Molecular characterization of the first saltwater crocodilepox virus genome sequences from the world’s largest living member of the Crocodylia

Abstract

Crocodilepox virus is a large dsDNA virus belonging to the genus Crocodylidpoxvirus, which infects a wide range of host species in the order Crocodylia worldwide. Here, we present genome sequences for a novel saltwater crocodilepox virus, with two subtypes (SwCRV-1 and -2), isolated from the Australian saltwater crocodile. Affected belly skins of juvenile saltwater crocodiles were used to sequence complete viral genomes, and perform electron microscopic analysis that visualized immature and mature virions. Analysis of the SwCRV genomes showed a high degree of sequence similarity to CRV (84.53% and 83.70%, respectively), with the novel SwCRV-1 and -2 complete genome sequences missing 5 and 6 genes respectively when compared to CRV, but containing 45 and 44 predicted unique genes. Similar to CRV, SwCRV also lacks the genes involved in virulence and host range, however, considering the presence of numerous hypothetical and or unique genes in the SwCRV genomes, it is completely reasonable that the genes encoding these functions are present but not recognized. Phylogenetic analysis suggested a monophyletic relationship between SwCRV and CRV, however, SwCRV is quite distinct from other chordopoxvirus genomes. These are the first SwCRV complete genome sequences isolated from saltwater crocodile skin lesions.

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Molecular characterization of
the rst saltwater crocodilepox
virus genome sequences from the
world’s largest living member of the
Crocodylia
Subir Sarker1, Sally R. Isberg2,3, Natalie L. Milic3, Peter Lock4 & Karla J. Helbig1
Crocodilepox virus is a large dsDNA virus belonging to the genus Crocodylidpoxvirus, which infects a
wide range of host species in the order Crocodylia worldwide. Here, we present genome sequences
for a novel saltwater crocodilepox virus, with two subtypes (SwCRV-1 and -2), isolated from the
Australian saltwater crocodile. Aected belly skins of juvenile saltwater crocodiles were used to
sequence complete viral genomes, and perform electron microscopic analysis that visualized immature
and mature virions. Analysis of the SwCRV genomes showed a high degree of sequence similarity to
CRV (84.53% and 83.70%, respectively), with the novel SwCRV-1 and -2 complete genome sequences
missing 5 and 6 genes respectively when compared to CRV, but containing 45 and 44 predicted unique
genes. Similar to CRV, SwCRV also lacks the genes involved in virulence and host range, however,
considering the presence of numerous hypothetical and or unique genes in the SwCRV genomes, it
is completely reasonable that the genes encoding these functions are present but not recognized.
Phylogenetic analysis suggested a monophyletic relationship between SwCRV and CRV, however,
SwCRV is quite distinct from other chordopoxvirus genomes. These are the rst SwCRV complete
genome sequences isolated from saltwater crocodile skin lesions.
e crocodilepox virus belongs to the genus Crocodylidpoxvirus, a member of the subfamily Chordopoxvirinae
in the family Poxviridae. Whilst currently only represented by Nile crocodilepox virus (CRV), isolated from Nile
crocodiles (Crocodylus niloticus), poxvirus-like lesions have been identied on many of the Crocodylia order
worldwide1–6. However, relatively little is known about the origins, worldwide host distribution and genetic diver-
sity of this class of viruses. Morphologically, the Nile crocodilepox virus (CRV) and other unclassied crocodilian
poxvirus virions are similar to orthopoxvirus virions, demonstrating a brick-like shape with rounded corners and
having a dumbbell-shaped central core and lateral bodies1,3,7. However, they also display the regular, crisscross
surface structure pattern characteristic of parapoxvirus virions7–11.
Poxviruses have been shown to infect the crocodilians as well as other reptiles, including Hermann’s tortoise,
wild ap-necked chameleon, and lizards around the world12. e rst report of poxvirus-associated disease in
a reptile was in captive caimans (Caiman crocodilus) in the USA, and the aected animals have been shown to
develop gray-white skin lesions on various parts of the body9. In Nile crocodiles (C. niloticus), poxvirus infec-
tions have been associated with high morbidity but low mortality, and the aected skin demonstrates brownish
wart-like lesions that can occur over the entire body5,13.
Saltwater crocodiles (Crocodylus porosus) also present with poxviral lesions and the virus is considered a
signicant pathogen causing substantial economic loss for producers in Australia1,3–5,14. C. porosus are farmed
mainly to produce high-quality leather for the international leather market and any disease that results in
1Department of Physiology, Anatomy and Microbiology, School of Life Sciences, La Trobe University, Bundoora,
VIC 3086, Australia. 2Centre for Crocodile Research, Noonamah, NT, Australia. 3School of Psychological and Clinical
Sciences, Charles Darwin University, Darwin, NT, Australia. 4La Trobe Institute for Molecular Science, La Trobe
University, Bundoora, VIC, Australia. Correspondence and requests for materials should be addressed to S.S. (email:
s.sarker@latrobe.edu.au)
Received: 15 November 2017
Accepted: 20 March 2018
Published: xx xx xxxx
OPEN
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downgrading of hides causes signicant nancial loss2,3. A recent study by Moore et al.3 conrmed that there
was no breach in the basement membrane of poxvirus infected saltwater crocodile skins using histopathological
examination on dierent phases of poxvirus lesion progression. is study also postulated that allowing enough
time for poxvirus infection to resolve might have no detrimental eect on skin quality if there are no obvious
poxvirus lesions developing in the interim. In contrast to this study, Huchzermeyer et al.15 showed that there was
a potential connection between poxvirus infection and deep lesions that remained as large depressed foci on the
tanned skin of C. niloticus. In addition, the raised nodules (6–8 mm)7,13, and brown discolouration seen on live
animal skins of C. niloticus5.
