PDF file: Annual Report 2002/2003 - Scottish Crop Research Institute

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Viruses - evolving organisms ? three. The smaller RNA, RNA-2, encodes the virus coat protein and other proteins involved in transmission by nematodes (Fig. 4), and there are large differences among the RNA-2 sequences of different TRV strains 7 . The variability of the part of RNA-2 that encodes the virus coat protein is reflected in the extreme serological diversity of virus strains; multiple serotypes of both TRV and PEBV are recognized. We also showed that RNA-2 encodes the determinants of nematode transmissibility 8 . At least 10 trichodorid species can act as vectors, and for TRV there is a substantial degree of specificity between vector species and virus serotype 9 . In some instances, the specificity between a single trichodorid species and a single virus serotype seems absolute 10 , but in others the correlation is less exact. This probably reflects the involvement of the other RNA-2-encoded genes in nematode transmission 11 . These observations suggest that the reason why tobravirus RNA-2 is so much more variable in sequence than RNA-1 is that this variation represents adaptation for transmission by different species of nematode vector, or even by different populations of a vector species. In this connection, it should be remembered that, unlike viruses with aerial vectors, soil-borne viruses have limited means for long distance dispersal to new sites. Trichodorids probably move laterally no more than 1 m/yr, and tobraviruses therefore have to rely on means such as soil movement or the movement of infected vegetative propagating material or seeds for their long distance spread. Tobravirus populations are thus effectively evolving in isolation from each other. In these circumstances, properties such as efficient transmission by the local trichodorid population and a wide host range have obvious value for survival. The diversity of viruses From examples such as those discussed above, it is clear that evolution driven by natural selection can account for the present day diversification of viruses and for the diversity found within genera and species. When the scope is widened however, the arguments become more speculative. One might ask, for example, why the tobravirus genome is in two parts. Here, it is instructive to compare the tobravirus genome with that of tobacco mosaic virus (TMV), a virus with perhaps the simplest genome of all, consisting of only a single piece of RNA. The TMV genome encodes only four gene products: two proteins involved in RNA replication, a cell-to-cell movement protein and a coat protein (Fig. 4). Tobraviruses have analogues of each of these gene products, which bear sufficient similarities to the TMV proteins to suggest that they have a common origin. The tobravirus genome also encodes three additional proteins: one involved in transmission through seeds and two involved in transmission by nematodes. There are no clues as to the origins of these additional genes, but it seems likely that a common ancestor of TMV and tobraviruses acquired them and subsequently evolved into the tobraviruses. What was the selection pressure that drove this augmented genome to divide into two parts? One possibility is that with the additional genes it was too large for efficient replication. RNA replication is relatively error prone, and as an RNA gets larger the probability of producing an error-free copy falls off rapidly. Another possibility is that hiving off the genes involved in nematode transmission into a separate piece of RNA facilitated their rapid adaptation to different vector populations. The functions of RNA replication, cell-to-cell movement and coat protein provided by the TMV genome are the minimum set of functions required by an autonomous RNA plant virus. Coat proteins though are multi-functional. They not only protect virus RNA from degradation, but are usually required for plant-to-plant transmission by vectors and for systemic movement of virus through the vascular system of an infected plant. However, a few plant viruses, notably the umbraviruses, do not have a coat protein gene 12 . Umbraviruses use the coat protein of a helper virus to encapsidate their RNA for plant-to-plant transmission by aphids, and thus can only be transmitted from plants that are doubly infected by the umbravirus and the helper. Umbravirus genomes do contain genes encoding RNA replication and cell-tocell movement proteins, and also include a gene known as ORF3 (Fig. 5), whose protein product provides some of the other functions of a coat protein. Thus the ORF3 protein forms nucleoprotein particles with the virus RNA 13 (Fig. 5), protects the RNA from degradation, and mediates its transport through the phloem 14 . One might speculate that the ORF3 protein is a proto-coat protein in the course of evolution, which has yet to develop the ability to form conventional virus particles and mediate vector transmission. Alternatively, it could be a relic of a fully functional coat protein that has lost its particle formation and vector transmission functions after the virus acquired the ability to use a helper virus protein for these purposes. Of course, neither of these ideas might be correct, and they may merely reflect the prejudice of a 74
