Emergence of SARS-CoV-2 through recombination and strong purifying selection

Abstract

COVID-19 has become a global pandemic caused by the novel coronavirus SARS-CoV-2. Understanding the origins of SARS-CoV-2 is critical for deterring future zoonosis, discovering new drugs, and developing a vaccine. We show evidence of strong purifying selection around the receptor binding motif (RBM) in the spike and other genes among bat, pangolin, and human coronaviruses, suggesting similar evolutionary constraints in different host species. We also demonstrate that SARS-CoV-2’s entire RBM was introduced through recombination with coronaviruses from pangolins, possibly a critical step in the evolution of SARS-CoV-2’s ability to infect humans. Similar purifying selection in different host species, together with frequent recombination among coronaviruses, suggest a common evolutionary mechanism that could lead to new emerging human coronaviruses.

Full text

Cite as: X. Li et al., Sci. Adv
10.1126/sciadv.abb9153 (2020).
RESEARCH ARTICLES
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Introduction
The severe respiratory disease COVID-19 was first noticed in
late December 2019 (1). It rapidly became epidemic in China,
devastating public health and economy. At the beginning of
May, COVID-19 had spread to ~150 countries and infected
over 3.3 million people (2). On March 11, 2020, the World
Health Organization (WHO) officially declared it a pandemic.
The etiological agent of COVID-19 (3), severe acute respir-
atory syndrome coronavirus 2 (SARS-CoV-2) (4), was identi-
fied as a new member of the genus Betacoronavirus, which
includes a diverse reservoir of coronaviruses (CoVs) isolated
from bats (5–7). While genetically distinct from the betacoro-
naviruses that cause SARS and MERS in humans (8, 9), SARS-
CoV-2 shares the highest level of genetic similarity (96.3%)
with CoV RaTG13, sampled from a bat in Yunnan in 2013 (8).
Recently, CoV sequences closely related to SARS-CoV-2 were
obtained from confiscated Malaya pangolins in two separate
studies (10, 11). These pangolin SARS-like CoVs (Pan_SL-CoV)
form two distinct clades corresponding to their locations of
origin: the first clade, Pan_SL-CoV_GD, sampled from
Guangdong (GD) province in China, is genetically more sim-
ilar to SARS-CoV-2 (91.2%) than the second clade, Pan_SL-
CoV_GX, sampled from Guangxi (GX) province (85.4%).
Understanding the origin of SARS-CoV-2 may help de-
velop strategies to deter future cross-species transmissions
and to establish appropriate animal models. Recombination
plays an important role in the evolution of coronaviruses (12,
13). Viral sequences nearly identical to SARS and MERS vi-
ruses were found in civets and domestic camels, respectively
(14, 15), demonstrating that they originated from zoonotic
transmissions with intermediate host species between the bat
reservoirs and humans—a common pattern leading to CoV
zoonosis (5–7). However, non-human viruses nearly identical
to SARS-CoV-2 have not yet been found. In this paper we
demonstrate, through localized genomic analysis, a complex
pattern of evolutionary recombination and strong purifying
selection between CoVs from distinct host species and that
cross-species infections that likely originated SARS-CoV-2.
Results
Acquisition of receptor binding motif through recombi-
nation
Phylogenetic analysis of 43 complete genome sequences from
three clades (SARS-CoVs and bat_SL-CoVs in clade 3; SARS-
CoV-2, bat_SL-CoVs and pan_SL-CoVs in clade 2; and two di-
vergent bat_SL-CoVs in clade 1) within the Sarbecovirus
group (9) confirms that RaTG13 is overall the closest se-
quence to SARS-CoV-2 (Fig. S1). Pan_SL-CoV_GD are the next
closest viruses, followed by Pan_SL-CoV_GX. Among the bat-
CoV sequences in clade 2 (Fig. S1), ZXC21 and ZC45, sampled
from bats in 2005 in Zhoushan, Zhejiang, China, are the most
divergent, with the exception of the beginning of the ORF1a
gene (region 1, Fig. 1A). All other Bat_SL-CoV and SARS-CoV
sequences form a separate clade 3, while clade 1 comprises
Emergence of SARS-CoV-2 through recombination and
strong purifying selection
Xiaojun Li1,†, Elena E. Giorgi2,†, Manukumar Honnayakanahalli Marichannegowda1, Brian Foley2, Chuan
Xiao3, Xiang-Peng Kong4, Yue Chen1, S. Gnanakaran2, Bette Korber2,5 Feng Gao1,6,*
1Department of Medicine, Duke University Medical Center, Dur ham, NC 27710, USA. 2Theoretical Biology and Biophysics, Los Alamos National Laboratory, Los Alamos, NM
87544, USA. 3Department of Chemistry and Biochemistry, The University of Texas at El Paso, El Paso, TX 79968, USA. 4Department of Biochemistry and Molecular
Pharmacology, Grossman School of Medicine, New York University, New York, NY 10016 5New Mexico Consortium, Los Alamos, New Mexico 87545, USA 6National
Engineering Laboratory for AIDS Vaccine, School of Life Sciences, Jilin University, Changchun 130012, China.