Although crocodile poxvirus has evolved to infect the species within the order Crocodylia worldwide, to date
only one crocodile poxvirus genome has been published; a Nile crocodilepox virus (CRV)1. Excepting the recent
study by Moore et al.3, who have demonstrated the genetic evidence of poxvirus in saltwater crocodile using a
PCR targeted to amplify partial sequences, there is no other study characterizing the complete genome of saltwa-
ter crocodilepox virus (SwCRV). erefore, the aim of the present study was to identify and characterize geneti-
cally and microscopically of the SwCRV genome sequences and virion morphology respectively that is associated
with clinical disease in Australian saltwater crocodiles sourced from the Darwin Crocodile Farm, Northern
Territory in 2017. To the best of our knowledge, this is the rst report of complete poxvirus genome sequences
from the saltwater crocodile.
Results
Identication of poxvirus infection in farmed saltwater crocodiles (C. porosus). Aected belly
skin of a juvenile saltwater crocodile demonstrated an early active poxvirus lesion in the mid-scale region
(Fig.1A), an active lesion on the upper scale margin (Fig.1B), and two linear blemishes ranging from the active
to expulsion stages (Fig.1C) of poxvirus lesions as dened by Moore et al.3. Transmission electron microscopy
(TEM) analysis of negatively stained exudate sourced from two dierent juvenile saltwater crocodiles was per-
formed. Two stages of viral enveloped including brick-shaped virus particles (Fig.1E and F) indicating active
poxvirus infection in these crocodiles were identied by TEM. According to Harrison et al.16, two dierent stages
of virus particles can be observed: the immature virion (IV) (Fig.1E, white arrow) and an intracellular mature
virion (IMV) (Fig.1E, orange arrow). Morphologically, the immature virions were brick shaped with rounded
corners, whereas the intracellular mature virion had a dumbbell-shaped central core with lateral bodies. e
Figure 1. Macroscopic (top panel) and transmission electron microscopic (bottom panel) analysis of saltwater
crocodile tissues infected with poxvirus. Belly skin of juvenile saltwater crocodile showing poxvirus lesions as
dened by Moore et al.3 (A) has an early active (black arrow) and active (white arrowhead) poxvirus lesion in
the mid-scale region, (B) has an active lesion on the upper scale margin, and (C) shows poxvirus lesions along
two linear blemishes ranging from the active to expulsion stages. Dierent stages of virus maturation (D–F)
including immature virion (IV) (E, white arrow), and intracellular mature virion (IMV) (F, orange arrow) were
imaged by transmission electron microscopy.
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virions had a length of approximately 200 to 250 nm and a width of 120 to 150 nm. To further conrm the pres-
ence of poxvirus, a PCR targeting a conserved region of the RNA polymerase subunit gene was performed17.
Genome structure and analysis of SwCRV. Poxvirus genomic material was isolated from two separate
poxvirus lesions on two separate crocodiles both from Darwin Crocodile Farm. e assembled saltwater croco-
dilepox virus subtypes 1 and 2 (SwCRV-1 and -2) complete genomes were linear double-stranded DNA molecules
of 187,976 and 184,894 bp, respectively. Nucleotide composition averaged over 62% G + C content for each of the
two isolates analysed here (Fig.2A), which is relatively high compared to other chordopoxviruses (ChPVs). Like
Nile crocodilepox virus (CRV), SwCRV genomes contained a large central coding region bounded by two identi-
cal inverted terminal repeat (ITR) regions. e assembled ITRs of SwCRV-1 and -2 contained 1,700 and 1,254 bp,
respectively. Large variations in the size of ITRs were detected between SwCRV-2 and CRV genomes, and this var-
iation has commonly been found in other ChPVs18,19. Importantly, each of the inverted repeats constitutes arrays
of direct repeats, and ve tandem repeats detected within each ITR region of SwCRV-1, which consisted of 18,
12, and three 6 bp repeat units that shared approximately 88–100% nucleotide identity with each other. Whereas,
ITRs of SwCRV-2 comprised four tandem repeats within each ITR region consisting of an 18, two 12, and one
6 bp repeat unit and sharing approximately 88–91% nucleotide identity with each other. ese direct repeat arrays
are smaller in size than those detected previously in other ChPVs, however we cannot rule out the possibility that
they extend beyond the sequenced portion of the genome.
