasmids and ssDNA viruses. Based on this fact, it has been suggested that ssDNA viruses might have evolved from RC transposons . Our finding of endogenous viral Rep-like genes, however, favors the hypothesis that Helitrons may have arisen from ssDNA viruses which were integrated into the genome of an early eukaryotic ancestor .
Identification of ssDNA virus-like transposable elements in eukaryotic genomes
In addition to the geminivirus-like transposon, we have also identified a parvovirus (linear ssDNA)-like repetitive element in the acorn worm (Saccoglossus kowalevskii) genome (see Additional file 2: Tabular data S3). Like parvoviruses, this repetitive element contains two large ORFs: one putative ORF encodes a protein containing parvovirus non-structural protein NS1 domain (Parvo_NS, pfam01057) and the other putative ORF encodes a protein containing parvovirus coat protein VP1 domain (Parvo_coat_N, pfam08398) (Figure 5B). It also possesses a palindromic hairpin structure at its 5' terminal sequence, which is commonly found in parvoviruses. There are over 50 copies of this repeat interspersed in the genome. Some of these contained degraded ORFs; and some contained only a single ORF or a fragment. Furthermore, we also identified numerous parvovirus non-structural protein-like sequences in genomes of the hydrozoan Hydra magnipapillata and the planarian Schmidtea mediterranea (Figure 5B). We noted that these have been annotated as integrated virus-like element: DENSOV_HM and PIVE in Repbase Update, respectively [Jurka J, Repbase Reports 8(3), 182-182 (2008), http://www.girinst.org/2008/vol8/issue3/DENSOV_HM.html; Rebrikov DV et al. Repbase Reports 8(2), 166-166 (2008), http://www.girinst.org/2008/vol8/issue2/PIVE.html].
Phylogenetic analysis revealed that PIVE is more closely related to planarian (Girardia tigrina) virus, Planaria asexual strain-specific virus-like element (PEVE) (see Additional file 1: Figure S7), which has not yet been found to integrate in host genome . The DENSOV_HM was located at the base of the papillomavirus clade and did not cluster within family Papillomaviridae. Furthermore, their genome structure is different. Therefore it may have originated from integration of the virus in new family infecting hydrozoan. The parvovirus-like sequences from acorn worm grouped together with PIVE_1p and PEVE small segment and these further clustered with parvoviruses. Considering that the genome structure of acorn worm repeated element is also similar to parvoviruses, it is most likely that it derived from parvovirus lineage infecting acorn worm distantly related to known parvoviruses.
Consequently, these findings provide direct evidence that eukaryotic transposons could originated from ssDNA viruses.
Preservation and expression of endogenous viral genes in host genomes
The fact that endogenous viral sequences are conserved and expressed in host organisms suggests that these viral genes have been coopted to assume cellular functions in eukaryotic genomes. It should be noted that expression of mRNA from endogenous viral sequences was also detected in the parasitic protozoan E. histolytica, although their long ORFs were defective (Figure 6). Perhaps selection to maintain these viral sequences has recently been lost.
We also detected endogenous viral sequence-related ESTs in some plants and animal species (see Additional file 2: Tabular data S5). Because the genome sequences of relevant host species are not available or available but not well matched with their ESTs, it remains to be established whether they represent authentic expressed endogenous viral sequences.
The host range of circular ssDNA viruses
Circoviruses are previously known to infect only birds and pigs . These viruses have been detected in dragonflies, fish and human most recently [47–49]. Geminiviruses and nanoviruses only have been found to infect higher plants [23, 50]. Recent metagenomic studies uncovered that these circular ssDNA viruses were commonly found in various environmental samples, but it is difficult to provide information on the host range and ecology for these viruses.
The endogenous circovirus-like sequences in honeybee mite were most closely related to cycloviruses (Additional file 1: Figure S4), members of a recently proposed genus in the family Circoviridae, which were commonly found in faecal samples of human and chimpanzee by viral metagenomics . In addition, the endogenous virus-like sequences from some species of various organisms (such as amphibians, algae, diatom, gastropod, etc.) were clustered with viral metagenomic sequences or circovirus-like genomes identified from environmental samples . These findings suggest that these various species are the definitive hosts of relevant environmental viruses.
Interestingly, although the endogenous circovirus/nanovirus-like sequences occurred widely in the genomes of eukaryotic species, ranging from unicellular organisms to mammals, we did not detect any of these sequences in plant, bird and pig genomes sequenced to-date. In contrast, the nematodes (roundworms) were not known to be infected by ssDNA virus, but endogenous circovirus-like sequences occurred in some nematode species. In addition, geminivirus-like sequences were found in some fungal genomes. So far, however no genetically related exogenous counterparts were found in these fungi, even though some of these, such as the rice blast fungus Magnaporthe oryzae, were widely studied. Likewise, Populus is not known to be infected by geminiviruses but harbored one endogenous geminiviral sequence. These observations suggest that some of the endogenous viral sequences could provide immune protection in the host similar to the endogenous retroviral capsid proteins in mice and sheep, which offer protection against exogenous retroviral infections [52, 53].
