Selection pressure and founder effects constrain genetic variation in differentiated populations of soilborne bymovirus Wheat yellow mosaic virus (Potyviridae) in China.

To study the population genetic structure and forces driving the evolution of Wheat yellow mosaic virus (WYMV), the nucleotide sequences encoding the coat protein (CP) (297 sequences) or the genome-linked virion protein (VPg) (87 sequences) were determined from wheat plants growing at 11 different locations distributed in five provinces in China. There were close phylogenetic relationships between all sequences but clustering on the phylogenetic trees was congruent with their provenance, suggesting an origin-dependent population genetic structure. There were low levels of genetic diversity, ranging from 0.00035 ± 0.00019 to 0.01536 ± 0.00043 (CP), and 0.00086 ± 0.00039 to 0.00573 ± 0.00111 (VPg), indicating genetic stability or recent emergence of WYMV in China. The results may suggest that founder effects play a role in shaping the genetic structure of WYMV. Between-population diversity was consistently higher than within-population diversity, suggesting limited gene flow between subpopulations (average FST 0.6241 for the CP and 0.7981 for the VPg). Consistent amino acid substitutions correlated with the provenance of the sequences were observed at nine positions in the CP (but none in the VPg), indicating an advanced stage in population structuring. Strong negative (purifying) selection was implicated on both the CP and VPg but positive selection on a few codons in the CP, indicating an ongoing molecular adaptation.

Detection of Viruses in Sweetpotato from Honduras and Guatemala Augmented by Deep-Sequencing of Small-RNAs.

Sweetpotato (Ipomoea batatas) plants become infected with over 30 RNA or DNA viruses in different parts of the world but little is known about viruses infecting sweetpotato crops in Central America, the center of sweetpotato domestication. Small-RNA deep-sequencing (SRDS) analysis was used to detect viruses in sweetpotato in Honduras and Guatemala, which detected Sweet potato feathery mottle virus strain RC and Sweet potato virus C (Potyvirus spp.), Sweet potato chlorotic stunt virus strain WA (SPCSV-WA; Crinivirus sp.), Sweet potato leaf curl Georgia virus (Begomovirus sp.), and Sweet potato pakakuy virus strain B (synonym: Sweet potato badnavirus B). Results were confirmed by polymerase chain reaction and sequencing of the amplicons. Four viruses were detected in a sweetpotato sample from the Galapagos Islands. Serological assays available to two of the five viruses gave results consistent with those obtained by SRDS, and were negative for six additional sweetpotato viruses tested. Plants coinfected with SPCSV-WA and one to two other viruses displayed severe foliar symptoms of epinasty and leaf malformation, purpling, vein banding, or chlorosis. The results suggest that SRDS is suitable for use as a universal, robust, and reliable method for detection of plant viruses, and especially useful for determining virus infections in crops infected with a wide range of unrelated viruses.

Evolution of cassava brown streak disease-associated viruses.

Cassava brown streak disease (CBSD) has occurred in the Indian Ocean coastal lowlands and some areas of Malawi in East Africa for decades, and makes the storage roots of cassava unsuitable for consumption. CBSD is associated with Cassava brown streak virus (CBSV) and the recently described Ugandan cassava brown streak virus (UCBSV) [picorna-like (+)ssRNA viruses; genus Ipomovirus; family Potyviridae]. This study reports the first comprehensive analysis on how evolution is shaping the populations of CBSV and UCBSV. The complete genomes of CBSV and UCBSV (four and eight isolates, respectively) were 69.0-70.3 and 73.6-74.4% identical at the nucleotide and polyprotein amino acid sequence levels, respectively. They contained predictable sites of homologous recombination, mostly in the 3'-proximal part (NIb-HAM1h-CP-3'-UTR) of the genome, but no evidence of recombination between the two viruses was found. The CP-encoding sequences of 22 and 45 isolates of CBSV and UCBSV analysed, respectively, were mainly under purifying selection; however, several sites in the central part of CBSV CP were subjected to positive selection. HAM1h (putative nucleoside triphosphate pyrophosphatase) was the least similar protein between CBSV and UCBSV (aa identity approx. 55%). Both termini of HAM1h contained sites under positive selection in UCBSV. The data imply an on-going but somewhat different evolution of CBSV and UCBSV, which is congruent with the recent widespread outbreak of UCBSV in cassava crops in the highland areas (>1000 m above sea level) of East Africa where CBSD has not caused significant problems in the past.

Simultaneous virus-specific detection of the two cassava brown streak-associated viruses by RT-PCR reveals wide distribution in East Africa, mixed infections, and infections in Manihot glaziovii.

