Introduction
Reassortment is an evolutionary mechanism of segmented RNA viruses that plays an important
but ill-defined role in virus emergence and interspecies transmission. Recent experimental
studies have greatly enhanced our understanding of the cellular mechanisms of reassortment
within a host cell. Our purpose here is to offer a brief introduction on the role
of reassortment in segmented RNA virus evolution, explain the host cellular mechanisms
of reassortment, and provide a synthesis of recent experimental findings and methodological
developments. While we focus our discussion on influenza viruses, wherein most of
the studies on reassortment have been carried out, the concepts presented are broadly
applicable to other multipartite genomes.
What Is Virus Reassortment?
Virus reassortment, or simply reassortment, is a process of genetic recombination
that is exclusive to segmented RNA viruses in which co-infection of a host cell with
multiple viruses may result in the shuffling of gene segments to generate progeny
viruses with novel genome combinations (Fig 1A) [1]. Reassortment has been observed
in members of all segmented virus families, including, for example, Bluetongue virus
[2], but reassortment is most prominently described for influenza viruses as a primary
mechanism for interspecies transmission and the emergence of pandemic virus strains
[3–5]. For instance, reassortment accelerates the rate of acquisition of genetic markers
that overcome adaptive host barriers faster than the slower process of incremental
increase due to mutation alone. The emergence of new influenza genes in humans and
their subsequent establishment to cause pandemics have been consistently linked with
reassortment of novel and previously circulating viruses [4–6].
10.1371/journal.ppat.1004902.g001
Fig 1
Reassortment of two tripartite genomes producing a novel reassortant.
A) Diagrammatic representation of the emergence of a novel reassortant strain with
genes derived from two parents. B) Phylogenetic discordance between segments 1 and
3 (left) and segment 2 (right) for three tripartite strains. Branches in bolder colors
represent parental strains, whereas lighter colors represent the acquisition of gene
segments from different parents to form a novel reassortant strain.
In contrast, recombination occurs through a template switch mechanism, also known
as copy choice recombination. When two viruses co-infect a single cell, the replicating
viral RNA-dependant-RNA-polymerase can disassociate from the first genome and continue
replication by binding to and using a second distinct genome as the replication template,
resulting in the generation of novel mosaic-like genomes with regions from different
sources [7,8] such as some circulating recombinant forms of HIV [9]. Although, in
principle, recombination can occur in both segmented and non-segmented viruses, reports
of recombination in segmented viruses have been frequently disputed [10,11] as weak
evidence that arose through laboratory or bioinformatic artifacts [12,13]. Here we
focus on virus reassortment using the well-studied influenza virus as an example.
How Do Segmented Viruses Reassort within a Host Cell?
Essential prerequisites for reassortant include the entry of more than one virus particle
into a single host cell, followed by the concomitant production of genome segments
within the host cell. Experimental systems have revealed a high frequency of multiple
infections [1,14], although there is some evidence suggesting the role of specific
viral proteins limiting further infection [15].
Ultimately, the definitive formation of viable infectious reassortants is dependent
on the incorporation of one copy of each segment into a virus particle. Two alternative
mechanisms for reassortment within the host cell have been proposed. The random packaging
model [16,17] posits that viral RNA is incorporated in virions without discrimination
(but not other viral or cellular RNA); hence, the likelihood of forming viable reassortants
with an entire genome set occurs by chance [16]. However, mounting evidence supports
an alternative selective packaging model [18–20], which proposes that a virus particle
packages eight unique viral RNA segments through specific packaging signals. Experimental
visualization of RNA interactions [18] during virus assembly has revealed detailed
interactive networks—i.e., epistatic interaction of virus packaging signals—among
virus segments, which are thought to play an important role in directing reassortment.
Through the experimental swapping of packaging signals between influenza viruses of
different types, Essere et al. [19] were able to overcome the bias observed towards
specific genotypes. In an extreme case, Baker et al. [19,21] have shown that the swapping
of packaging signals of two different species of influenza viruses enabled reassortment
to form viable particles that have not been observed in nature, indicating a central
role for these packaging signals in reassortment. Intuitively, the emergence of differences
in the packaging signals of diverging virus lineages may lead to virus speciation.