The origin and evolution of circoviruses and nanoviruses
Based on the different phylogenies between the N-terminal and the C-terminal regions of circovirus Rep, Gibbs and Weiller  suggested that circovirus Rep proteins may have evolved by a recombination event between the Rep protein of nanoviruses and an RNA binding protein encoded by picorna-like viruses after the nanoviruses switched hosts to infect a vertebrate. However, it seems unlikely that the virus recombination event took place in a vertebrate considering the fact that endogenous circovirus-like sequences were widely found in nonvertebrate species. To examine more thoroughly the origin and evolution of circoviruses and nanoviruses, we selected representative Rep-like proteins from viruses, plasmids and bacterial genomes and used sufficient samples to construct phylogenetic trees. In consideration of a possible recombination event, we aligned and performed phylogenetic analysis corresponding to full-length Rep genes, the N-terminal and C-terminal regions respectively. As shown in Additional file 2: Figure S8, circovirus-like sequences and viral Rep-like sequences from bacterial plasmid and bacterial genomes clustered together in all trees. However, while nanovirus-like sequences clustered with circovirus-like sequences in the N-terminal tree they were grouped with geminivirus-like sequences in the C-terminal tree. In the full-length Rep tree, nanovirus-like sequences were placed between the geminivirus-like and circovirus-like sequences, possibly due to the compromise of different phylogenetic signals from the two parts of nanovirus-like Reps. Therefore, if a recombination event had occurred, it is likely to have taken place in the nanovirus-like Reps rather than in the circovirus-like Reps.
It has been proposed that eukaryotic ssDNA viruses may have evolved from prokaryotic plasmids or phages . In our phylogenetic trees, the virus-like sequences from bacterial plasmid and bacterial genomes were generally located at the base of circovirus-like sequences, suggesting that circoviruses might have originated from relevant bacterial plasmids. Considering that the nanovirus-like sequences clustered with circovirus-like sequences in the N-terminal tree, it is most likely that the nanoviruses shared the most recent common ancestor with circovirus-like viruses and subsequently the C-terminal sequences of ancestor nanoviral Reps may have recombined with those of geminivirus-like viruses or plasmids. But the possibility that nanovirus-like Reps were the result of convergent evolution cannot be ruled out. The Canarypox virus and the ancestor of picorna-like viruses may have captured the helicase domain sequences from circovirus-like viruses by recombination.
The origin and evolution of geminiviruses
Based on the observations that geminiviruses occupied a common ecological niche with phytoplasmas and their Reps shared a most recent common ancestor with phytoplasmal plasmids in phylogenetic analysis, Krupovic et al  proposed that the geminiviruses may have originated from phytoplasmal plasmid followed by acquisition of the capsid gene from an ssRNA plant virus. However, in view of the recent reports on the geminivirus-like mycovirus and numerous related sequences in fungal genomes, the evolutionary relationships among these geminivirus-like elements need to be revaluated. To address this question, we constructed phylogenetic trees for the representative Rep-like proteins from plants, fungi, phytoplasma and algae using the full-length Rep genes, the N-terminal RCR catalytic domain and C-terminal helicase domain sequences respectively (Additional file 2: Figure S9). In all trees, the plant geminiviral Reps clustered together with fungal Reps, suggesting that they shared a more recent common ancestor with those from fungi rather than from phytoplasmal plasmids. Furthermore, although the Rep protein of SsHADV-1 is related to geminiviruses, the genome organization of SsHADV-1 and particle morphology is distinct from those of geminiviruses . Although the capsid protein of SsHADV-1 lacks sequence similarity with those of any geminiviruses, its most similar sequences are commonly found in environmental samples. In addition, sequences related to SsHADV-1 were widely found in fungal genomes and diverse metagenomic samples. These results suggest that SsHADV-1 and related viruses from fungi and environment may have evolved independently rather than being descendent from geminiviruses or vice versa. Therefore, it is possible that the ancestor of geminiviruses and related fungal viruses may have occurred prior to the separation of plants and fungi, and subsequently perhaps they had a unique path to evolution in their hosts.
Our study provided comprehensive and convincing evidence that the genes of small circular ssDNA viruses have been transferred into a broad range of eukaryotic genomes, and some of the transferred genes were also conserved and functional in host genomes. This discovery extends the host range of circular ssDNA viruses and offers insight into the origin and evolution of relevant viruses. Furthermore, our finding also revealed that the capture and functional assimilation of exogenous viral genes may represent an important force in the evolution of eukaryotes.
