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since 14/11/96
Members of a single orbivirus serogroup (species) have:
1. Significant levels of serological cross-reaction between
"conserved" virus structural or non-structural proteins, to at
least some other members of the group, which demonstrate the existence
of a "common antigen".
2. Significant levels of RNA sequence homology (in a conserved genome
segment) to at least some other members of the group (for example >77%
homology in segment 3 of different BTV strains). Members of different serogroups
will show lower levels of homology in the same conserved segments (for
example, <74% homology in segment 3 between even the closely related
Wallal and Warrego serogroup viruses). High levels of RNA sequence homology
could also be expected in other conserved genome segments.
3. Each orbivirus serogroup contains a number of different serotypes,
which can be distinguished using serum neutralization assays. Viruses which
are of the same serotype will also be of the same serogroup.
4. Viruses which belong to the same serogroup (species) may be capable
of reassorting genome segments during co-infection of the same cells. Reassortment
has not been demonstrated between viruses of different serogroups (species).
5. The position and the identity of those RNA bases which are conserved
at the termini of different genome segments also appears to be conserved
between different members of a serogroup (species). The members of some
different serogroups may also share the same conserved sequences. However,
it is expected that in most cases these sequences will be different between
serogroups.
6. Genome segments from viruses within a single orbivirus serogroup
(species) frequently show similar migration patterns during agarose gel
electrophoresis (AGE).
Significant levels of serological cross-reaction between "conserved"
virus structural or non-structural proteins, to at least some other members
of the group, which demonstrate the existence of a "common antigen".
Viruses which belong to different serogroups (species) usually do not show
significant serological cross reactions. In rare cases specific individual
viruses can show a low level cross-reaction with a member of another serogroup.
This is usually only a "one-way" reaction and is only evident
between serogroups that are recognized as "close" (for example,
BTV, EHDV, AHSV, EEV or Eubenangee).
These assays can use: polyclonal antibodies contained in convalescent
sera; antisera raised against infected tissue culture materials; antisera
raised against purified viral particles or viral proteins, which are known
serogroup specific antigens; antisera raised against recombinant bacteria
or virus expressed orbivirus proteins, which are known serogroup specific
antigens; antisera raised against expressed virus or core like particles;
or monoclonal antibodies against viral proteins which are known serogroup
specific antigens. The antigens used in these assays can include: infected
tissue culture materials; purified viral particles; purified viral proteins,
which are known serogroup specific antigens; recombinant bacteria or virus
expressed orbivirus proteins, which are known serogroup specific antigens;
or expressed virus or core like particles.
Significant levels of serological cross-reaction between virus structural
or non-structural proteins to at least some other members of the group,
that can be detected using a range of different serological assays, including
CF, AGID and ELISA. These cross-reactions are frequently two way. However,
serogroups may also include viruses, which cross react poorly with each
other but both react well with a third serogroup member. The antigens used
in these serogroup specific reactions do not include hyper variable proteins,
such as the outer capsid proteins which are involved in serotype specific
reactions with neutralizing antibodies, or other proteins which show a
high degree of variation (such as the NS3 protein of AHSV). The most suitable
proteins for analysis of intra or inter serogroup serological reactions
are the conserved core or nonstructural proteins, such as the outer core
protein VP7 and the "tubule" protein NS1 of BTV, EHDV or AHSV.
These proteins are particularly useful since they are both produced in
relatively large amounts during replication and VP7 in particular is immunodominant.
Another of the proteins which is highly conserved is the major inner
core scaffolding protein of BTV "VP3". Analysis of representative
RNA sequences from specific segments can now be made relative quickly using
RT PCR. Comparisons with data previously obtained for segment 3 from viruses
of different orbivirus serogroups have provided phylogenetic trees and
allow the relationship between new viruses and the members of established
species to be studied (Parkes and Gould, 1996; reviewed by Mertens 1994).
The amount of data that is available concerning reassortment of genome
segments between viruses from different orbivirus serogroups is
limited. This may in part be due to an assumption that it does not occur
and therefore few experiments have been attempted. Attempts to generate
reassortant progeny between members of even some of the closely related
serogroups have failed (for example, EHDV, BTV and Eubenangee; Mertens
et al., 1984). However, reassortment has frequently been demonstrated
between members of single orbivirus serogroups, in tissue culture cells
and in infected individuals of both vector and mammalian host species.
Incompatibility of the components of the genome segment selection, packaging
or replication mechanisms may prevent the production of reassortants between
the members of different serogroups. Functional constraints, could also
limit the viability or any reassortant progeny viruses produced.
Analysis of the terminal sequences of different orbiviruses have shown
that the BTV isolates analyzed all have the same two conserved hexanucleotide
sequences at the ends of the genome segments. It has been suggested for
several members of the Reoviridae that these conserved termini may
play a specific role in recognition assembly and replication of the progeny
virus genome. Their identity may therefore be one of the essential factors
permitting genome segment reassortment between different viruses. However,
similar terminal sequences to those found in BTV serogroup viruses were
also demonstrated in an EHDV (Mertens et al., 1994). AHSV and Eubenangee
serogroup viruses have fewer conserved bases. These data indicate that
although the identity of the terminal sequences may be conserved within
a serogroup, two serogroups may also share the same sequences. Similarities
in the near terminal regions, identified in a single segment may not always
demonstrate conservation of the same bases in all ten segments.
The serotype of each distinct virus strain within each orbivirus serogroup
appears to be controlled by the genome segments which code for the protein
components of the outer capsid layer. The serogroup is determined by the
other genome segments which code for the more conserved core and non-structural
proteins. The identity of each serotype appears to be specific to its own
serogroup. The identification of a serotype can therefore be used as a
method that also identifies the virus serogroup. The exclusivity of serotypes
to a particular serogroup also suggests that reassortment between serogroups
is either not possible or is a rare event.
Genome segments from viruses within a single orbivirus serogroup (species)
frequently show similar migration patterns during agarose gel electrophoresis
(AGE) (Pedley et al., 1988). These patterns are much less variable
than when the RNA segments are compared by high percentage PAGE, for example
the genome profiles of the different BTV isolates which have been compared
by AGE are indistinguishable. The EHDV serogroup viruses are slightly more
variable but show a high degree of similarity in the migration of most
of the genome segments. Although viruses from closely related orbivirus
serogroups may have relatively similar dsRNA migration patterns during
AGE, in the majority of cases where they have been compared they are distinguishable.
Mertens, P.P.C. (1994). Orbiviruses and coltiviruses -
general features. In: Encyclopedia of Virology, edited by Webster, R.G.
and Granoff, A. London: Academic Press, 2, 941-956.
Pedley, S., Mohammed, E.H. and Mertens, P.P.C. (1988). Analysis
of the genome segments of six different bluetongue virus isolates using
RNA-RNA hybridisation: A generalised coding assignment for bluetongue viruses.
Virus Research 10: 381-390.
Mertens, P.P.C. and Sangar, D.V. (1985). Analysis of the terminal sequences
of the genome segments of four orbiviruses. Virology 140: 55-67.
Parkes, H., and Gould, A.R., (1996). Characterisation of Wongorr virus,
an Australian orbivirus. Virus Research 44: 111-122.
Comments to Dr. Peter P.C. Mertens (peter.mertens@bbsrc.ac.uk)