†These authors contributed equally.
*Corresponding author. Email: fgao@duke.edu
COVID-19 has become a global pandemic caused by the novel coronavirus SARS-CoV-2. Understanding the
origins of SARS-CoV-2 is critical for deterring future zoonosis, discovering new drugs, and developing a
vaccine. We show evidence of strong purifying selection around the receptor binding motif (RBM) in the
spike and other genes among bat, pangolin, and human coronaviruses, suggesting similar evolutionary
constraints in different host species. We also demonstrate that SARS-CoV-2’s entire RBM was introduced
through recombination with coronaviruses from pangolins, possibly a critical step in the evolution of SARS-
CoV-2’s ability to infect humans. Similar purifying selection in different host species, together with
frequent recombination among coronaviruses, suggest a common evolutionary mechanism that could lead
to new emerging human coronaviruses.
Science Advances Publish Ahead of Print, published on May 29, 2020 as doi:10.1126/sciadv.abb9153
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BtKY72 and BM48-31, the two most divergent Bat_SL-CoV se-
quences in the Sarbecovirus group (Fig. S1). Recombination
in the first SARS-CoV-2 sequence (Wuhan-Hu-1) with other
divergent CoVs has been previously noted (3). Here, to better
understand the role of recombination in the origin of SARS-
CoV-2 among these genetically similar CoVs, we compare Wu-
han-Hu-1 to six representative Bat_SL-CoVs, one SARS-CoV,
and the two Pan_SL-CoV_GD sequences using SimPlot anal-
ysis (16). RaTG13 has the highest similarity across the genome
(8), with two notable exceptions where a switch occurs (Fig.
1A). In phylogenetic reconstructions, SARS-CoV-2 clusters
closer to ZXC21 and ZC45 than RaTG13 at the beginning of
the ORF1a gene (region 1, Fig. 1B), and, as previously reported
(10, 17), to a Pan_SL-CoV_GD in region 2 (Figs. 1C and S2),
which spans the receptor angiotensin-converting enzyme 2
(ACE2) binding site in the spike (S) glycoprotein gene. When
comparing Wuhan-Hu-1 to Pan_SL-CoV_GD and RaTG13, as
representative of distinct host-species branches in the evolu-
tionary history of SARS-CoV-2, using the recombination de-
tection tool RIP (18), we find significant recombination
breakpoints before and after the ACE2 receptor binding mor-
tif (RBM) (19, 20) (Fig. S2A). This suggests that SARS-CoV-2
carries a history of cross-species recombination between the
bat and the pangolin CoVs.
Pan_SL-CoV sequences are generally more similar to
SARS-CoV-2 than other CoV sequences, with the exception of
RaTG13 and ZXC21, but are more divergent from SARS-CoV-
2 at two regions in particular: the beginning of the ORF1b
gene and the highly divergent N terminus of the S gene (re-
gions 3 and 4, respectively, Fig. 1A). Within-region phyloge-
netic reconstructions show that Pan_SL-CoV sequences
become as divergent as BtKY72 and BM48-31 in region 3 (Fig.
1D), while less divergent in region 4, where Pan_SL-CoV_GD
clusters with ZXC21 and ZC45 (Fig. 1E). Together, these ob-
servations suggest ancestral cross-species recombination be-
tween pangolin and bat CoVs in the evolution of SARS-CoV-
2 at the ORF1a and S genes. Furthermore, the discordant phy-
logenetic clustering at various regions of the genome among
clade 2 CoVs also supports extensive recombination among
these viruses isolated from bats and pangolins.
The SARS-CoV-2 S glycoprotein mediates viral entry into
host cells and therefore represents a prime target for drug
and vaccine development (12, 19). While SARS-CoV-2 se-
quences share the greatest overall genetic similarity with
RaTG13, this is no longer the case in parts of the S gene. Spe-
cifically, amino acid sequences of RBM in the S1 subunit are
nearly identical to those in two Pan_SL-CoV_GD viruses, with
only one amino acid difference (Q498H)—although the RBM
region has not been fully sequenced in one of Guangdong
pangolin virus (Pan_SL-CoV_GD/P2S) (Fig. 2A). Pangolin
CoVs from Guangxi are much more divergent. Phylogenetic
analysis based on the amino acid sequences of this region
shows three distinct clusters of SARS-CoV, SARS-CoV-2 and
bat-CoV only viruses, respectively (Fig. 2B). Interestingly,
while SARS-CoV and SARS-CoV-2 viruses use ACE2 for viral
entry, all CoVs in the third cluster have a 5-aa deletion and a
13-14-aa deletion in RBM (Fig. 2A) and do not infect human
target cells (5, 21, 22).
