Direct method for quantitation of extreme polymerase error frequencies at selected single base sites in viral RNA

Altered replicase specificity is responsible for resistance to defective interfering particle interference of an Sdi- mutant of vesicular stomatitis virus

The in vitro resistance of an Sdi- mutant of vesicular stomatitis virus to interference by wild-type defective interfering (DI) particles was expressed quantitatively in a cell-free replication system derived from mutant-infected cells. Added wild-type DI particle templates were replicated very poorly by extracts of Sdi- mutant-infected cells. However, the addition of purified viral polymerase (a complex of L and NS proteins) from wild-type vesicular stomatitis virus allowed efficient replication of wild-type DI particle genomes in these cell extracts. Added wild-type NS protein alone did not complement DI particle genome replication in these cell extracts, but it did complement a defect in the in vitro transcriptional activity of Sdi- mutant virus. These results clearly implicate the vesicular stomatitis virus polymerase complex in the inability of Sdi- mutants to replicate DI particles and in the quantitative escape from DI particle interference in evolving virus populations.

Gene encoding capsid protein VP1 of foot-and-mouth disease virus: a quasispecies model of molecular evolution

A phylogenetic tree relating the VP1 gene of 15 isolates of foot-and-mouth disease virus (FMDV) of serotypes A, C, and O has been constructed. The most parsimonious tree shows that FMDV subtypes and isolates within subtypes constitute sets of related, nonidentical genomes, in agreement with a quasispecies mode of evolution of this virus. The average number of nucleotide replacements per site for all possible pairs of VP1 coding segments is higher among representatives of serotype A than serotype C or O. In comparing amino acid sequences, the values of dispersion index (variance/mean value) are greater than 1, with the highest values scored when all sequences are considered. This indicates an accumulation of mutations at a limited number of residues, suggesting that distributions of sequences fluctuate around points of high stability. Evolution of FMDV follows a path very distant from that of a star phylogeny, and it has not been possible to derive conclusions on constancy of evolutionary rates with the test applied to the analysis. FMDVs, as other RNA viruses, are of limited genetic complexity and their population sizes are extremely large. Their evolution concerns complex, indeterminate mixtures of genomes rather than a single, determinate species.

Determination of the mutation rate of a retrovirus

The mutation rate of Rous sarcoma virus (RSV) was measured. Progeny descended from a single virion were collected after one replication cycle, and seven regions of the genome were analyzed for mutations by denaturing-gradient gel electrophoresis. In all, 65,250 nucleotides were screened, yielding nine mutations, and the RSV mutation rate was calculated as 1.4 x 10(-4) mutations per nucleotide per replication cycle. These results indicate that RSV is an extremely mutable virus. We speculate that the mutation rate of a virus may correlate inversely with the effectiveness of vaccination against a given virus and suggest that prevention of retrovirus-mediated disease via vaccination may prove difficult.

RNA genome stability of Toscana virus during serial transovarial transmission in the sandfly Phlebotomus perniciosus

We have carried out a T1 ribonuclease fingerprinting analysis of the RNA genomes of Toscana virus isolates from successive generations of an experimentally virus-infected laboratory colony of Phlebotomus perniciosus sandflies. This analysis detected no virus RNA genome changes during transovarial transmission of the virus over 12 sandfly generations (a period of almost 2 years). These results demonstrate that although RNA viruses can exhibit high rates of mutational change under a variety of conditions, Toscana virus RNA genomes can be maintained in a stable manner during repeated transovarial virus transmission in the natural insect host. The implications of these results for insect RNA virus evolution are discussed.

Evolution of sex in RNA viruses

The distribution of deleterious mutations in a population of organisms is determined by the opposing effects of two forces, mutation pressure and selection. If mutation rates are high, the resulting mutation-selection balance can generate a substantial mutational load in the population. Sex can be advantageous to organisms experiencing high mutation rates because it can either buffer the mutation-selection balance from genetic drift, thus preventing any increases in the mutational load (Muller, 1964: Mut. Res. 1, 2), or decrease the mutational load by increasing the efficiency of selection (Crow, 1970: Biomathematics 1, 128). Muller's hypothesis assumes that deleterious mutations act independently, whereas Crow's hypothesis assumes that deleterious mutations interact synergistically, i.e., the acquisition of a deleterious mutation is proportionately more harmful to a genome with many mutations than it is to a genome with a few mutations. RNA viruses provide a test for these two hypotheses because they have extremely high mutation rates and appear to have evolved specific adaptations to reproduce sexually. Population genetic models for RNA viruses show that Muller's and Crow's hypotheses are also possible explanations for why sex is advantageous to these viruses. A re-analysis of published data on RNA viruses that are cultured by undiluted passage suggests that deleterious mutations in such viruses interact synergistically and that sex evolved there as a mechanism to reduce the mutational load.