Further analysis of ITRs regions of SwCRV, detected a putative concatemer resolution motif (Supplementary
FigureS1) in the very terminus of each ITR, and this motif has been found to be essential for the resolution of
concatemeric DNA molecules during Vaccinia virus replication20. is motif spans nucleotides 126 to 145 and
79–98 (SwCRV-1 and -2, respectively) of each terminal nucleotide of the SwCRV two-DNA strand region, which
is consistent with the previous reports in the case of CRV and other members of the ChPVs1,21. e concate-
mer resolution motifs of SwCRV were 100% similar to CRV, and consistently these motifs have been detected
within the 150-bp terminal-most regions of the two DNA strands of ChPV members1,21. Furthermore, a hairpin
loop-like secondary structure has been predicted in the terminal region of the SwCRV genome, which is consist-
ent with previous reports21,22, however, its functionality needs to be assessed further.
Despite the high G+C content of SwCRV and the paucity of stop codons, 218 and 215 methionine-initiated
ORFs encoding proteins of 50 to 1894 amino acids (aa) in length were observed in SwCRV-1 and -2, respectively,
Figure 2. Comparative genome architectures of the SwCRV-1 and -2. (A) Sequence alignment of saltwater
crocodilepox virus subtype 1 (SwCRV-1, GenBank accession number MG450915) to the reference Nile
crocodilepox virus (CRV, DQ356948) genome. (B) Sequence alignment of saltwater crocodilepox virus
subtype 2 (SwCRV-2, GenBank accession number MG450916) to the reference Nile crocodilepox virus (CRV,
DQ356948) genome. e alignment was performed using the global alignment program contained in CLC
Genomic Workbench (tool for Classical sequence analysis). e middle graphs in A and B represent the
sequence conservation between the aligned SwCRV and CRV sequences at a given coordinate on the base
sequence. e bottom graphs in A and B represent the gap fractions which are mostly for insertion and deletion
between two representative viral genomes. (C) A sequence alignment using MAFFT in Geneious (version
10.2.3), and comparative ORF map of SwCRV and CRV. Protein coding ORFs, with blue arrows depicting the
direction of transcription, whereas the orange blocks depicted Inverted Terminal Repeats (ITRs), respectively.
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and were numbered from le to right (Fig.2B, and Supplementary TableS1). Comparison of the predicted ORF pro-
tein sequences to the non-redundant protein sequence database at the National Center for Biotechnology Information
(NIH, Bethesda, MD) using BLASTP identied homologs with signicant protein sequence similarity (E value ≤ e-4)
for 173 and 171 genes in SwCRV-1 and -2, respectively (Supplementary TableS1). Interestingly, there were 45 and 44
predicted protein coding genes in SwCRV-1 and -2, respectively, that were not present in any other poxvirus, nor did
they match any sequences in the NR protein database using BLASTP; these unique ORFs encoded proteins of 50 to 876
aa in length (Supplementary TableS1). Among the predicted protein coding genes without detectable homologs, three
contained predicted transmembrane segments detected by HHpred (Supplementary TableS1).
Among the predicted protein-coding genes of the SwCRV-1 and -2 genomes, respectively 173 and 171 pre-
dicted gene were homologs to other ChPV gene products (Supplementary TableS1). Among these conserved
chordopoxvirus gene products, the highest number of protein-coding genes in SwCRV-1 and -2 genomes (168
and 167, respectively) demonstrated homology to the Nile crocodilepox virus. In contrast to CRV, the genomes of
SwCRV-1 and CRV-2 included two additional genes encoding virion protein (SwCRV-1, ORF145 and SwCRV-2,
ORF146) and early transcription factor VETFs (SwCRV-1, ORF147 and SwCRV-2, ORF144) which were iden-
tied as homologs of gene products of the Orf parapox virus in sheep (Supplementary TableS1). Furthermore,
the SwCRV-1 genome included an insertion sequence encoding an uncharacterized protein (ORF156), also from
the Orf parapox virus in sheep (Supplementary TableS1). As expected from the higher percent nucleotide iden-
tity between SwCRV and CRV (84.53 and 83.70%, respectively), SwCRV-1 and -2 were found to be consider-
ably closer to CRV. In comparison to CRV, ve ORFs with unknown functions (CRV024, CRV029, CRV156,
CRV171 and CRV172) were commonly missing in both the SwCRV-1 and -2 genomes, and a further ORF of CRV
(CRV035) was missing in SwCRV-2. A further 2 genes (CRV-ORF41 and ORF58) were signicantly fragmented
in both of the SwCRV genomes. All conserved genes of SwCRV-1 and -2 showed the highest sequence similarity
to the orthologs from CRV and orthopoxvirus, and these observations imply that the conserved SwCRV-1 and
-2 genes share an evolutionary history (Supplementary TableS1) with them. Overall, the central genomic core
region of the SwCRV-1 and -2 contained homologues of conserved poxvirus genes involved in basic replicative
mechanisms (RNA transcription and modication, and DNA replication), in virion structure, and in morpho-
genesis of intracellular mature, intracellular enveloped, and extracellular enveloped virions (IMV, IEV and EEV,
respectively). e terminal regions being highly divergent, containing most of the predicted unique genes (Fig.2B
and Supplementary TableS1); which is consistent with other ChPVs23,24.