Although both SARS-CoV and SARS-CoV-2 use the human
ACE2 as their receptors (8, 23), they show a high level of ge-
netic divergence (Figs. 1 and S1). However, structures of the
S1 unit of the S protein from both viruses are highly similar
(20, 24–26), with the exception of a loop that bends differ-
ently (Fig. 3A). The root-mean-square deviation (RMSD) be-
tween the two S proteins are 1.2Å over 174 Cα residues (24).
This suggests that conformational similarity of the binding
motif enables viral entry through molecular recognition of
ACE2. These structural studies also thoroughly analyzed the
contact residues between the S protein and human ACE2 (20,
24). Previously structural and mutagenesis studies have iden-
tified two hot-spots, K31 and K353, at the S/ACE2 interface in
SARS-CoV. In SARS-CoV-2, these two hot-spots were slightly
weakened due to different residues on its S protein but the
loop that takes different conformations from SARS-CoV pro-
vides additional interaction that strengthens the interaction
(26). Among 17 distinct amino acids between SARS-CoV-2 and
RaTG13 in the RBM region (Fig. 2A), five contact sites based
on the structural studies (24) are different, likely impacting
RaTG13’s binding to ACE2 (Fig. 3B and Table S1). The single
amino acid difference at position 498 (Q or H) between SARS-
CoV-2 and Pan_SL-CoV_GD is at the edge of the ACE2 con-
tact interface; neither Q or H at this position form hydrogen
bonds with ACE2 residues (Fig. 3C). Thus, a functional RBM
nearly identical to the one in SARS-CoV-2 is naturally present
in Pan_SL-CoV_GD viruses. The very distinctive RaTG13
RBM suggests that this virus will not likely infect human cells
efficiently. Indeed, a recent study showed that the RaTG13
pseudovirus is much less efficient than SARS-CoV-2 pseudo-
viruses in using ACE2 to infect cells, and this is most likely
due to the L486F and Y493Q substitutions, which result in
lower ACE2 binding in RaTG13 (26). Therefore, it is likely that
the acquisition of a complete functional RBM by a RaTG13-
like CoV through a recombination event with a Pan_SL-
CoV_GD-like virus enabled it to more efficiently use ACE2 for
human infection.
Three small insertions are identical in SARS-CoV-2 and
RaTG13 but not found in other CoVs in the Sarbecovirus
group (27, 28). The RaTG13 sequence was sampled in 2013,
years before SARS-CoV-2 was first identified. It is unlikely
that both SARS-CoV-2 and RaTG13 independently acquired
identical insertions at three different locations in the S gene.
Thus, it is plausible that a RaTG13-like virus served as a pro-
genitor to generate SARS-CoV-2 by gaining a complete hu-
man ACE2 binding RBM from Pan_SL-CoV_GD-like viruses
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through recombination. Genetic divergence at the nucleic
acid level between Wuhan-Hu-1 and Pan_SL-CoV_GD viruses
is significantly reduced from 13.9% (Fig. 1E) to 1.4% at the
amino acid level (Fig. 2B) in the RBM region, indicating re-
combination between RaTG13-like CoVs and Pan_SL-
CoV_GD-like CoVs. Furthermore, SARS-CoV-2 has a unique
furin cleavage site insertion (PRRA) not found in any other
CoVs in the Sarbecovirus group (Fig. S3) (27), although simi-
lar motifs are also found in MERS and more divergent bat
CoVs (29). This PRRA motif makes the S1/S2 cleavage in
SARS-CoV-2 much more efficient than in SARS-CoV and may
expand its tropism and/or enhance its transmissibility (20).
A recent study of bat CoVs in Yunnan, China, identified a
three-amino acid insertion (PAA) at the same site (30). Alt-
hough it is not known if this PAA motif can function like the
PRRA motif, the presence of a similar insertion at the same
site indicates that such insertion may already be present in
the wild bat CoVs. The more efficient cleavage of S1 and S2
subunits of the spike glycoprotein (29) and efficient binding
to ACE2 by SARS-CoV-2 (20, 25) may have allowed SARS-CoV-
2 to jump to humans, leading to the rapid spread of SARS-
CoV-2 in China and the rest of the world.