The expanding cassava brown streak disease (CBSD) epidemic in East Africa is caused by two ipomoviruses (genus Ipomovirus; Potyviridae), namely, Cassava brown streak virus (CBSV), and Ugandan cassava brown streak virus (UCBSV) that was described recently. A reverse transcription polymerase chain reaction (RT-PCR) based diagnostic method was developed in this study for simultaneous virus-specific detection of the two viruses. Results showed that CBSV and UCBSV are distributed widely in the highlands (> 1000 m above the sea level) of the Lake Victoria zone in Uganda and Tanzania and also in the Indian Ocean costal lowlands of Tanzania. Isolates of UCBSV from the Lake Victoria zone were placed to two phylogenetic clusters in accordance with their origin in Uganda or Tanzania, respectively. Mixed infections with CBSV and UCBSV were detected in many cassava plants in the areas surveyed. CBSV was also detected in the perennial species Manihot glaziovii (DNA-barcoded in this study) in Tanzania, which revealed the first virus reservoir other than cassava. The method for detection of CBSV and UCBSV described in this study has important applications for plant quarantine, resistance breeding of cassava, and studies on epidemiology and control of CBSD in East Africa.

Detection, distribution, and genetic variability of European mountain ash ringspot-associated virus.

European mountain ash ringspot-associated virus (EMARAV) was recently characterized from mountain ash (rowan) (Sorbus aucuparia) in Germany. The virus belongs tentatively to family Bunyaviridae but is not closely related to any classified virus. How commonly EMARAV occurs in ringspot disease (EMARSD) affected mountain ash trees was not reported and was investigated here. Virus-specific detection tools such as reverse transcription-polymerase chain reaction and dot blot hybridization using digoxigenin-labeled RNA probes were developed to test 73 mountain ash trees including 16 trees with no virus-like symptoms from 16 districts in Finland and Viipuri, Russia. All trees were infected with EMARAV. Hence, EMARAV is associated with EMARSD and can also cause latent infections in mountain ash. Symptom expression and the variable relative concentrations of viral RNA detected in leaves showed no correlation. Infectious EMARAV was detected also in dormant branches of trees in winter. Subsequently, genetic variability, geographical differentiation, and evolutionary selection pressures were investigated by analyzing RNA3 sequences from 17 isolates. The putative nucleocapsid (NP) gene sequence (944 nucleotides) showed little variability (identities 97 to 99%) and was under strong purifying selection. Amino acid substitutions were detected in two positions at the N terminus and one position at the C terminus of NP in four isolates. The 3' untranslated region (442 nucleotides) was more variable (identities 94 to 99%). Six isolates from a single sampling site exhibited as wide a genetic variability as isolates from sites that were hundreds of kilometers apart and no spatial differentiation of populations of EMARAV was observed.

Genetically distinct strains of Cassava brown streak virus in the Lake Victoria basin and the Indian Ocean coastal area of East Africa.

Six isolates of Cassava brown streak virus (CBSV, genus Ipomovirus; Potyviridae) from the Lake Victoria basin in Uganda and Tanzania were characterized. Virus particles were 650 nm long. The complete coat protein (CP)-encoding sequences (1,101 nucleotides, nt) were 90.7-99.5 and 93.7-99.5% identical at the nt and amino acid (aa) levels, respectively. The 3' untranslated region was 225, 226 or 227 nt long. These eight isolates were only 75.8-77.5% (nt) and 87.0-89.9% (aa) identical when compared to the partial CP sequences (714 nt) of six CBSV isolates characterized previously from the costal lowlands of Tanzania and Mozambique. Hence, two genetically different and geographically separated populations of CSBV exist in East Africa.

Natural wild hosts of sweet potato feathery mottle virus show spatial differences in virus incidence and virus-like diseases in Uganda.

Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) is globally the most common pathogen of sweetpotato. An East African strain of SPFMV incites the severe 'sweetpotato virus disease' in plants co-infected with Sweet potato chlorotic stunt virus and threatens subsistence sweetpotato production in East Africa; however, little is known about its natural hosts and ecology. In all, 2,864 wild plants growing in sweetpotato fields or in their close proximity in Uganda were observed for virus-like symptoms and tested for SPFMV in two surveys (2004 and 2007). SPFMV was detected at different incidence in 22 Ipomoea spp., Hewittia sublobata, and Lepistemon owariensis, of which 19 species are new hosts for SPFMV. Among the SPFMV-positive plants, approximately 60% displayed virus-like symptoms. Although SPFMV incidence was similar in annual and perennial species, virus-like diseases were more common in annuals than perennials. Virus-like diseases and SPFMV were more common in the eastern agroecological zone than the western, central, and northern zones, which contrasted with known incidence of SPFMV in sweetpotato crops. The data on a large number of new natural hosts of SPFMV detected in this study provide novel insights into the ecology of SPFMV in East Africa.