Rapid selection of genetic and antigenic variants of foot-and-mouth disease virus during persistence in cattle

Rapid evolution of foot-and-mouth disease virus (FMDV) is documented during persistent infections of cattle. The carrier state was established experimentally with plaque-purified FMDV of serotype C3. Virus was recovered from the esophageal pharyngeal area of the animals up to 539 days postinfection. Analysis of capsid proteins by electrofocusing and by electrophoretic mobility of the genomic poly(C)-rich tract suggested heterogeneity in several isolates and sequential dominance of viral subpopulations. Nucleotide sequences of the VP1-coding region of the parental FMDV C3 clones and of seven isolates from the carrier cattle showed point mutations that represented rates of fixation of mutations of 0.9 X 10(-2) to 7.4 X 10(-2) substitutions per nucleotide per year; 59% of the base changes led to amino acid substitutions, some of which were located within residues 135 to 151, a region involved in neutralization of FMDV. In the esophageal pharyngeal fluid samples, FMDV C3-neutralizing activity was present. Antigenic variation was demonstrated with monoclonal antibodies raised against FMDV C3. Two isolates from carrier cattle differed from the parental virus by 10(2)- or 10(3)-fold decreased reactivity with neutralizing monoclonal antibodies. We suggest that persistent, inapparent infections of ruminants, in addition to being a reservoir of virus, may promote the rapid selection of antigenically variant FMDVs.

Antigenic analysis of African measles virus field isolates: identification and localisation of one conserved and two variable epitope sites on the NP protein

Measles virus isolates from epidemics in the Cameroons (1983) and Gabon (1984) were analysed by a panel of monoclonal antibodies against four of the virion proteins. We observed no antigenic variation in the haemagglutinin, the fusion glycoprotein, or in the matrix protein. However, both inter- and intra-epidemic variation was observed in the nucleoprotein (NP). On the basis of strain reactivity and a competition binding assay, three epitopic sites were designated on the NP. One site was found on all the measles virus strains examined, whereas the other two were variable. Examination of proteolytic cleavage of the NP in situ (on the ribonucleoparticle) showed that the conserved site is located on a large fragment which remains bound to the viral genome. The peptide removed by proteolysis contained the two variable epitopes. The variability of the NP is discussed in relationship to its biological activity.

VP1 of serotype C foot-and-mouth disease viruses: long-term conservation of sequences

The nucleotide sequences of the VP1-coding regions of several isolates of serotype C3 foot-and-mouth disease virus (FMDV) were determined. The deduced amino acid sequences were compared with those of serotype C1 FMDV. The results provide evidence for two different lineages of FMDV C3 and document the potential for both long-term conservation and rapid evolution of FMDV.

Point mutations in glycoprotein gene of vesicular stomatitis virus (New Jersey serotype) selected by resistance to neutralization by epitope-specific monoclonal antibodies

Antigenic variants of the New Jersey serotype of vesicular stomatitis virus (VSV-NJ) were isolated and cloned by selecting virus plaques resistant to neutralization by high-titered monoclonal antibodies (MAbs) directed to glycoprotein (G) epitopes V, VI, VII, or VIII. The G proteins of each neutralization-resistant virus variant also exhibited markedly reduced antigenic reactivity with each corresponding epitope-specific MAb as determined by enzyme-linked immuno-absorbent assay and by Western blot analysis. Loss of antigenic reactivity of certain mutant G proteins to a MAb other than the one used to select the mutant virus suggested close antigenic proximity, particularly for epitopes VI and VII. The virion RNAs coding for the entire G gene of the wild-type virus and 10 MAb-induced mutants were sequenced by primer DNA extension using the dideoxy method. Each mutant G gene exhibited only a single nucleotide change, leading in each case to a single amino acid substitution, as follows: Glu210----Lys for all three mutants selected by MAb14 (epitope VII); Pro268----Thr for one mutant selected by MAb12 (epitope VI); Ser277----Lys for all three mutants selected by MAb15 (epitope VIII); and Glu364----Lys for all three mutants selected by MAb11 (epitope V). These neutralizing MAb-selected mutations are clustered in the middle third of the 517-amino acid VSV-NJ G protein, presumably resulting in conformational changes that alter recognition of one or more antigenic determinants by a specific monoclonal antibody.