Strong purifying selection among SARS-CoV-2 and
closely related viruses
Recombination from Pan_SL-CoV_GD at the RBM and at the
unique furin cleavage site insertion prompted us to examine
the SARS-CoV-2 sequences within these regions. Amino acid
sequences from SARS-CoV-2, RaTG13, and all Pan_SL-CoV vi-
ruses (group A) are identical or nearly identical in the region
before and after the RBM and at the region after the furin
cleavage site (S2 subunit), while all other CoVs (group B) are
very distinctive (Fig. 4A and S4). The average of all pairwise
dN/dS ratios, defined as ω, among SARS-CoV-2, RaTG13, and
Pan_SL-CoV viruses at the S2 subunit is ω = 0.013, compared
to the much higher values ω =0.053 in the S1 region preceding
the furin cleavage site, and ω = 0.042 at the S2 subunit for all
other CoVs (Fig. 4B). The much lower ω value at the S2 subu-
nit among the SARS-CoV-2, RaTG13, and Pan_SL-CoV viruses
indicates that this region is under strong purifying selection
within these sequences. A plot of synonymous and nonsynon-
ymous substitutions relative to Wuhan-Hu-1 highlights the
regional differences across the region before and after the
furin cleavage site (Fig. 4A): the S2 subunit is highly con-
served among the SARS-CoV-2, RaTG13, and Pan_SL-CoV vi-
ruses (group A), while far more nonsynonymous mutations
are observed in the rest of the CoV sequences (group B). The
shift in selective pressure at the S1/S2 cleaveage site among
these related viruses versus other CoVs begins near codon
368 (Fig. 4B): the two graphs show the cumulative plots of
the average behavior of each codon for all pairwise compari-
sons in the input data, for synonymous mutations, non-
synonymous mutations and indels of group A sequences and
group B sequences. The non-synonymous plot shows a
marked change in slope (vertical step) in the group A se-
quences at codon 368, but not in group B sequences. Simi-
larly, when looking at all the dS/dN ratios (ω) for each group
A sequence compared to the Wuhan-Hu-1 sequence, we see
that these ratios are much lower in the 5′-end of the region,
before codon 368 (nucleic acid position 1104), compared to
the 3′-end, and no such difference is observed in the group
B sequences (Fig. 4C).
This strong purifying selection observed in the S2 subunit
of the S gene is not surprising given its role in cell entry by
fusing the viral and host cell membranes (5, 19). Following
the binding of RBD to the ACE2 receptor, heptad repeat re-
gions 1 (HR1) and 2 (HR2) within the S2 subunit rearrange to
form the fusion core, bringing together the viral and cell
membranes for fusion and infection (Fig. S5A). Due to the
mechanistic constraints for this assembly for fusion, the pro-
tein segments that take part in this assembly are well pre-
served (20, 31). Furthermore, some regions of the S2 subunit
are covered by S1 in the trimer conformation of the spike pro-
tein (Fig. S5B). Based on the currently available, but incom-
plete, cryo-EM structure of the spike trimer, we estimate that
60%-65% of S2 amino acids are buried. This adds further
structural constraints on changing amino acids in S2.
While hundreds of new SARS-CoV-2 sequences are added
to the GISAID repertoire every day (32), we note that the
RBM region currently remains highly conserved. No amino
acid within 6 Angstroms of the ACE2 binding site has re-
peated variations, with the exception of G476S, a very rare
mutation found in 8 sequences from a local cluster in Wash-
ington state, out of 6,400 total sequences from GISAID (April
13, 2020). In addition, we observe similar patterns of purify-
ing selection pressure in other parts of the genome, including
the E and M genes, as well as the partial ORF1a and ORF1b
genes (Fig. S6 and S7). Interestingly, the viruses affected by
purifying selection pressure varies depending on which genes
are analyzed. SARS-CoV-2, RaTG13, all Pan_SL-CoV and the
two bat CoVs (ZXC21 and ZC45) are under the similar purify-
ing selection in both the E and M genes (Figs. 5A and S6). In
the S2 subunit, similar purifying selection are only observed
for SARS-CoV-2, RaTG13, and all Pan_SL-CoV (Fig. 5B). A few
viruses including only SARS-CoV-2, RaTG13, and pangolin
CoVs from Guangdong are under similar purifying selection
in the partial regions of ORF1a and ORF1b (Figs. 5C and S7).
Strong purifying selection pressure on SARS-CoV-2, RaTG13
and Pan_SL-CoV_GD viruses, as indicated by consistently
low ω values, suggests that these complete and partial genes
are under similar functional/structural constraints among
the different host species. In two extreme cases, amino acid
sequences of the E gene and the 3′ end of ORF1a are identical
among the compared CoV sequences, although genetic
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distances are quite large among these viruses at the nucleic
acid level (Fig. 5A and 5C). Such evolutionary constraints in
many parts of the viral genome, especially at functional do-
mains in the S gene which plays an important role in cross-
species transmission (5, 12), coupled with frequent recombi-
nation, may facilitate cross-species transmissions between
RaTG13-like bat and/or Pan_SL-CoV_GD-like viruses.
