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Biological variability and phylogenetic relationships of beet polerovirus species

Sébastien Hauser, Mark Stevens, Christophe Mougel, Etienne Herrbach,
Helen G. Smith, Christiane Fritsch and Olivier Lemaire
Vection et lutte intégrée, INRA, BP 507, 68021 Colmar (France)
Hauser@colmar. inra. fr

The yellowing disease of sugar beet can be caused by different viruses belonging to the Polerovirus genus within the Luteoviridae. These viruses are currently classified as Beet mild yellowing virus (BMYV) and Beet western yellows virus (BWYV). Host range, serological and molecular variability studies have shown that the strains of these viruses could be reclassified into three distinct species. The first species, BMYV, infects sugar beet and shepherd's purse. The second species, Brassica yellowing virus (BrYV), does not infect beet, but infects a large number of plants belonging to the Brassica genus. The third, Beet chlorosis virus (BChV), infects beet but not shepherd's purse.

The sequences of the BMYV, BrYV and BChV isolates studied showed that the coat protein (CP) amino acid regions are highly conserved (more than 90% homology), whereas the P0 sequences are variable (less than 30 % homology). Such distinct phylogeny between the two regions within same genomes suggests modular evolution of these species. Beet poleroviruses may have the same ancestor for the 3' part of the genome, which encodes the structural proteins. However they may have distinct ancestors for the 5' part of the genome encoding the functional proteins (replication and putatively host specificity).

A dot plot comparison of the full length nucleotide sequence of an European isolate of Beet chlorosis virus (BChV-2a) and of an American isolates (BChV-CR) revealed less homology in the 3' than in 5' part of the genomes. The 5' part of their genomes seems to be under high selection pressure, in order to maintain replication efficiency and probably host range specificity. The genetic diversity in the 3' part could be explained by adaptation to host plants and to aphid vectors in the areas where the particular isolate is prevalent.

New studies are underway at INRA Colmar to better understand the relationships between 5' sequence variability of the genome and the biological properties, and to clarify the relationships between Beet poleroviruses and other Luteovirus supergroup members.

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

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To contact the organizers:
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