Turnip mosaic potyvirus (TuMV) is probably the most widespread and damaging virus that infects cultivated brassicas worldwide. Previous work has indicated that the virus originated in western Eurasia, with all of its closest relatives being viruses of monocotyledonous plants. Here we report that we have identified a sister lineage of TuMV-like potyviruses (TuMV-OM) from European orchids. The isolates of TuMV-OM form a monophyletic sister lineage to the brassica-infecting TuMVs (TuMV-BIs), and are nested within a clade of monocotyledon-infecting viruses. Extensive host-range tests showed that all of the TuMV-OMs are biologically similar to, but distinct from, TuMV-BIs and do not readily infect brassicas. We conclude that it is more likely that TuMV evolved from a TuMV-OM-like ancestor than the reverse. We did Bayesian coalescent analyses using a combination of novel and published sequence data from four TuMV genes [helper component-proteinase protein (HC-Pro), protein 3(P3), nuclear inclusion b protein (NIb), and coat protein (CP)]. Three genes (HC-Pro, P3, and NIb), but not the CP gene, gave results indicating that the TuMV-BI viruses diverged from TuMV-OMs around 1000 years ago. Only 150 years later, the four lineages of the present global population of TuMV-BIs diverged from one another. These dates are congruent with historical records of the spread of agriculture in Western Europe. From about 1200 years ago, there was a warming of the climate, and agriculture and the human population of the region greatly increased. Farming replaced woodlands, fostering viruses and aphid vectors that could invade the crops, which included several brassica cultivars and weeds. Later, starting 500 years ago, inter-continental maritime trade probably spread the TuMV-BIs to the remainder of the world.

Elongated particles of simple RNA viruses of plants are composed of an RNA molecule coated with numerous identical capsid protein subunits to form a regular helical structure, of which tobacco mosaic virus is the archetype. Filamentous particles of the closterovirus beet yellows virus (BYV) reportedly contain '4000 identical 22-kDa (p22) capsid protein subunits. The BYV genome encodes a 24-kDa protein (p24) that is structurally related to the p22. We searched for the p24 in BYV particles by using immunoelectron microscopy with specific antibodies against the recombinant p24 protein and its N-terminal peptide. A 75-nm segment at one end of the 1370-nm filamentous viral particle was found to be consistently labeled with both types of antibodies, thus indicating that p24 is indeed the second capsid protein and that the closterovirus particle, unlike those of other plant viruses with helical symmetry, has a "rattlesnake" rather than uniform structure.The orthodox design of elongated RNA-containing plant viruses, exemplified in molecular biology textbooks by the structure of tobacco mosaic tobamovirus, is the helical array of an RNA molecule coated with numerous identical capsid protein (CP) subunits encoded by a single viral gene (1, 2). Beet yellows virus (BYV), a member of the closterovirus group, is transmitted by aphids semipersistently to a wide range of plant species and causes important losses in sugar beet crops (3). The virus has very flexuous filamentous particles 1250-1450 nm long, with a helix pitch of 3.75 nm and 8.5 CP subunits per helix turn (3, 4). Although evolutionarily related to the "tobamo-like" plant viruses, BYV has a genome that is 2.5 times larger than tobacco mosaic virus (5, 6). This difference may be due, in part, to acquisition of foreign coding sequences via RNA recombination and to gene duplication in closterovirus genomes (6, 7). The BYV genome contains a peculiar tandem of genes, one for the 22-kDa CP (p22) and the other for a structurally related 24-kDa protein (p24). It has been suggested that the p24 gene evolved by duplication of and subsequent divergence from the downstream CP gene (8). Similar tandem arrangement of normal and aberrant CP genes is found in another aphid-transmissible closterovirus, citrus tristeza virus (CTV; ref. 8). Interestingly, the genes for CP and its duplicate have diverged much more and are transposed in the whitefly-transmissible lettuce infectious yellows closterovirus compared to BYV and CTV (7). Heretofore, only a single CP species has been found in closterovirus particles, and it has been speculated that the CP duplicates may be involved in long-distance and cell-to-cell movement of viral infection rather than in virion formation (7-9).We show here the presence of a distinct tail at one end of the BYV particles that is specifically labeled with antibodies to the p24. Thus, closterovirus particles, unlike those of other plant viruses with helical symmetry, apparently have a morphologically polar structure composed of two CPs, for which we pro...

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A disease characteriscd by severely stunted pl~nts with small dark green Icaveb was found in groundnut (Arachis h y p g a~a l in sandy soils in Punjab State. India. The disease occurred in patches in the field and reappeared in the same positions in succeeding groundnut crops. Plants infected early did not produce mature pods. Seeds sown in soil collected from infected fields produced plants with typical disease symptoms. Phaseolus vulgaris cv. Local and Chenopodium quinoa were found to be good diagnostic hosts. The disease was shown to be caused by a rod-shaped virus c. 24 nm in diameter with predominant particle lengths of c. 249 and 184 nm when stained in uranyl acetate. The virus, named Indian peanut clump virus (IPCV), resembled peanut clump virus (PCV) reported from W. Africa in symptomatology on groundnuts, particle morphology and soil-borne nature. However, it is not serologically related to two W. African PCV isolates tested, or to tobacco rattle (PRN and CAM strains) or pea early browning virus (Dutch isolate) in microprecipitin, enzyme linked immunosorbent assay and immunosorbent electron microscopy tests.