Frequent recombination between SARS-CoVs and
bat_SL-CoVs
Previous studies using more limited sequence sets found that
SARS-CoVs originated through multiple recombination
events between different bat-CoVs (10, 12, 21, 33, 34). Our
phylogenetic analyses of individual genes confirm this and
show that SARS-CoV sequences tend to cluster with
YN2018B, Rs9401, Rs7327, WIV16 and Rs4231 (group A) for
some genes and Rf4092, YN2013, Anlong-112 and GX2013
(group B) for others (Fig. S8). SimPlot analysis using both
groups of bat_SL-CoVs and the closely related bat CoV YNLF-
34C (34) shows that SARS-CoV GZ02 shifts in similarity
among different bat SL-CoVs at various regions of the ge-
nome (Fig. 6A). In particular, phylogenetic reconstruction of
the beginning of ORF1a (region 1) confirms that SARS-CoVs
cluster with YNLF-34C (34), and this cluster is distinctive
comparing to all other CoVs (Fig. 6B). YNLF-34C is more di-
vergent from SARS-CoV than other bat-CoV viruses before
and after this region, confirming the previously reported
complex recombinant nature of YNLF-34C (34) (Fig. 5A). At
the end of the S gene (region 2), SARS-CoVs cluster with
group A CoVs, forming a highly divergent clade (Fig. 6C). In
region 3 (ORF8), SARS-CoVs and group B CoVs, together with
YNLF-34C, form a very divergent and distinctive cluster (Fig.
6D). To further explore the recombinant nature of SARS-
CoVs, we compared GZ02 to representative bat CoV se-
quences using the RIP recombination detection tool (18). We
identified four significant breakpoints (at 99% confidence)
between the two parental lineages (Fig. S9A), further sup-
ported by phylogenetic analysis (Fig. S9B-S9D). In addition,
the two aforementioned groups of bat CoVs (shown in light
brown and light blue in the trees) show similar cluster
changes across the five recombinant regions, suggesting mul-
tiple events of historic recombination among bat SL-CoVs.
These results demonstrate that SARS-CoV shares a recombi-
nant history with at least three different groups of bat-CoVs
and confirms the major role of recombination in the evolu-
tion of these viruses.
Of the bat SL-CoVs that contributed to the recombinant
origin of SARS-CoV, only group A viruses bind to ACE2.
Group B bat SL-CoVs do not infect human cells (5, 21, 22) and
have two deletions in the RBM (Figs. 1E and 2A). The short
deletion between residues 445 and 449, and in particular the
loss of Y449, which forms three hydrogen bonds with ACE2,
will significantly affect the overall structure of the RBM (Figs.
3C and 3D). The region encompassing the large deletion be-
tween residues 473 and 486 contains the loop structure that
accounts for the major differences between the S protein of
SARS-CoV and SARS-CoV-2 (Fig. 3A) and strengthen the in-
teraction of the latter to ACE2 (26). This deletion causes the
loss of contact site F486 and affects the conserved residue
F498’s hydrophobic interaction with residue M82 on ACE2
(Fig. 3D). These two deletions will render RBM in those CoVs
incapable of binding human ACE2. Therefore, recombination
may play a role in enabling cross-species transmission in
SARS-CoVs through the acquisition of an S gene type that can
efficiently bind to the human ACE2 receptor.
ORF8 is one of the highly variable genes in coronaviruses
and its function has not yet been well elucidated (5, 12, 35).
Recombination breakpoints within this region show that re-
combination occurred at the beginning and the end of ORF8
(Fig. S10), where nucleic acid sequences are nearly identical
among both SARS-CoVs and group B bat CoVs. Moreover, all
compared viruses form three highly distinct clusters (Fig.
6D), suggesting that the ORF8 gene may be biologically con-
strained and evolves through modular recombination. The
third recombination region at the beginning of ORF1a is near
where SARS-CoV-2 also recombined with other bat CoVs (re-
gion 1 in Fig. 1A). This region is highly variable (5, 12) and
recombination within this part of the genome was also found
in other CoVs, suggesting that it may be a recombination
hotspot and may factor into cross-species transmission.
Discussion
There are three important aspects to betacoronavirus evolu-
tion that should be carefully considered in phylogenetic re-
constructions among more distant coronaviruses. First, there
is extensive recombination among all of these viruses (10, 12,
21, 33, 34) (Figs. 1 and 5), making standard phylogenetic re-
constructions based on full genomes problematic, as different
regions of the genome have distinct ancestral relationships.
Second, between more distant sequences, synonymous sub-
stitutions are often fully saturated, which can confound anal-
yses of selective pressure and adds noise to phylogenetic
analysis. Finally, there are different selective pressures at
work in different lineages, which is worth consideration
when interpreting trees.