Comparison between SwCRV-1 and -2 genomic structures. At the genomic level, SwCRV-1 and -2
genomes shared 97.80% nucleotide identity, and contain 205 genes with the same relative order and orientation,
of which 43 were unique to SwCRV. Recent studies by Sarker et al.25 postulated that a 1% dierence between
genomes corresponds to approximately 10 mutations in an average sized gene any of which could signicantly
aects gene function if an early STOP codon is introduced to the gene sequence. Likewise, small changes within
the promoter and enhancer regions of genes can signicantly alter gene expression levels. Comparison of the
two SwCRV genomes showed that three ORFs (SwCRV1–044, 045 and 156) with unknown functions were miss-
ing in SwCRV-2 (Supplementary TableS1). Furthermore, there were several occurrences of gene translocations,
including the position of genes, observed between the two SwCRV genomes (highlighted as grey shading in
Supplementary TableS1). Collectively these points of dierence support inclusion of SwCRV-1 and SwCRV-2 as
separate subtypes within the same species.
SwCRV proteins of special interest. F-box proteins. F-box proteins are critical components for catalys-
ing the ubiquitination of proteins, and serve as substrate-specic/recognition subunits in many critical cellular
functions26–28. However, the function of F-box proteins found in poxviruses is not well understood. As with CRV,
SwCRV also contains 9 genes encoding F-box domains (Supplementary TableS1), which comprise the largest
gene family within the ChPVs. However, the F-box proteins of SwCRV were relatively diverse in comparison to
the homologous proteins encoded by CRV. e amino acid identities of encoded F-box genes between SwCRV
and CRV ranges from 76.6 to 96.9%, with the highest diversity being demonstrated in ORF211 and ORF208 for
the SwCRV-1 and -2, respectively. Seven of the SwCRV F-box proteins were located in tandem in the le-terminal
genomic region, and ranged in size from 189 to 288 amino acids in length. Whereas, SwCRV1-ORF211 and
SwCRV2-ORF208 encoded F-box proteins were located in the right-terminal genomic region, and showed signif-
icant divergence with respect to the homologous CRV F-box proteins (Supplementary TableS1).
GyrB-like ATPase domain gene family. Both SwCRV-1 and -2 contained 7 genes (SwCRV1-ORF109, 110, 112,
114, 116, 117, 119, and SwCRV2-ORF107, 108, 110, 112, 114, 115, 117, respectively) encoding DNA gyrase B subunit
(GyrB)-like ATPase domain that shared approximately 11 to 31% amino acid identity with each other (Supplementary
TableS1). The SwCRV GyRB-like ATPase domain gene family shared significant protein homology to the CRV
GyrB-like ATPase domain of type II DNA topoisomerases (topo II), with amino acid identities ranging from 81 to 95%.
Similar to CRV1, SwCRV GyrB-like ATPase domain ORFs were tandemly arranged in the central genomic location.
However, arrangement of the repeats does not share co-linearity with other ChPVs (Supplementary TableS1). In con-
trast to other ChPVs, the GyrB-like ATPase domain gene family was commonly found in the CRV genome, along with
the newly sequenced SwCRV genomes from the present novel host species (C. porosus). is suggests that GyrB-like
ATPase domain genes family is unique for the genus crocodylipoxvirus.
B22R-like proteins. B22R-like genes are the largest known poxvirus genes that encode proteins of unknown
function, but which are predicted to contain transmembrane domains1. SwCRV-1 and -2 contained four copies
of the B22R gene (SwCRV1-ORF51–53, 55, and SwCRV2-ORF49–51, 53, respectively), that shared between 67
to 86% amino acid identity to the CRV B22R genes (Supplementary TableS1). In contrast to CRV, SwCRV1- and
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-2 contained an additional copy of a predicted B22R-like gene (SwCRV1–052 and SwCRV2–050, respectively),
which also shared 80% protein homolog to CRV041. Comparable to CRV and avipoxviruses, SwCRV B22R-like
genes were located in a genomic region, which was distinct from mammalian ChPV B22R genes homologues
(Supplementary TableS1). Importantly, one of the predicted B22R-protein coding genes in SwCRV-1 and -2
(SwCRV1–053 and SwCRV2–051, respectively) encodes a product that is signicantly shorter in length (1385
AA) than the encoded product of the homologous gene in CRV (1874 AA), and demonstrated a low-level of
protein similarity (>67%) within the B22R-like gene family.
Evolutionary relationships of SwCRV. Considering the fact that the saltwater crocodilepox virus sub-
types 1 and 2 (SwCRV-1 and -2) were genomically most closely related to CRV, multiple-nucleotide alignments
from selected complete poxvirus genome sequences were used to construct a ML phylogenetic tree and calculate
the distance matrix. e unrooted phylogenetic tree (Fig.3) derived from these complete genome sequence align-
ments conrmed that these two SwCRV subtypes, isolated from saltwater crocodiles, were mostly related to CRV,
and revealed the highest closest homology to the CRV sequence (84.53% and 83.70% sequence identity between
CRV and SwCRV-1 and -2, respectively, Fig.3). A higher distance between the SwCRV and other selected poxvi-
rus genome sequences was observed by highlighting the low level of sequence identity (22.32 to 33.07%).