Multiple viral mutations rather than host factors cause defective measles virus gene expression in a subacute sclerosing panencephalitis cell line

A measles virus (MV) genome originally derived from brain cells of a subacute sclerosing panencephalitis patient expressed in IP-3-Ca cells an unstable MV matrix protein and was unable to produce virus particles. Transfection of this MV genome into other cell lines did not relieve these defects, showing that they are ultimately encoded by viral mutations. However, these defects were partially relieved in a weakly infectious virus which emerged from IP-3-Ca cells and which produced a matrix protein of intermediate stability. The sequences of several cDNAs related to the unstable and intermediately stable matrix proteins showed many differences in comparison with a stable matrix protein sequence and even appreciable heterogeneity among themselves. Nevertheless, partial restoration of matrix protein stability could be ascribed to a single additional amino acid change. From an examination of additional genes, we estimated that, on average, each MV genome in IP-3-Ca cells differs from the others in 30 to 40 of its 16,000 bases. The role of extreme variability of RNA virus genomes in persistent viral infections is discussed in the context of the pathogenesis of subacute sclerosing panencephalitis and of other human diseases of suspected viral etiology.

Genetic diversity of enzootic isolates of vesicular stomatitis virus New Jersey

The RNA genomes of 43 vesicular stomatitis virus (VSV) isolates of the New Jersey (NJ) serotype were T1-ribonuclease fingerprinted to compare the extent of genetic diversity of virus from regions of epizootic and enzootic disease activity. Forty of these viruses were obtained from Central America during 1982 to 1985. The other three were older isolates, including a 1970 isolate from Culex nigripalpus mosquitos in Guatemala, a 1960 bovine isolate from Panama, and a 1976 isolate from mosquitos (Mansonia indubitans) in Ecuador. The data indicate that extensive genetic diversity exists among virus isolates from this predominantly enzootic disease zone. Six distinct T1 fingerprint groups were identified for the Central American VSV NJ isolates from 1982 to 1985. The 1960 VSV NJ isolate from Panama and the 1976 isolate from Ecuador formed two additional distinct fingerprint groups. This finding is in sharp contrast to the relatively close genetic relationship existing among VSV NJ isolates obtained from predominantly epizootic disease areas of the United States and Mexico during the same period (S. T. Nichol, J. Virol. 61:1029-1036, 1987). In this previous study, RNA genome T1 fingerprint differences were observed among isolates from different epizootics; however, the isolates were all clearly members of one large T1 fingerprint group. The eight T1 fingerprint groups described here for Central American and Ecuadorian viruses are distinct from those characterized earlier for virus isolates from the United States and Mexico and for the common laboratory virus strains Ogden and Hazelhurst. Despite being isolated 14 years earlier, the 1970 insect isolate from Guatemala is clearly a member of one of the 1982 to 1985 Central American virus fingerprint groups. This indicates that although virus genetic diversity in the region is extensive, under certain natural conditions particular virus genotypes can be relatively stably maintained for an extended period. The implications of these findings for the evolution of VSV NJ and epizootiology of the disease are discussed.

Genetic and immunogenic variations among closely related isolates of foot-and-mouth disease virus

Genetic heterogeneity among closely related isolates of foot-and-mouth disease virus (FMDV) has been measured by direct sequencing of the VP1-coding-region RNA for three new FMDVs of serotype C1 and by additional sequences of RNA from previously reported isolates, all belonging to a single episode of disease [Sobrino et al., Gene 50 (1986) 149-159]. In the ten viruses compared, eight different VP1 are represented. The changes include amino acid substitutions at a critical antigenic determinant of VP1. We document that variations present in such natural isolates result in changes of the immunogenic properties of the viruses. Vaccines prepared with two of the FMDV C1 analyzed induce complete protection against an homologous virus but only partial protection against an heterologous virus in swine, the host from which these viruses were isolated.