The currently sampled pangolin CoVs are too divergent
from SARS-CoV-2 to be its recent progenitors, but it is note-
worthy that these sequences contain an RBM that can most
likely bind to human ACE2. While RaTG13 is the most closely
related CoV sequence to SARS-CoV-2, it has a distinctive
RBM. In addition, a recent study showed that the RaTG13
pseudovirus is much less efficient than the SARS-CoV-2 pseu-
dovirus in using ACE2 to infect cells (26). SARS-CoV-2 has a
nearly identical RBM to the one found in the pangolin CoVs
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from Guangdong. Thus, it is plausible that RaTG13-like bat-
CoV viruses may have obtained the RBM sequence binding to
human ACE2 through recombination with Pan_SL-CoV_GD-
like viruses. We hypothesize that this, and/or other ancestral
recombination events between viruses infecting bats and
pangolins, may have played a key role in the evolution of the
strain that lead to the introduction of SARS-CoV-2 into hu-
mans. It is also possible that other not yet identified hosts
infected with CoVs that can jump to human populations
through cross-species transmission if they can successfully
infect human cells through ACE2 or other receptors. Interest-
ingly, an analysis of 6,400 SARS-CoV-2 sequences from
GISAID (Global Initiative on Sharing All Influenza Data) (36,
37) identifies only one very rare mutation, G476S that is di-
rectly in a ACE2 contact residue. It was found in a local clus-
ter of sequences from Washington state. However, it is at the
periphery of the receptor contact surface, and so may not sig-
nificantly impact the virus’s receptor binding affinity.
All three human CoVs (SARS, MERS and SARS-2) are the
result of recombination among CoVs. Recombination in all
three viruses involved the S gene, likely a precondition to zo-
onosis that enabled efficient binding to human receptors (5,
12). Extensive recombination among bat coronaviruses and
strong purifying selection pressure among viruses from hu-
mans, bats and pangolins may allow such closely related vi-
ruses to readily jump between species and adapt to the new
hosts. Many bat CoVs have been found able to bind to human
ACE2 and replicate in human cells (10, 21, 22, 38–40). Sero-
logical evidence has revealed that additional otherwise unde-
tected spillovers have occurred in people in China living in
proximity to wild bat populations (41). Continuous surveil-
lance of coronaviruses in their natural hosts and in humans
will be key to rapid control of new coronavirus outbreaks.
While the SARS and MERS originating strains have been
found in civets and dromedary camels respectively (14, 15), so
far, efforts to identify a similarly close link in the original
pathway of SARS-CoV-2 into humans have failed. If the new
SARS-CoV-2 strain did not cause widespread infections in its
natural or intermediate hosts, such a strain may never be
identified. The close proximity of animals of different species
in a wet market setting may increase the potential for cross-
species spillover infections, by enabling recombination be-
tween more distant coronaviruses and the emergence of re-
combinants with novel phenotypes. While the direct reservoir
of SARS-CoV-2 is still being sought, one thing is clear: reduc-
ing or eliminating direct human contact with wild animals is
critical to preventing new coronavirus zoonosis in the future.
Materials and Methods
Sequences analysis
All 43 CoV complete genome sequences were obtained from
GenBank and GISAID (Global Initiative on Sharing All
Influenza Data) (36, 37), and were selected to be representa-
tive of the diversity (Tables S2 and S3). Pan_SL-
CoV_GD/P1La sequence was generated by combining
Pan_SL-CoV_GD/P1L (10) with some additional sequences
from the NCBI BioProject database PRJNA5732983 (11, 42) to
have a maximal coverage of the complete genome sequence
for analysis. A new CoV sequence from pangolin
(EPI_ISL_410721) (43) was not included here as it became
available after we had already completed the analyses in this
study. Once it became available, we observed that it was as
close to SARS-CoV-2 as the sequences we had already used
and hence did not change the interpretation of our results.
Whole genome sequences were first aligned using Clustal X2
(44). The alignments for all coding regions were manually op-
timized based on the amino acid sequence alignment using
SeaView 5.0.1.
Recombination Analyses
SimPlot 3.5.15 (16) was used to determine the percent identity
of the query sequence to reference sequences. Potential re-
combinant regions among analyzed sequences were identi-
fied by sliding a 400bp-window at a 50bp-step across the
alignment using the Kimura 2-parameter model. Phyloge-
netic trees were constructed by the maximum likelihood
method using the GTR model (45), and their reliability was
estimated from 1,000 bootstrap replicates. The positions of
analyzed sequence regions were based on those in the refer-
ence SARS-CoV-2 Wuhan-Hu-1 (MN908947). Recombination
regions and breakpoints were also analyzed using the LANL
database (46) tool RIP (18) with a 400bp window. Regions
between breakpoints were identified using a 99% confidence
threshold.