In order to screen for the circular ssDNA virus-related sequences in eukaryotic genomes, we performed tBLASTn searches against different NCBI sequence databases (http://blast.ncbi.nlm.nih.gov/Blast.cgi) using as queries the representative peptide sequences derived from viruses in families Anelloviridae, Circoviridae, Geminiviridae and Nanoviridae. NCBI databases used for sequence searches included nr (all GenBank + RefSeq Nucleotides + EMBL + DDBJ + PDB sequences + HTGS phase 3 but excluding HTGS phase 0,1,2, EST, GSS, STS, PAT, WGS), refseq_genomic (genomic entries from NCBI's Reference Sequence project), NCBI Genomes/chromosome (a database with complete genomes and chromosomes from the NCBI Reference Sequence project), wgs (a database for whole genome shotgun sequence entries), gss (Genome Survey Sequence, includes single-pass genomic data, exon-trapped sequences, and Alu PCR sequences), htgs (unfinished High Throughput Genomic Sequences: phases 0, 1 and 2), and the eukaryotes genomic BLAST database. All non-redundant matches from these searches with E-values ≤1e-5 were extracted along with 1 kb of flanking regions, and then were used to screen the non-redundant (NR) protein database using BLASTx. All genomic sequences from host genomes that unambiguously matched viral proteins were considered as candidate endogenous viral sequences. These candidate endogenous viral sequences were used to research the databases for other homologous sequences that would have been missed during initial searches using the known extant viruses. All database searches were performed online and were completed in June 2010.
Examining possible chimeras or errors in assembling of endogenous viral sequences
To rule out the possibility that these endogenous viral sequences were chimeric clones or misassembled from contaminated sequences of exogenous incidental viral sequences, we searched against archival data of the eukaryotic genome sequencing using their endogenous viral sequences and flanking cellular sequences as megaBLAST queries on the NCBI Trace Archive (http://www.ncbi.nlm.nih.gov/blast/mmtrace.shtml) with the cut-off value: > 95% nt identity, respectively; and carefully examined the junctions between endogenous viral sequences and cellular sequences. The statistics of junction coverages that show the number of trace records containing the junctions between endogenous viral sequences and cellular sequences are listed in Additional file 2: Tabular data S2.
Sequence comparison and phylogenetic analysis
The putative peptides of endogenous viral sequences were obtained according to BLASTx hits and manual editing. The in-frame stop codons were indicated as X. Multiple alignments of protein sequences were constructed either using MCOFFEE (when the number of sequences < 50)  or using COBALT  (http://www.ncbi.nlm.nih.gov/tools/cobalt/cobalt.cgi?link_loc=BlastHomeAd) and manually edited. To give the best alignment, the alignment parameter Constraint E-value and Word Size were adjusted for different datasets when using COBALT. Although many of the endogenous viral sequences are of different lengths in alignments, it is now well known that sequences of very different lengths can be accurately placed on phylogenies . Hence, all the putative peptides of endogenous viral sequences were used for the phylogenetic analysis with proteins of representative exogenous viruses to determine the full-scale evolutionary relationships among them. Maximum likelihood (ML) phylogenies were estimated using amino acid sequence alignments with PhyML-mixtures [59, 60], assuming the EX2 mixture model  and SPR tree topologies search strategy . Gaps in alignment are systematically treated as unknown characters. The reliability of internal branches was evaluated based on approximate likelihood ratio test (aLRT) statistics .
Detection of expression of endogenous viral sequences from host genomes
To investigate whether endogenous viral sequences could be expressed in host genomes, we first, used the endogenous viral sequences to screen the NCBI EST database using the method described in Genome screening. Subsequently, we used the identified virus-related ESTs to compare with host genomes and virus genomes by megaBLAST to determine whether they were expressed sequences from host genomes or the result of laboratory contamination.
PCR amplification and DNA sequencing
Genomic DNA samples of dog (Canis lupus familiaris) and cat (Felis catus) were obtained from Zyagen Laboratories (USA). To PCR amplify the candidate DNA fragments from these DNA samples, primers pairs were designed based on the virus-like sequences and their flanking cellular sequences, see Additional file 3: Table S1 for the primers pairs used. PCR products were fractionated by gel electrophoresis on 1% agarose gels and stained with ethidium bromide. DNA was sequenced by Sanger methods at the Beijing Genomics Institute (BGI). New sequences generated in this study were deposited in the GenBank under accession numbers: JF414126-JF414131.
List of abbreviations
approximate likelihood ratio test
Basic Local Alignment Search Tool
Expressed Sequence Tags
Genomic Survey Sequence
High Throughput Genomic Sequence
horizontal gene transfer
International Committee on Taxonomy of Viruses
National Center for Biotechnology Information
open reading frame
Polymerase chain reaction
rolling circle replication
subtree prune and regraft
terminal inverted repeats
target site duplications
replication initiator protein
Whole Genome Shotgun.
We thank the anonymous reviewers for their constructive and helpful comments.
This research was supported in part by the Program for New Century Excellent Talents in University (NCET-06-0665), the Commonweal Specialized Research Fund of China Agriculture (3-21) and the Huazhong Agricultural University Scientific & Technological Self-innovation Foundation.
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