Last updated 5/2/1997
Members of a single cypovirus species (type) will have:
1. Similar electrophoretic migration of at least 7 genome segments, as analysed using either an agarose, or a low percentage (not above 5%) polyacrylamide gel system. Consequently, significant differences in the electrophoretic migration of at least three genome segments is required to distinguish two different "electropherotypes" (species).
2. A high degree of sequence conservation in all of the genome segments (estimated to be >80%). While members of different species will show significantly lower conservation (estimated to be <70%).
3. High levels of serological cross reaction by ELISA or AGID, for example using polyclonal antisera to purified virions or polyhedrin proteins. The cross reaction between the members of different electropherotypes (species) will be very significantly lower, in most cases below detectable levels.
4. Current limited evidence suggests that the conserved terminal sequences are likely to be the same within an electropherotype (species) but may be different between different types.
The current separation of a "species" within the genus Cypovirus is based primarily on the identification of significant differences in the electrophoretic migration of the genome segments by analysis of the genome "profile" using either low (< 5 %) polyacrylamide gels, or agarose gel electrophoresis systems (Payne and Rivers, 1976; Payne et al., 1977; Payne and Mertens, 1983; Mertens et al., 1979). These gel systems minimise the differences in segment migration due to variation in secondary structure, which can be more easily detected by using higher percentage polyacrylamide gel systems. Significant differences in the relative migration of at least three genome segments is currently regarded as sufficient to distinguish two "electropherotypes" or species. The paper originally proposing this classification and identifying the first eleven species or types is by (Payne and Rivers, 1976) Subsequent studies have already lead to the recognition (using the above definition) of one further electropherotype (Payne et al., 1977) and with the two new types (see proposals) the total currently stands at 14 (N.B. now accepted by ICTV).
The amount of comparable RNA sequence data that is currently available for different members of the cypovirus genus is still very limited. At this stage there is insufficient sequence data to give a clear quantitative picture of the levels of RNA sequence homology that is present or may be expected in the different genome segments from different cypovirus types (species). The sequence comparison of segment 10 of type 1 and 5 is reported to show "no homology" (Fossiez et al., 1998). However, blotting and cross hybridisation studies have been used to compare the genome segments from six distinct cypovirus isolates of four different serotypes (three different isolates of type 1 and isolates of types 2,5 and 12: Mertens et al manuscript in preparation) and from type 1 and 5 (Galinski et al., 1983). These studies demonstrated that the level of cross-hybridisation between any of the genome segments of different "types" was "low", indicating that the RNA sequence homology was also low. By comparison to results from similar experiments with 6 different bluetongue virus (BTV) isolates (Mertens et al 1987) and BTV RNA sequence data (reviewed by Mertens, 1994), it is estimated that within a single cypovirus type the genome segments will all share > 80% RNA sequence homology. It is also estimated that the remaining genome segments of viruses from different cypovirus "types" will share <70% RNA sequence homology. Although in most cases the level of homology will be significantly lower between the different types (Galinski et al., 1983), these estimated figures take into account the relatively closer relationship that was observed between type 1 and type 12 viruses. These two virus types are recognised to be more closely related (although still only distantly) than other the types (species) both by cross hybridisation and in terms of genome segment migration (only three different segments different). With the possible exception of slightly higher levels of conservation in one of the larger segments, there was no evidence for highly conserved and variable (serotype specific) genome segments, as is seen in some of the other genera which have mammalian hosts (for example the orbiviruses).
In the absence of suitable cell culture systems which will support multiple rounds of cypovirus replication, or even the replication of all of the different types, there is no accepted serum neutralisation assay system for serotyping these viruses. It is also arguable that the absence of neutralising antibodies in the insect systems makes the whole concept of serotype less relevant to this particular genus. In serological comparisons made with the same group of six viruses described above (Mertens et al., 1979) there was little or no cross reaction by ELISA, or AGID between the virion or polyhedral proteins from viruses of different electropherotypes (species). The only viruses which showed any significant cross-reactions were those within a single type and (at a much lower level) between the type 1 and type 12 viruses.