Direct measurement of the poliovirus RNA polymerase error frequency in vitro

The fidelity of RNA replication by the poliovirus-RNA-dependent RNA polymerase was examined by copying homopolymeric RNA templates in vitro. The poliovirus RNA polymerase was extensively purified and used to copy poly(A), poly(C), or poly(I) templates with equimolar concentrations of noncomplementary and complementary ribonucleotides. The error frequency was expressed as the amount of a noncomplementary nucleotide incorporated divided by the total amount of complementary and noncomplementary nucleotide incorporated. The polymerase error frequencies were very high and ranged from 7 x 10(-4) to 5.4 x 10(-3), depending on the specific reaction conditions. There were no significant differences among the error frequencies obtained with different noncomplementary nucleotide substrates on a given template or between the values determined on two different templates for a specific noncomplementary substrate. The activity of the polymerase on poly(U) and poly(G) was too low to measure error frequencies on these templates. A fivefold increase in the error frequency was observed when the reaction conditions were changed from 3.0 mM Mg2+ (pH 7.0) to 7.0 mM Mg2+ (pH 8.0). This increase in the error frequency correlates with an eightfold increase in the elongation rate that was observed under the same conditions in a previous study.

Heterogeneity of genome rearrangements in rotaviruses isolated from a chronically infected immunodeficient child

Rotaviruses with genome rearrangements, isolated from a chronically infected immunodeficient child, were adapted to growth in BSC-1 cells. Preparations of viral RNA from fecal extracts showed a mixed atypical rotavirus RNA profile, which was due to the presence of at least 12 subpopulations of viruses grossly differing in genotype. Besides various forms of genome rearrangements involving segment 8-, 10-, and 11-specific sequences, reassortment in vivo was likely to have occurred during the emergence of these viruses. The protein products of viral genomes with various forms of segmental rearrangements seemed to be largely unaltered. Genome rearrangement is proposed to be a third mechanism directing the evolution of rotaviruses.

The isolation of interferon-inducing mutants of vesicular stomatitis virus with altered viral P function for the inhibition of total protein synthesis

We have previously reported that T1026, a temperature-sensitive (ts) noncytocidal mutant of VSV, and its ts revertant, T1026-R1, are nonconditional mutants in the VSV function "P" for the inhibition of total protein synthesis (viral plus cellular) in infected cells (C. P. Stanners, A. M. Francoeur, and T. Lam, 1977, Cell 11, 273-281; C. P. Stanners, S. Kennedy, and L. Poliquin, 1987, Virology 160, 255-258). We have also shown that P- mutants such as these are superior interferon inducers relative to their parental P+ wild-type virus, HR, and that P- mutants may be distinguished from P+ virus using the plaque interferon production of PIF assay. (A. M. Francoeur, T. Lam, and C. P. Stanners, 1980, Virology 105, 526-536). In order to carry the analysis of VSV P function further, a number of independent mutants in the VSV P function are required. We show here that the PIF assay may be used to isolate spontaneously occurring interferon-inducing mutants (PIF+ mutants) from wild-type VSV (PIF- virus) populations. About one-half of the PIF+ mutants isolated with the PIF assay were found to have alterations in the VSV P function. As well as mutants that were defective for the inhibition of total protein synthesis, the assay yielded a new class of VSV P function mutants which appear to inhibit protein synthesis more severely than does P+ virus. The majority of newly isolated PIF+ mutants was also found to be temperature sensitive for growth. The ts phenotype, however, could be reverted for most PIF+ mutants with little effect on the PIF or P phenotype. These findings show that interferon induction and P function are related functions of VSV; this fact has allowed the isolation of a repertoire of mutants with widely varying P function.

Comparison of the virulent Asibi strain of yellow fever virus with the 17D vaccine strain derived from it

We have sequenced the virulent Asibi strain of yellow fever virus and compared this sequence to that of the 17D vaccine strain, which was derived from it. These two strains of viruses differ by more than 240 passages. We found that the two RNAs, 10,862 nucleotides long, differ at 68 nucleotide positions; these changes result in 32 amino acid differences. Overall, this corresponds to 0.63% nucleotide sequence divergence, and the changes are scattered throughout the genome. The overall divergence at the level of amino acid substitution is 0.94%, but these changes are not randomly distributed among the virus protein. The capsid protein is unchanged, while proteins NS1, NS3, and NS5 contain 0.5% amino acid substitutions, and proteins ns4a and ns4b average 0.8% substitutions. In contrast, proteins ns2a and ns2b have 3.0 and 2.3% amino acid divergence, respectively. The envelope protein also has a relatively high rate of amino acid change of 2.4% (a total of 12 amino acid substitutions). The large number of changes in ns2a and ns2b, which are largely conservative in nature, may result from lowered selective pressure against alteration in this region; among flaviviruses, these polypeptides are much less highly conserved than NS1, NS3, and NS5. However, many of the amino acid substitutions in the E protein are not conservative. It seems likely that at least some of the difference in virulence between the two strains of yellow fever virus results from changes in the envelope protein that affect virus binding to host receptors. Such differences in receptor binding could result in the reduced neurotropism and vicerotropism exhibited by the vaccine strain.

