Variations of virion sizes in different clones of Venezuelan equine encephalomyelitis virus

Summary

An electron microscopic study of suspensions of Venezuelan equine encephalomyelitis (VEE) virus showed that mononucleoid virions, contained in the virus population, were heterogeneous in size (the coefficient variation of (V) of the long axis dimension was 10%). In contrast, virions present in cloned populations isolated from this suspension were relatively homogeneous (varied from 4 to 7%). Relative homogeneity and variations of virion sizes in different clones reflected a genotypical diversity of the original virus population. Among the clones tested some contained relatively small particles (a mean value of a long axis reaching 72.68±5.13 mμ and some relatively large virus particles (86.12±4.01 mμ). Typical size values of virions in genetically pure lines remained unchanged during serial cultivation in chick fibroblast cell cultures. When virus was exposed to 5-fluorouracil, mutations based on thermoresistance, plaque size and sensitivity to the inhibitory action of agar polysaccharides occurred simultaneously with an alteration of virion size. These findings led us to consider the size of mononucleoid virions as a genetic marker of VEE virus. An inverse correlation (a correlation coefficient r=−0.98) was observed between virion size and plaque size in clones isolated from the “wild” population; mutations towards an enlargement of virions were always accompanied by a decrease in plaque size.