The limited cypovirus RNA sequence data that is available does indicate that the conserved groups of six or seven base pairs, which are present at the termini (+ve 5', or 3', respectively) of the cypovirus genome segments may be different in different types (species). (type 1 +ve5'AGUAAA .............GUUAGCC 3'; Arella et al., 1988 ; or type 5 +ve 5'AGTTTA ...........GAGTTGC3' ; Fossiez et al., 1989; Payne and Mertens, 1983). In view of the potential importance which has frequently been suggested of such conserved termini in the identification of viral mRNA during replication and assembly of the viral genome, they may represent one determining factor governing the ability of different cypovirus strains to reassort genome segments during co-infection. If that is the case, as also seems likely from studies with other members of the Reoviridae, identification of these terminal sequence may also be a distinguishing feature of each cypovirus species, although possibly not an "exclusive" one. In other genera of the Reoviridae the number and positions of the conserved bases relative to the termini can vary between different species (serogroups). It seems likely that the members of a single cypovirus species will all have the same conserved RNA terminal sequences, although it is possible that the members of two different species could also share the same conserved termini.
There does not appear to be an exclusive species relationship between the cypoviruses and their hosts and as such identification of the host from which the virus was originally isolated or in which it was grown, may be useful information to help identify strains but has little taxonomic value. This can lead to some complications since in much of the early literature the viruses are named in this way.
Arella, M., Lavallee, C., Belloncik, S., and Furuichi, Y. (1988). Molecular cloning and characterisation of cytoplasmic polyhedrosis virus polyhedrin and a viable deletion mutant gene. Journal of Virology 62: 211-217.
Fossiez, F., Belloncik, S., and Arella, M. (1989). Nucleotide sequence of the polyhedrin gene of Euxoa scandens cytoplasmic polyhedrosis virus (Escpv) Virology 169: 462-465.
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.
Mertens, P.P.C., Crook, N.E., Rubinstein, R., Pedley, S. and Payne, C.C. (1989). Cytoplasmic polyhedrosis virus classification by electropherotype: Validation by serological analyses and agarose gel electrophoresis. Journal of General Virology 70: 173-185.
Mertens, P.P.C., Pedley, S., Cowley, J. and Burroughs, J.N. (1987). A comparison of six different bluetongue virus isolates by crosshybridisation of the dsRNA genome segments. Virology 161: 438-447.
Payne, C.C., Piasecka-Serafin, M., and Pilley, B. (1977). The properties of two recent isolates of cytoplasmic polyhedrosis viruses. Intervirology 8: 155-163.
Payne, C.C. and Mertens, P.P.C. (1983). Cytoplasmic polyhedrosis viruses, in "the Reoviridae". (Ed. W.K. Joklik) 425-504, Plenum Press, New York.
Payne, C.C., and Rivers, C.F. (1976). A provisional classification of cytoplasmic polyhedrosis viruses based on the sizes of the RNA genome segments. Journal of General Virology 33: 71-85.
To recognise a cypovirus, isolated from the nest of Polistes hebraeus a new and distinct electropherotype as cypovirus type 13 and therefore as an additional cypovirus species.
Fouilaud,M. and Morel,G., (1994). Characterisation of cytoplasmic and nuclear polyhedrosis viruses recovered from the nest of Polistes hebraeus F.(Hymenoptera; Vespidae). Journal of Invertebrate Pathology 42: 401-404.
To recognise a new cypovirus isolated from Heliothis armigera as a distinct electropherotype as cypovirus type 14 and therefore as an additional cypovirus species.
Belloncik, S., Liu, J., Su, D. and Arella, M. (1996). Identification and characterisation of a new cypovirus type 14, isolated from Heliothis armigera. Journal of Invertebrate Pathology 67: 41-47.