To better understand these evolutionary relationships, the complete coding sequences of the DNA polymerase
and DNA Topoisomerase genes were utilised to assemble phylogenetic trees and calculate distance matrixes, as
has been performed previously24,29. A robust clade support (100%) between the two SwCRV and CRV was shown
using a phylogenetic tree analysis in combination with a pairwise amino acid comparison using the DNA poly-
merase gene (Fig.4A), with the DNA polymerase gene orthologs of SwCRV demonstrating amino acid identities
ranging between 43.76 and 94.24%, to other chordopoxviruses (Supplementary FigureS2A). Interestingly, we also
observed a closer and well-resolved evolutionary relationship using the DNA topoisomerase coding sequences
(Fig.4B), which demonstrated a very strong clade support (100%) between the SwCRV’s and CRV (Fig.4B), with
the protein sequence of the DNA topoisomerase gene of SwCRV exhibiting the highest (>96% aa identity) simi-
larity with CRV (Supplementary FigureS2B).
Discussion
is paper fully characterises the genomes of the rst saltwater crocodilepox virus subtypes 1 and 2 (SwCRV-1
and -2) directly from typical poxvirus lesions on juvenile C. porosus belly skins. Poxvirus infection in juvenile salt-
water crocodile was rst reported in 199230 and a very recent study has conrmed the presence of poxvirus-like
particles in the lesion by using electron microscopy, and via PCR amplication of the RNA polymerase subunit
gene3. At this stage, no taxonomic classication has been granted for SwCRV by the International Committee on
Taxonomy of Viruses (ICTV; https://talk.ictvonline.org/taxonomy/)6 due to the lack of genome sequence data
and unclear evolutionary history with other members of the family Poxviridae. In this study, we determined
two novel SwCRV subtypes 1 and 2 (SwCRV-1 and -2) complete genome sequences with predicted full-length
coding regions including ITRs, and these SwCRV genomes should be considered as new species under the genus
Crocodylidpoxvirus. We also further established the ancestry history of SwCRV with other closely related mem-
bers of the Chordopoxvirinae subfamily.
e nucleotide sequences of SwCRV-1 and -2 are signicantly dierent to the Nile crocodilepox virus (CRV)
(>84% and >83% similarity, respectively) but had close similarity to each other (>97%). SwCRV-1 and SwCRV-2
both displayed genetic distance and a novel genome structure in comparison to CRV, including the absence of
5 and 6 genes respectively, and the signicant fragmentation of a further 2 genes most likely to cause them to be
non-functional. With the exception of three predicted protein coding genes which have been detected to contain
transmembrane segments using HHpred, SwCRV-1 and -2 contained 45 and 44 predicted protein-coding genes
respectively, which are not found in any other known proteins in the NIH database, and is overall suciently
genetically dierent to be considered a separate virus species.
Similar to CRV, SwCRV also lacks the genes involved in virulence and host range, including those involving
the interferon response, intracellular signalling, and host immune response modulation. Nonetheless, the novel
SwCRV-1 and -2 complete genome sequences contained an additional 45 and 44 predicted ORFs, respectively
with unknown functions, which are found to be unique at this time. It might be possible that these unique ORFs
in SwCRV potentially have roles to modulate host immune response, which needs to be examined further. Recent
studies in eukaryotes have revealed that the small ORFs could represent potential steps in gene, peptide and pro-
tein evolution31,32. For example, small ORFs (sORFs; < 100 amino acids) in Saccharomyces cerevisiae are essential
for viability33, whereas, several peptides encoded by sORFs are involved in activating transcription factors related
to epidermal morphogenesis in Drosophila34.
Like many other members of the Poxviridae, crocodile poxvirus encodes multiple proteins which may have
functions related to ubiquitin ligase (E3) enzyme components of the ubiquitin proteolytic pathway. is path-
way is a potent mechanism for these viruses to selectively target proteins to alter protein functions and/or sort
specic protein targets to the proteasome for degradation. Although these proteins are known to be important
in some poxvirus lifecycles, their exact function remains unknown1,35–39. Similar to CRV, SwCRV contains a
number of proteins related to ubiquitination, including proteins containing F-box motifs, Really Interesting New
Gene (RING) nger motifs and one homologue of anaphase-promoting complex (APC)/C subunit 11 (Apc11)
(Supplementary TableS1). F-box-like motifs were also recently detected in poxvirus ankyrin repeat (ANK) pro-
teins, however, clear compositional dierences to typical F-box proteins raise questions regarding the classica-
tion and function of this motif 38. In-vivo studies on Orf virus demonstrate that poxviral F-box-like motif acts
as a functional F-box, and it interacts dependently on the poxviral F-box-like motif and the adaptor subunit of
the complex (SKP1)38. However, the function and signicance of the F-box-like motif more broadly in crocodile
poxviruses remains to be discovered.