Molecular epizootiology and evolution of vesicular stomatitis virus New Jersey

Vesicular stomatitis virus (VSV) has been shown previously to be capable of undergoing rapid mutational change during sequential experimental infections in various tissue culture cell systems (J. Holland, K. Spindler, F. Horodyski, E. Grabau, S. Nichol, and S. Vandepol, Science 215:1577-1585, 1982). The present study was undertaken to determine the degree of genetic diversity and evolution of the virus under natural infection conditions and to gain insight into the epizootiology of the disease. Between 1982 and 1985, numerous outbreaks of VSV of the New Jersey serotype were reported throughout regions of the United States and Mexico. A T1 RNase fingerprint analysis was performed on the RNA genomes of 43 virus isolates from areas of epizootic and enzootic virus activity. This indicates that virus populations were genetically relatively homogeneous within successive U.S. virus epizootics. The data included virus isolates from different epizootic stages, geographical locations, host animals, and host lesion sites. In contrast, only distant genome RNA T1 fingerprint similarities were observed among viruses of the different U.S. epizootics. However, Mexican viruses isolated before or concurrent with U.S. epizootics had very similar RNA genome fingerprints, suggesting that Mexico may have been the possible origin of virus initiating recent U.S. VSV New Jersey outbreaks. Comparison of T1 fingerprints of viruses with enzootic disease areas revealed a greater extent of virus genetic diversity in these areas relative to that observed in epizootic areas. The evolutionary significance of these findings and their relationship to experimental data on VSV evolution are discussed.

The hemagglutinin-neuraminidase (HN) gene of Newcastle disease virus strain Italien (ndv Italien): comparison with HNs of other strains and expression by a vaccinia recombinant

A cDNA library was constructed with poly(A+) mRNA from cells infected with the virulent Italien NDV strain. A clone that hybridized to the HN gene mRNA was sequenced. A long open reading-frame encodes for a protein of 571 amino acids, with a calculated molecular weight of 61,900, including 13 cysteine residues and six potential glycosylation sites. To define the sequence changes that occurred in the avian paramyxovirus hemagglutinin-neuraminidase (HN) during the evolution of virulence, we have studied the HNs of the virulent Italien NDV strain, the mesovirulent Beaudette strain and the nonvirulent Hitchner strain. The majority of amino acid variations are conservative changes but they cluster at 4 preferential sites in the putative head of HN. The clusters of amino acid substitutions are intimately associated or overlap with regions of HN rich in charged amino acid residues and in cysteines. The latter are conserved not only between HNs from all 3 NDV strains but also between HNs of 4 different paramyxoviruses, NDV, SV 5, Sendai and PI 3. The HN coding sequence was inserted into the genome of vaccinia virus under the control of vaccinia P 7.5 K transcriptional regulatory sequences. Expression of native HN proteins at the surface of recombinant HN vaccinia-infected cells was demonstrated by indirect immunofluorescence with 2 anti-HN monoclonals.

Sequence analysis of the E2 gene of a hyperglycosylated, host restricted mutant of Sindbis virus and estimation of mutation rate from frequency of revertants