Such a phenomenon could explain the lack of reassortment between the two influenza
virus species (A and B) that share structural and functional similarities and that
occupy the same ecological niche. Despite a lack of a mechanistic understanding of
the function of packaging signals, these observational studies highlight important
implications for viral evolution through epistatic interaction between gene segments
and the emergence of novel reassortants.
How Is Reassortment Detected?
The identification of reassortment is important to detect novel reassortants with
increased transmissibility, increased pathogenicity, or those that escape antibody
recognition or are resistant to antivirals. Reassortment is most commonly detected
through incongruencies in phylogenetic relationships among the different segments
of a viral genome [22–26], as gene segments from the same virus isolate occupy conflicting
phylogenetic positions due to differences in their evolutionary histories (Fig 1B).
Early studies identified reassortment by manually detecting phylogenetic incongruence
of different viral segments. However, this method becomes impractical for studying
large datasets, especially those with complex reassortment histories with nested reassortments
or when there is a lack of phylogenetic support for reassortment among closely related
sequences [27]. This has led to the development of several automated reassortment
detection methodologies [28–31], but the phylogeny-based methods have remained the
most robust and popular method for detecting reassortment [29,30]. Several extensions
of the phylogenetic method have also been successfully applied to estimate past reassortment
of viral lineages, including the coalescent-based Bayesian phylogenetics that infer
and compare the time of most recent common ancestor (TMRCA) of each segment to infer
possible reassortment [32], multi-dimensional scaling of tree distances [25,32], and
more recently, using time-resolved Bayesian phylogenetics and trait state changes
[33–35]. In addition, several distance-based methods exist [27], where degrees of
similarity between pairs of viral genomes are used to infer reassortment [36,37].
Recently, a study has used a novel method based on the rapid rate of amino acid replacement
post reassortment as a method of detecting a reassortment event [27]. While all the
studies listed above are aimed at identifying reassortment events and strains, methodologies
that infer an explicit rate of reassortment are rare, but examples include [33,34,38].
What Do Genomic Studies Tell Us about Reassortment?
Influenza exhibits high levels of mixed infections in all major hosts [39–42], with
up to 25% of all infections in avian hosts involving multiple influenza subtypes.
However, large-scale genomic studies have identified various levels of restrictions
on random reassortment between co-circulating influenza viruses, which differ depending
on host, subtype, and preferential genetic combinations [35,36,43–46]. The greatest
frequency of influenza reassortment is observed in their natural reservoir, wild aquatic
birds [40], where viruses of different subtypes frequently exchange gene segments.
However, reassortment is more restrictive in other hosts, particularly humans. Reassortment
between human seasonal influenza viruses of different subtypes (A/H1 and A/H3 viruses)
is rare [47] despite co-circulation over 40 years and extensive evidence of mixed
infection [39]. Furthermore, studies of human influenza viruses have shown that certain
combinations of gene segments were consistently detected in surveillance, suggesting
either preferential assortment of these gene segments or a fitness advantage to these
combinations. Convincing evidence comes from the two co-circulating and frequently
reassorting lineages of influenza B viruses [35,48], but virions consistently contained
the polymerase basic 1, 2, and the hemagglutinin (HA) genes (PB1-PB2-HA) from a single
lineage [35]. Similarly, preferential combinations of segments are transiently observed
for human influenza A viruses [45,46].
What Are the Consequences of Virus Reassortment?
The tremendous genomic novelty generated by reassortment confounds all current methods
of virus control. Evolutionary studies indicate an advantage for gene lineages with
reassorting backgrounds. Specifically, a significant increase in transient amino acid
mutations is observed following reassortment [27], primarily in the surface glycoprotein
hemagglutinin, the major immunogenic protein of influenza that leads to antigenic
change [25,32]. This suggests that the placement of the HA within novel genetic backgrounds
through reassortment greatly affects virus fitness and directly influences antigenic
variation, contributing to the long-term evolution of the virus. However, reassortment
could lead to evolutionary change due to various other factors, including selection
pressure induced by herd immunity; the residues being under weak selective constraint;
or compensation for fitness costs of mutations accruing elsewhere in the genome [25].
Similarly, the emergence of drug-resistant mutations may be acquired following reassortment,
as shown for the emergence of amantadine-resistant H3N2 viruses [49] and oseltamivir-resistant
seasonal H1N1 viruses [50]. These studies suggest that reassortment confounds available
methods of virus control, although detailed examination of the role of reassortment
in driving genome-wide evolution is still needed.