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e SwCRV genomes also encode for an APC11-like protein (79 amino acids), which is a member of the
anaphase promoting complex/cyclosome, the largest known cellular ubiquitin ligase complex, which is involved
in controlling cell-cycle regulation40. e swCRV APC-like protein is homologous to Nile crocodilepox virus
CRV047, Molluscum contagiosum virus (MOCV) MC026L, parapox virus 014, and Squirrelpox virus 026L. Only
a small number of poxviruses encode Apc11 homologues, a protein which demonstrates sequence similarities to
the mammalian cellular APC/C subunit 11 (Apc11), which has evolved from a RING-type E3 ligase, and perhaps
suggests that the ring box E3 subunits originated from a diverse host species36,41. One of the hallmark functions
of the RING-H2 domain of cellular Apc11 proteins is to allow ubiquitination of protein substrates aer a direct
interaction with E2 enzymes to recruit it to the APC42–44. However, sequence analysis has demonstrated that
the poxvirus APC11-homolouge proteins contain sequence mutations in the E2 ligase binding domain, render-
ing them unable to bind E2 ligases39. Both SwCRV-1 and SwCRV-2 display high similarity to the CRV protein
CRV047 (92.7%, Supplementary TableS1), and also contain sequence mutations in the hypothesised E2 ligase
binding domain of the APC11-like protein. e functionality of these APC11- homologues in other poxviruses
is not well understood but recent studies looking at PACR function, the Orf virus APC11-like protein, demon-
strated that expression of the viral protein led to cell cycle deregulation and acted as a negative regulator of APC
function. Deletion of this gene from the Orf virus also led to reduced viral replication41. us, it has been hypoth-
esised that poxvirus APC-11-homolouges may act as viral mimics of the mammalian APC11 gene, to negatively
regulate cell cycle and promote viral replication.
Figure 3. Phylogenetic tree among selected complete genome sequences of poxviruses. e ML tree was
constructed from a multiple-nucleotide alignment from the selected complete genome sequences of poxviruses.
e numbers on the le show bootstrap values as percentages, and the clade consisted with SwCRV was
highlighted using saltwater crocodile shading (taken and supplied by author S.R.I.). e GenBank accession
details for poxviruses were used: CRV (Nile crocodilepox virus, DQ356948); MOCV1 (Molluscum contagiosum
virus subtype 1, MCU60315); MOCV2 (Molluscum contagiosum virus subtype 2, KY040274); SQPV (Squirrel
poxvirus, HE601899); BPSV (Bovine papular stomatitis virus, KM875470); PPRD (Parapoxvirus red deer,
KM502564); ORFV (Orf virus, DQ184476); PCPV (Pseudocowpox virus, GQ329670); SwCRV-1 (saltwater
crocodilepox virus subtype 1, MG450915); SwCRV-2 (saltwater crocodilepox virus subtype 2, MG450916);
DPV (Deerpox virus, AY689436); GPV (Goatpox virus, KC951854); MYXV (Myxoma virus, KP723391); RFV
(Rabbit broma virus, AF170722); LSDV (Lumpy skin disease virus, NC_003027); FWPV (Fowlpox virus,
AF198100); SWPV-1 (Shearwaterpox virus-1, KX857216); YMTV (Yaba monkey tumor virus, NC_005179);
MPXV (Monkeypox virus, JX878407); VACV (Vaccinia virus, AY678275); YKPV (Yoka poxvirus, HQ849551);
SWPV (Swinepox virus, NC_003389).
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Similar to other chordopoxviruses, SwCRV also contained four copies of the B22R gene that shared approxi-
mately between 67 to 86% amino acids identity to the CRV B22R gene. In contrast to CRV, both SwCRV genomes
had an additional copy of the predicted B22R-like gene. B22R is a surface glycoprotein that is well conserved in the
ChPVs genus and has only one possible homolog outside the poxvirus family, in cyprinid herpesvirus 3, member of
the family Alloherpesviridae45. B22R is present in every chordopoxvirus genus except parapoxvirus, and is the largest
known protein encoded by poxviruses. It is predicted to contain carboxyl-terminal transmembrane domains and
cysteine residues which may mediate disulde bond formation, however, its function is still unknown45,46.
Figure 4. Phylogenetic tree and pairwise comparison of the DNA polymerase and DNA topoisomerase genes.
e ML tress were constructed from the protein sequences of selected poxviruses using DNA polymerase
genes (A), and DNA topoisomerase genes (B). e numbers on the le show bootstrap values as percentages,
and SwCRV clade was highlighted using saltwater crocodile shading (taken and supplied by author S.R.I.). e
abbreviations for poxviruses were used: CRV (Nile crocodilepox virus); MOCV1 (Molluscum contagiosum
virus subtype 1); MOCV2 (Molluscum contagiosum virus subtype 2); SQPV (Squirrel poxvirus); BPSV (Bovine
papular stomatitis virus); PPRD (Parapoxvirus red deer); ORFV (Orf virus); PCPV (Pseudocowpox virus);
SwCRV-1 (saltwater crocodilepox virus subtype 1); SwCRV-2 (saltwater crocodilepox virus subtype 2); DPV
(Deerpox virus); GPV (Goatpox virus); MYXV (Myxoma virus); RFV (Rabbit broma virus); LSDV (Lumpy
skin disease virus); FWPV (Fowlpox virus); SWPV-1 (Shearwaterpox virus-1); YMTV (Yaba monkey tumor
virus); MPXV (Monkeypox virus); VACV (Vaccinia virus); YKPV (Yoka poxvirus); SWPV (Swinepox virus).