SVap15/21, a strain of Sindbis virus (SV) derived from our standard laboratory strain of SV (SVstd) after repeated passage on Aedes albopictus cells, grows normally in mosquito cells but is host restricted (hr) in vertebrate cells. It is also temperature sensitive (ts) and produces pinpoint plaques on vertebrate cells (sp). E2 glycoprotein of SVstd differs from that of the more widely used SVHR (from which SVstd was derived) by an additional (i.e., third) N-linked glycan. The E2 of SVap15/21, in turn, differs from that of SVstd by the addition of a fourth glycan. We have determined the nucleotide sequence of the E2 genes of SVap15/21 and of SVstd, as well as that of our isolate of SVHR. The nucleotide sequence of the SVstd E2 gene predicted the occurrence of an additional N-linked glycan attachment site, not present in the SVHR E2, at Asn232 (Asp in SVHR). The sequence of the SVap15/21 E2 gene demonstrated three mutations relative to the SVstd gene, including one that predicted the occurrence of another potential N-linked glycan attachment site at Asn275. Sequence analysis of 15 revertants of SVap15/21 which are no longer host-restricted revealed that all had lost the glycosylation site at Asn275, confirming the connection between the hyperglycosylation and the host dependent block in assembly. Most of these revertants had also lost the temperature sensitivity and small plaque traits (i.e., were ts+ and lp). Each revertant of this class was characterized by one of three different mutations in two separate codons (Asn275 and Thr277), resulting in the loss of the glycosylation site at Asn275. A fourth mutation, resulting in an Asn275----Tyr substitution, was associated with a hr+ ts phenotype in three isolates. Finally an additional mutation in a different part of the E2 gene was found in two hr+ ts sp isolates that had also lost the glycosylation site at Asn275 through a mutation resulting in a Thr277----Ile substitution. Knowledge of the nucleotide sequences associated with the ts defect in SVap15/21 and with its reversion permit an estimation of the mutation rate of this virus. This calculation indicates a mutation rate of less than 10(-6) errors per base incorporation.

Measurement of the mutation rates of animal viruses: influenza A virus and poliovirus type 1

Epidemiologic and genetic evidence suggests that influenza A viruses evolve more rapidly than other viruses in humans. Although the high mutation rate of the virus is often cited as the cause of the extensive variation, direct measurement of this parameter has not been obtained in vivo. In this study, the rate of mutation in tissue culture for the nonstructural (NS) gene of influenza A virus and for the VP1 gene in poliovirus type 1 was assayed by direct sequence analysis. Each gene was repeatedly sequenced in over 100 viral clones which were descended from a single virion in one plaque generation. A total of 108 NS genes of influenza virus were sequenced, and in the 91,708 nucleotides analyzed, seven point changes were observed. A total of 105 VP1 genes of poliovirus were sequenced, and in the 95,688 nucleotides analyzed, no mutations were observed. We then calculated mutation rates of 1.5 X 10(-5) and less than 2.1 X 10(-6) mutations per nucleotide per infectious cycle for influenza virus and poliovirus, respectively. We suggest that the higher mutation rate of influenza A virus may promote the rapid evolution of this virus in nature.

Genetic variability of the AIDS virus: nucleotide sequence analysis of two isolates from African patients

To define further the genetic variability of the human AIDS retrovirus, we have cloned and sequenced the complete genomes of two isolates obtained from Zairian patients. Their genetic organization is identical with that of isolates from Europe and North America, confirming a common evolutionary origin. However, the comparison of homologous proteins from these different isolates reveals a much greater extent of genetic polymorphism than previously observed. It is nevertheless possible to define conserved domains in the viral proteins, especially in the envelope, that could be of interest for the understanding of the molecular mechanisms of viral pathogenicity and for the development of diagnostic and therapeutic reagents.

Fixation of mutations in the viral genome during an outbreak of foot-and-mouth disease: heterogeneity and rate variations

Rates of fixation of mutations during the evolution of the foot-and-mouth disease virus (FMDV) C1 in nature have been estimated by hybridization of viral RNA to cloned cDNAs representing defined FMDV genome segments, and comparison of the selected RNAs by T1 RNase oligonucleotide fingerprinting. Values ranged from less than 0.04 X 10(-2) to 4.5 X 10(-2) substitutions per nucleotide per year (s/nt/yr), depending on the time period and the genomic segment considered. Rates for viral structural protein genes were up to sixfold higher than for nonstructural protein genes. Values in excess of 10(-2) s/nt/yr have been measured for the RNA region that encodes VP1-VP3. The nucleotide sequences of the major immunogenic region of capsid protein VP1 have been determined for six new FMDV C1 isolates, and they are compared with the two previously known sequences of FMDV C1 (C-S8 and C1-O). Both oligonucleotide fingerprinting of selected RNA fragments and direct nucleotide sequencing demonstrate that genetic heterogeneity exists among three viruses isolated on the same day, introducing a significant indetermination in the evaluation of fixation rates of mutations. During the FMDV C1 outbreak, amino acid substitutions did occur that are known to affect the immunological properties of the virus. The proportion of mutations between two viral RNAs does not increase significantly with the time elapsed between the two isolations, suggesting a cocirculation of multiple, related, nonidentical FMDVs ('evolving quasispecies') as the mode of evolution of this agent.