Selection Analyses
Cumulative plots of the average behavior of each codon for
all pairwise comparisons in the input data, for insertions and
deletions (indels), synonymous (syn), and nonsynonymous
(nonsyn) mutations and values of the ratios of the rate of syn-
onymous nucleotide substitutions per synonymous site and
nonsynonymous substitutions per nonsynonymous site
(dN/dS, or ω) were obtained using the LANL database tool
SNAP (47). In order to avoid counting instances where syn-
onymous mutations were saturated, averages of all pairwise
dN/dS ratios were calculated excluding pairs that yielded dS
values greater than 1.
Structure modeling of receptor binding
To investigate the single mutation Q498H in RBM between
SARS-CoV-2 and Pan_SL-CoV_GD, Q498 in the crystal struc-
ture of S/ACE2 complex was mutated to H498 using Chimera
(48). Local energy minimization (only H498 was allowed to
move) was computed using Chimera’s built-in functions. To
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investigate the impact of the deletion between residue 473 to
486 to the binding interface between SARS-CoV-2 and human
ACE2, a homology model with the deletion was generated us-
ing I-TASSER (49). The top five best models provided by the
server have Confidence Score (C-score) of 0.86, -2.33, -4.01, -
4.17, and -4.49. The C-score was used to estimate the quality
of the models, which should be between -5.0 to 2; the higher
the value, the higher the confidence in the model (49). Based
on the C-score, model 1 was used in Fig. 3D. The interaction
of the RBD of RaTG13 and ACE2 was modeled on PDB 6M0J,
a structure of RBD of SARS-CoV-2 in complex with human
ACE2 (24) using ICM software package (50), and the muta-
tional differences of the Gibbs free energy (Table S1) were cal-
culated with the built-in algorithm.
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ACKNOWLEDGMENTS
We thank all those who have contributed SARS-CoV-2 genome sequences to the
GISAID database (https://www.gisaid.org). We also thank Dr. Xinquan Wang
from Tsinghua University for sharing the PDB 6M0J structure with us before its
official release date. Funding: EEG, BK, SG and BF acknowledge support by the
Laboratory Directed R esearch and Development program of Los Alamos
National Laboratory under project number 20200554ECR. Author
contributions: Project conceptualization: F.G., B.K., E.E.G; Structure analysis:
C.X., X-P.K., S.G.; Sequence analysis: F.G., B.K., X.L., E.E.G., M.H.M., Y.C., B.F;
Phylogenetic analysis: F.G., B.K., X.L., E.E.G., M.H.M., Y.C.; Recombination
analysis: F.G., E.E.G., B.K., X.L., M.H.M., B.F.; Manuscript writing: F.G., B.K.,
E.E.G. Manuscript editing: F.G., B.K., E.E.G., X.L., C.X., X-P.K.; F.G. and B.F.
supervised the project. Competing interests: All authors declare no competing
interests. All data are available in the main text or the supplementary materials.
Data and materials availability: All data needed to evaluate the conclusions in
the paper are present in the paper and/or the Supplementary Materials.
Additional data related to this paper may be requested from the authors.
SUPPLEMENTARY MATERIALS
advances.sciencemag.org/cgi/content/full/sciadv.abb9153/DC1
Submitted 26 March 2020
Accepted 19 May 2020
Published First Release 29 May 2020
10.1126/sciadv.abb9153
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Fig. 1. SARS-CoV-2 recombination with Pan_SL-CoV and Bat_SL-CoV.
(
A
) SimPlot
genetic similarity plot between SARS-CoV-2 Wuhan-Hu-
1 and representative CoV
sequences, using a 400-bp window at a 50-bp step and the Kimura 2-parameter model.
Phylogenetic trees of regions of disproportional similarities, showing high similarities
between SARS-CoV-2 and ZXC21 (
B
) or GD/P1La (
C
), high genetic divergences of all
Pan_SL-CoV sequences (
D
), and high similarities between GD/P1La and to divergent
bat_SL-CoV sequences (
E
). All positions are relative to Wuhan-Hu-1. In Fig. 1A we use the
ORF1a and ORF1b nomenclature consistent with the original publication from of the Wuhan
virus (3), however, the NCBI betacoronavirus reference sequences (see SAR-CoV-2,
NC_045512.2, for an example) designate a single longer stretch called ORF1ab (from 266 to
21,555) that spans both 1a and 1b.
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Fig. 2. Impact of SARS-CoV-2 recombination on coreceptor binding
. (
A
) Amino acid sequences of the
receptor binding motif (RBM) in the spike (S) gene among Sarbecovirus CoVs compared to Wuhan-Hu-1
(top). Dashes indicate identical amino acids, dots indicate deletions. ACE2 critical contact sites highlighted
in blue, two large deletions in green. (
B
) Phylogenetic tree analysis of amino acids sequences of RBM. Viruses
with the ability to bind ACE2 form two distinct clusters (one including SARS_CoVs and the other including
SARS_CoV-2s). Bat-SL-CoVs with large deletions forms another distinct cluster.