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With the exception of bacteria like Treponema pallidum subsp. Pallidum, ermotoga maritima, Bacillus
subtilis and Halomonas variabilis47–50, the GyrB-like ATPase domain has only been found in CRV and SwCRV
genomes which makes them unique amongst the chordopoxviruses. GyrB-like ATPase domain is a member of
the type-II topoisomerase family of the ATP-dependent enzymes that catalyze topological DNA rearrangement
in bacteria47. In the case of Nile crocodilepox virus (CRV), it has been suggested that GyrB-like ATPase domain
may have energy-dependent functions potentially involving novel host-virus interactions1, however, its actual
functions in the poxviruses is still unknown and requires further examination.
e phylogenetic distribution of Crocodylidpoxvirus genomes indicates that Nile crocodiles, saltwater croc-
odiles and perhaps other Crocodylia species could be important hosts for Crocodylidpoxvirus dispersal around
the globe. As shown in Fig.3, it is reasonable to postulate that these viruses perhaps originated from a common
ancestor that diverged from a CRV-like progenitor. e reservoir hosts of these crocodile poxviruses may be
the specic crocodile species, and/or other captive or wild animal species that are in close propinquity with the
farmed crocodile host species. Without further experimentation, we cannot trace the actual source of poxvirus
infection in the saltwater crocodile. Similar to other poxviruses25,51, it will not be surprising if vectors such as
mosquitoes are playing a part during the transmission of poxvirus within the saltwater crocodile population.
Examining the phylogenetic tree and distance matrix using DNA polymerase and DNA topoisomerase genes
of the swCRV genomes and CRV, it was demonstrated that there was an obvious trend associated within the pox-
virus species isolated under the genus Crocodylidpoxvirus (Fig.4 and Supplementary FigureS2). Well-supported
phylogenetic trees were constructed using both the DNA polymerase and DNA topoisomerase genes and they
show that SwCRV is closely related to CRV. ese results further conclude that SwCRV may be more closely
linked at a conserved gene level than across the length of the genome, highlighting the importance of complete
genome characterization in comparison to single gene phylogenies. However, given their genetic diversity and
geographically discrete distributions, it is perhaps not surprising that the saltwater crocodile and the Nile croco-
dile species may be exposed to dierent poxvirus infections.
Conclusions
ese are the rst crocodilepox virus genome sequences isolated from a poxvirus infection of the Australian
saltwater crocodile. e novel complete genome sequences of SwCRV subtypes 1 and -2 (SwCRV-1 and -2) are
signicantly divergent, but most similar to the CRV. Together with the sequence similarities observed between
SwCRV and other chordopoxviruses, this study concluded that the SwCRV 1 and -2 complete genome sequences
described here are not closely related to any other chordopoxvirus complete genomes isolated from avian, rep-
tilian or other natural host species. As such, they should be considered as dierent subtypes of the same species,
tentatively named as saltwater crocodilepox virus subtype 1 and saltwater crocodilepox virus subtype 2. Similar
to CRV, SwCRV also lacks genes predicted to involve host immune response, however, it is completely reasonable
to hypothesise that the genes encoding these functions are present but not recognized. erefore, further studies
for understanding the functions of hypothetical and/or uncharacterized proteins will be important to uncover
whether or not these proteins are playing any role in host immune response modulation.
Materials and Methods
Source of samples. Exudate from characteristic poxvirus lesions on the belly skin of juvenile saltwater croc-
odiles were collected from Darwin Crocodile Farm (Noonamah, Northern Territory, Australia), as described by
Moore et al.3, Northern Territory, Australia; (Fig.1). Animal sampling was performed to comply with approved
guidelines set by the Australian Code of Practice for the Care and Use of Animals for Scientic Purposes (1997)
and approved by the Charles Darwin University Animal Ethics Committee (A16005).
DNA extraction, llumina library preparation and sequencing. Tissue from individual pox lesions was
aseptically dissected and mechanically homogenized in lysis buer using disposable tissue grinder pestles and trans-
ferred into a 1.5 mL microcentrifuge tube (Eppendorf). Virion enrichment and DNA extraction from the sample
was performed according to the protocol described by Sarker et al.25,52. DNA libraries were prepared according to
published protocol53 using the Illumina Nextera XT DNA Library Prep V3 Kit starting with one ng of total genomic
DNA (gDNA) as measured by Qubit (Invitrogen) and sequenced on the Illumina MiSeq platform.
Bioinformatics. Sequencing data was analysed according to a previously established pipeline25,53 and using
CLC Genomics workbench 9.5.4. A total of 2,263,362 and 770,348 pairs of 301-bp reads for the SwCRV-1 and
-2, were obtained respectively. Preliminary quality evaluation for all raw reads was generated, pre-processed to
remove ambiguous base calls and poor quality reads, and trimmed as previously described53. Trimmed sequence
reads were aligned to the Australian saltwater crocodile genome (Crocodylus porosus, GenBank accession number
NW_017728899) to remove host DNA contamination, with a total of 4.77% reads being excluded from further
analysis. Reads were assembled, and contigs with high condence were chosen for downstream analysis as previ-
ously described53. BLASTN analysis of the resulting contigs conrmed the closest match to be Nile crocodilepox
virus. e Nile crocodilepox virus genome sequence was used as a reference genome, with selected contigs being
mapped, edited, ordered and orientated as previously described53. A dra genome was created from de novo con-
tigs were used as a reference sequence to assemble against the trimmed reads to conrm further. is produced an
average coverage of 1905.9 and 476.58 for the genome sequences of SwCRV-1 and -2, respectively (Supplementary
FigureS3). e consensus genome sequences of SwCRV-1 and -2 were extracted from CLC Genomics workbench
9.5.4 using the following criteria (low coverage threshold 20, remove region with low coverage and join aer
removal). e nal consensus genome sequences were 187,976 and 184,894 bp for SwCRV-1 and -2, respectively
obtained from the Australian saltwater crocodile.