Viruses - evolving organisms ? a) b) ORF 1 ORF 3 ORF 2 ORF 4 2 Ranomenjanahary S; Rabindran R; Robinson D J (<strong>2002</strong>). Occurrence of three distinct begomoviruses in cassava in Madagascar. Annals of Applied Biology, 140, 315-318. 3 Zhou X; Liu Y; Calvert L; Munoz C; Otim-Nape G W; Robinson D J; Harrison B D (1997). Evidence that DNA-A of a geminivirus associated with severe cassava mosaic disease in Uganda has arisen by interspecific recombination. Journal of General Virology 78, 2101-2111. 4 Harrison B D; Zhou X; Otim-Nape G W; Liu Y; Robinson D J (1997). Role of a novel type of double infection in the geminivirusinduced epidemic of severe cassava mosaic in Uganda. Annals of Applied Biology 131, 437-448. Figure 5 a) Diagram of the genome of an umbravirus. Coloured blocks represent genes encoding replication proteins (green), cell-to-cell movement protein (purple) and ORF3 protein (red). b) Electron micrograph of nucleoprotein filaments containing ORF3 protein and viral RNA in the cytoplasm of an infected cell. virologist that it is “normal” for a virus to have a coat protein. Moreover, it is difficult to see how they could be tested experimentally. Although this kind of speculation about the origins of the many types of virus that now exist is an interesting intellectual exercise, it is of little relevance to practical problems of virus pathology. There may be no conclusive answer to the question whether or not a virus is an organism; indeed, as Professor Joad might have said, it all depends what you mean by an organism. But for many purposes, they can be treated as if they were organisms. In particular, the appearance of new variants and the forces that drive these changes seem to follow the same evolutionary models as apply to conventional organisms. If they were pressed on more philosophical questions though, most virologist would simply go along with Lwoff’s dictum that “Viruses are viruses”. References 1 Ogbe F O; Atiri G I; Robinson D; Winter S; Dixon A G O; Quin F M; Thottappilly G (1999). First report of East African cassava mosaic begomovirus in Nigeria. Plant Disease 83, 398. 5 Zhou X; Liu Y; Robinson D J; Harrison B D (1998). Four DNA- A variants among Pakistani isolates of cotton leaf curl virus and their affinities to DNA-A of geminivirus isolates from okra. Journal of General Virology 79, 915-923. 6 Sanz A I; Fraile A; García-Arenal F; Zhou X; Robinson D J; Khalid S; Butt T; Harrison B D (2000). Multiple infection, recombination and genome relationships among begomovirus isolates found in cotton and other plants in Pakistan. Journal of General Virology 81, 1839-1849. 7 Robinson D J & Harrison BD (1985). Unequal variation in the two genome parts of tobraviruses and evidence for the existence of three separate viruses. Journal of General Virology 66, 171-176. 8 Ploeg A T; Robinson D J; Brown D J F (1993). RNA-2 of tobacco rattle virus encodes the determinants of transmissibility by trichodorid nematodes. Journal of General Virology 74, 1463-1466. 9 Ploeg A T; Brown D J F; Robinson D J (1992). The association between species of Trichodorus and Paratrichodorus vector nematodes and serotypes of tobacco rattle tobravirus. Annals of Applied Biology 121, 619-630. 10 Ploeg A T; Brown D J F; Robinson D J (1992). Acquisition and subsequent transmission of tobacco rattle virus isolates by Paratrichodorus and Trichodorus nematode species. Netherlands Journal of Plant Pathology 98, 291-300. 11 MacFarlane S A (1999). Molecular biology of the tobraviruses. Journal of General Virology 80, 2799-2807. 12 Taliansky M E; Robinson D J; Murant A F (1996). Complete nucleotide sequence and organization of the RNA genome of groundnut rosette umbravirus. Journal of General Virology 77, 2335-2345. 13 Taliansky, M; Roberts I M; Kalinina N; Ryabov E V; Raj S K; Robinson D J; Oparka K J (<strong>2003</strong>). An umbraviral protein, involved in long-distance RNA movement, binds viral RNA and forms unique, protective ribonucleoprotein complexes. Journal of Virology 77, 3031-3040. 14 Ryabov E V; Robinson D J; Taliansky M (2001). Umbravirusencoded proteins both stabilize heterologous viral RNA and mediate its systemic movement in some plant species. Virology 288, 391-400. 75
Page 1 and 2:
Scottish Crop Research Institute ®
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The Scottish Crop Research Institut
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Contents Biomathematics and Statist
Page 7 and 8:
Introduction upon the Commissioner
Page 9 and 10:
Introduction ® Scottish Crop Resea
Page 11 and 12:
Report of the Director John R. Hill
Page 13 and 14:
Director’s Report tion from dismi
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Director’s Report paper A revised
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Director’s Report and the relatio
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Director’s Report Member States A
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Director’s Report US subsidies fr
Page 23 and 24: Director’s Report regions of the
Page 25 and 26: Director’s Report of silver and m
Page 27 and 28: Director’s Report tions. Obscenel
Page 29 and 30: Director’s Report participation.