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Fig. 3. Structure analysis of the RBM and ACE2 interface. (A) SARS-CoV and
SARS-CoV-2 receptor binding domains (RBD). Human ACE2 in green (PDB
6M0J) at the top and the RBD of the S-protein at the bottom; SARS-CoV S-protein
(PDB 2AJF) in red, and SARS-CoV-2 S-protein (PDB 6M0J) in magenta with RBM
in blue. All structure backbones shown as ribbons with key residues at the
interface shown as stick models, labeled using the same color scheme. (
B
)
Impact of different RBM amino acids between SARS-CoV-2 RaTG13 on ACE2
binding. (
C
) Impact of an amino acid at position 498 (Q in SARS-CoV-2, top, and
H in RaTG13, bottom) on ACE2 binding. Same color-coding as in (
A
) with
additional hydrogen bonding as light blue lines. (
D
) Impact of two deletions on
ACE2 binding interface in some bat-SL-CoVs, positions indicated in yellow, and
modeled structure with long deletion between residue 473 in light blue.
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Fig. 4. Strong purifying selection after furin cleavage in S gene among SARS-CoV-2 and
closely related viruses.
(
A
) Phylogenetic tree (left) and Highlighter plot (right) of sequences
around the RBM and furin cleavage site compared to SARS-CoV-2 Wuhan-Hu-1 (na positions
22541-24391). ACE2 receptor binding motif (RBM) and furin cleavage site highlighted in
light-gray boxes. Mutations compared to Wuahn-Hu-1 are light blue for synonymous, red for
non-synonymous. Dominance of synonymous mutations within group A compared to group
B highlighted on the right. (
B
) Cumulative plots of each codon average behavior for all
pairwise comparisons for indels and synonymous (light blue) and non-synonymous (red)
mutations, by group. The abrupt slope change of the nonsynonymous curve in group A at
around codon 368 (na 1104) is indicative of a shift in localized accumulations of non-
synonymous mutations after the furin cleavage site. Group B instead lacks this abrupt
change in slope at the same position. Values of ω denote average ratios of the rate of
nonsynonymous substitutions per nonsynonymous site (dN/dS) for each group and region.
(
C
) Sequence dS/dN ratios compared to Wuhan-Hu-1 within codons 1-368 (na 1-1104,
green) and codons 369-620 (na 1105-1893, dark blue) in group A and group B sequences.
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Fig. 5. Strong purifying selection on complete and partial gene regions among SARS-CoV-2,
RaTG13 and Pan_SL-CoV viruses.
Purifying selection pressure on complete and partial genes among
different viruses (red boxes) as evident by shorter branches in amino acid trees compared to nucleic
acid trees. Distinct purifying selection patterns are observed among different viruses: (
A
) SARS-CoV-
2, RaTG13, all Pan_SL-CoV and bat CoV ZXC21 and ZC45; (
B
) SARS-CoV-2, RaTG13, all Pan_SL-CoV
sequences; (
C
) SARS-CoV-2, RaTG13 and Pan_SL-CoV_GD. Cumulative plots of the average behavior
of each codon for all pairwise comparisons for synonymous mutations, non-synonymous mutations
and indels within each gene region. ω denotes the average ratio of the rate of nonsynonymous
substitutions per nonsynonymous site (dN/dS) for each group.
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Fig. 6. Multiple recombination of SARS-CoVs with different bat_SL-CoVs.
(
A
) SimPlot
genetic similarity plot between SARS-CoV GZ02 and SARS_SL-CoVs, using a 400-bp
window at a 50-bp step and the Kimura 2-parameter model. Group A CoVs (YN2018B,
Rs9401, Rs7327, WIV16 and Rs4231) are shown in blue, group B CoVs (Rf4092, YN2013,
Anlong-112 and GX2013) in orange, YNLF-34C in green, and outlier control HKU3-12 in
red. Phylogenetic trees for high similarity regions between GZ02 and YNLF-34C (
B
),
group A (
C
), and group B (
D
). All positions are relative to Wuhan-Hu-1.
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Emergence of SARS-CoV-2 through recombination and strong purifying selection
Chen, S. Gnanakaran, Bette Korber and Feng Gao
Xiaojun Li, Elena E. Giorgi, Manukumar Honnayakanahalli Marichannegowda, Brian Foley, Chuan Xiao, Xiang-Peng Kong, Yue
published online May 29, 2020
ARTICLE TOOLS http://advances.sciencemag.org/content/early/2020/05/28/sciadv.abb9153
MATERIALS
SUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2020/05/28/sciadv.abb9153.DC1
REFERENCES http://advances.sciencemag.org/content/early/2020/05/28/sciadv.abb9153#BIBL
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