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Genetic diversity and evolution of closteroviruses

Alexander V. Karasev
Biotechnology Foundation Laboratories at Thomas Jefferson University,
Department of Microbiology and Immunology, 1020 Locust St., Philadelphia, PA 19107;
AKarasev@hendrix.jci.tju.edu

Closteroviruses are a diverse group of positive-strand RNA viruses with flexuous, filamentous virions, having a genome of up to 20 kb in size. They were recently compiled into a separate family Closteroviridae, comprising viruses with either mono- or bipartite genomes. Phylogenetically, based on amino acid sequence analyses of their replicative proteins, closteroviruses belong to an alphavirus-like supergroup of positive-strand RNA viruses, although they resemble animal arteri- and coronaviruses in terms of genome organization and expression. In infected plants closteroviruses exist as complex population mixtures including up to nine subgenomic RNAs and multiple defective RNAs, and represent an attractive model to study virus evolution. Closteroviruses infect a wide range of crops, are transmitted by insects, and demonstrate extreme biological and genetic diversity, suggesting a rapid, ongoing evolution within the family. However, due to their phloem tropism, low yield, large genomes, and other technical obstacles, it is generally difficult to quickly assess evolutionary relationships between members of the family using traditional methods of virus phylogeny utilizing replicative proteins. A unique feature of closteroviruses is a highly conserved heat shock related protein (HSP70) gene present in all known representatives of the family Closteroviridae. Here, I present phylogenetic analysis of HSP70 proteins encoded by mono- and bipartite closteroviruses and compare it to phylogenies based on analyses of replication-associated proteins. The resulting phylogenetic trees show remarkable similarity suggesting that easily acquired HSP70 sequences can be used for elucidation of the closteroviral evolutionary pathways. Several important conclusions can be drawn from these trees. Phylogenetic analyses of replicative genes as well as HSP70 demonstrate that closteroviruses co-evolved with their insect vectors, resulting in three major lineages, i.e. aphid-, mealybug-, and whitefly-transmitted viruses. Phylogenetic grouping also allows to predict possible vectors for several closteroviruses infecting vegetatively propagated crops for which natural vectors have been currently unknown. It appears that a group of bipartite, whitefly-transmitted closteroviruses compiled in the genus Crinivirus represents a relatively recently formed branch, probably with the greatest evolutionary potential. Evolution of closteroviruses seems to proceed through acquisition of new genes, sometimes of a host origin, and their subsequent diversification. These new acquisitions led to considerable expansion of a monopartite closterovirus genome size, record-breaking for alpha-like viruses.

Abstract - Presented at the Virus Evolution Workshop
Ardmore, OK
October 21 - 24th, 1999

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e-mail: mroossinck@noble.org

Dr. Marilyn Roossinck
Plant Biology Division
The Noble Foundation
P.O. Box 2180
Ardmore, OK 73402

phone: 580 224-6630