Page 31 and 32: Director’s Report heads of state
Page 33 and 34: Director’s Report Philippines (2.
Page 35 and 36: Director’s Report Food Aid Famine
Page 37 and 38: Director’s Report adoption of unh
Page 39 and 40: Director’s Report economy propose
Page 41 and 42: Director’s Report (herbicides, in
Page 43 and 44: Director’s Report by a combinatio
Page 45 and 46: Director’s Report mended a study
Page 47 and 48: Director’s Report notes, bulletin
Page 49 and 50: Director’s Report research instit
Page 51 and 52: Director’s Report Ireland. Also i
Page 53 and 54: Director’s Report tats, most nota
Page 55 and 56: Director’s Report shares of total
Page 57 and 58: Director’s Report tonnes in 1999.
Page 59 and 60: Director’s Report Relative Import
Page 61 and 62: Director’s Report cheap food-stuf
Page 63 and 64: Director’s Report culturally-rela
Page 65 and 66: Director’s Report on large parts
Page 67 and 68: Director’s Report minimum input s
Page 69 and 70: Director’s Report organic farming
Page 71 and 72: Viruses - evolving organisms ? Viru
Page 73: Viruses - evolving organisms ? CLCu
Page 77 and 78: Barley Crop Development formation o
Page 79 and 80: Barley Crop Development Year Total
Page 81 and 82: Barley Crop Development on a year t
Page 83 and 84: GM herbicide-tolerant crops Some pe
Page 85 and 86: GM herbicide-tolerant crops and som
Page 87 and 88: GM herbicide-tolerant crops mitted
Page 89 and 90: GM herbicide-tolerant crops 12 Squi
Page 91 and 92: Mechanisms & Processes Peronospora
Page 93 and 94: Mechanisms & Processes Proteomic an
Page 95 and 96: Mechanisms & Processes High-through
Page 97 and 98: Mechanisms & Processes Studying pla
Page 99 and 100: Mechanisms & Processes Phytoene pea
Page 101 and 102: Mechanisms & Processes New discover
Page 103 and 104: Mechanisms & Processes in these cel
Page 105 and 106: Mechanisms & Processes d). This dat
Page 107 and 108: Genes to Products ic enzymes (use o
Page 109 and 110: Genes to Products Carotenogenesis i
Page 111 and 112: Genes to Products accumulation in s
Page 113 and 114: Genes to Products Retrotransposons
Page 115 and 116: SH43 SL03 SH44 SH12 B20 SL22 B47 B0
Page 117 and 118: Management of genes and organisms i
Page 119 and 120: Environment Functional diversity in
Page 121 and 122: Environment Plant root biomechanics
Page 123 and 124: Environment Microbial and microfaun
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Environment The role and importance
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Environment duced from barley mixtu
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Environment IOBC GMO Guidelines Pro
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Environment Mathematical modelling
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Biomathematics and Statistics Scotl
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Research services Analytical facili
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Research services Division of Finan
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Research services Information Techn
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Research services Estate, Glasshous
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Research services Engineering and M
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Research services MRS will: strengt
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Research services Health and Safety
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Publications Blok, V.C. 2002. Progr
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Publications Phenotype/genotype ass
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Publications for virus-transmitting
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Publications Muckenschnabel, I., Go
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Publications rich grassland. Procee
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Publications Woodford, J.A.T., Fent
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Statement of health & safety policy
Page 163 and 164:
The Governing Body The Governing Bo
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The Governing Body development and
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Staff Theme 2 - Genes to Products T
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Staff Division of Finance and Admin
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Staff Resignations Name Unit Band M
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Staff Editorial Duties Name Positio
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Staff Name Position Committee or Or
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SCRI Research Programme 2002-2003 S
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Research Projects SCR/587/02 SCR/58
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Meteorological Records D. Petrie &
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List of Abbreviations AAB ACRE ADAS
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