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Genome annotations. e SwCRV genomes were annotated using GATU54 and further verication of the
predicted ORFs were performed as previously described53, with the exception that overlapping ORFs showing
unique or potential poxviruses orthologs using BLASTP serach were annotated to investigate further whether
or not the missing ChPVs conserved genes in SwCRV were present. Future in vitro experimental work will need
to be performed to assess the potential functionality of overlapping reading frames. Additionally, HHpred using
default parameters55 was used to search protein homologs for unique ORFs predicted in this study. e correct
methionine start site, signs of truncation, correct stop codons, and validity of overlaps for SwCRV annotations
were further examined using other poxvirus ortholog alignments. e tandem direct repeats were identied using
the Tandem Repeats Finders56, and concatemer resolution motifs were analysed using Geneious (version 10.2.3).
Hairpin loop-like secondary structure in the ITRs region of SwCRV were also predicted using tools available in
Geneious (version 10.2.3)57.
Analysis of genome sequences and generation of phylogenetic trees. Nucleotide sequences of
poxviruses including a representative virus of each genus of the Chordopoxvirinae with newly sequenced SwCRV
genomes were downloaded from GenBank: CRV (Nile crocodilepox virus, DQ356948); SQPV (Squirrel poxvi-
rus, HE601899); MOCV1 (Molluscum contagiosum virus subtype 1, MCU60315); MOCV2 (Molluscum conta-
giosum virus subtype 2, KY040274); PPRD (Parapoxvirus red deer, KM502564); BPSV (Bovine papular stomatitis
virus, KM875470); ORFV (Orf virus, DQ184476); PCPV (Pseudocowpox virus, GQ329670); SwCRV-1 (saltwa-
ter crocodilepox virus subtype 1, MG450915); SwCRV-2 (saltwater crocodilepox virus subtype 2, MG450916);
DPV (Deerpox virus, AY689436); GPV (Goatpox virus, KC951854); RFV (Rabbit broma virus, AF170722);
MYXV (Myxoma virus, KP723391); LSDV (Lumpy skin disease virus, NC_003027); FWPV (Fowlpox virus,
AF198100); SWPV1 (Shearwaterpox virus-1, KX857216); YMTV (Yaba monkey tumor virus, NC_005179);
MPXV (Monkeypox virus, JX878407); VACV (Vaccinia virus, AY678275); YKPV (Yoka poxvirus, HQ849551);
SWPV (Swinepox virus, NC_003389).
Phylogenetic analysis and amino acid sequence alignment was performed as previously stated53. Except in
the case of DNA polymerase and DNA topoisomerase, tree topology with 500 bootstrap re-samplings under LG
substitution model was chosen to generate ML tree using tools available in Geneious (version 10.2.3).
Transmission electron microscopy. Exudate removed from pox lesions was suspended 1:10 in phosphate
buered saline (PBS), homogenised, claried and adsorbed onto 400-mesh copper EM grids, prior to staining and
imaging on a JEOL JEM-2100 transmission electron microscope as previously described53.
Data availability. e complete genome sequences of the SwCRV-1 and -2 were deposited in GenBank
under the accession number MG450915 and MG450916, respectively. Raw sequencing data from this study
has been deposited in the NCBI Sequence Read Achieve (SRA) under the accession number of SRP128935
(BioProject ID: PRJNA429047; BioSample accessions: SAMN08328748 and SAMN08328749) (http://www.ncbi.
nlm.nih.gov/sra/).
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Acknowledgements
e authors are extremely grateful to Securing Food, Water and Environment, RFA, ABC Research Funding
Scheme 2017, and the centre for Crocodile Research, Noonamah, Australia for funding this research (Project
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SCIenTIfIC REPORtS | (2018) 8:5623 | DOI:10.1038/s41598-018-23955-6
ID: 0001027183). Additional funding for this project was generously provided by Rural Industries Research and
Development Corporation (Project ID: PRJ-010453). e authors also like to acknowledge the Latrobe Institute
for Molecular Science (LIMS) Bioimaging Facility for the use of Transmission Electron Microscope.
Author Contributions
S.S. and K.H. conceived the experiment; S.I. collected the samples and provided photos for macroscopic lesion;
S.S. and P.L. performed TEM; S.S. conducted sequencing experiment, analysed the data and wrote the manuscript;
all authors participated in reviewing and editing the nal manuscript.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-018-23955-6.
Competing Interests: e authors declare no competing interests.
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