Discussion paper: The naming of Potato virus Y strains infecting potato

Potato virus Y (PVY) strain groups are based on host response and resistance gene interactions. The strain groups PVY(O), PVY(C) and PVY(N) are well established for the isolates infecting potato in the field. A switch in the emphasis from host response to nucleotide sequence differences in the virus genomes, detection of isolates recombining sequences of different strains, and the need to recognize isolates that cause necrotic symptoms in potato tubers have led to the assignment of new acronyms, especially to isolates of the PVY(N) strain group. This discussion paper proposes that any newly found isolates should be described within the context of the original strain groups based on the original methods of distinguishing strains (i.e., tobacco and potato assays involving use of 'differential' potato cultivars). Additionally, sequence characterization of the complete genomes of isolates is highly recommended. However, it is acceptable to amend the names of PVY isolates with additional, specific codes to show that the isolate differs at the molecular, serological or phenotypic level from the typical strains within a strain group. The new isolates should preferably not be named using geographical, cultivar, or place-association designations. Since many new variants of PVY are being discovered, any new static classification system will be meaningless for the time being. A more systematic investigation and characterization of PVY from potato at the biological and molecular levels should eventually result in a biologically meaningful genetic strain concept.

Point mutations in the potato leafroll virus major capsid protein alter virion stability and aphid transmission

The coat protein (CP) of potato leafroll virus (PLRV) is the primary component of the capsid, and is a multifunctional protein known to be involved in vector transmission and virus movement within plant hosts, in addition to particle assembly. Thirteen mutations were generated in various regions of the CP and tested for their ability to affect virus-host and virus-vector interactions. Nine of the mutations prevented the assembly of stable virions. These mutants were unable to infect systemically four different host species. Furthermore, although virus replication and translation of the CP were similar for the mutants and wild-type virus in individual plant cells, the translation of the CP readthrough product was affected in several of the mutants. Four of the mutants were able to assemble stable particles and infect host plants systemically, similarly to the wild-type virus; however, two of the mutants were transmitted less efficiently by aphid vectors. Based on a computer-generated model of the PLRV CP, the mutations that prevented virion assembly were associated with subunit interfaces, while the amino acid alterations in the assembly-competent mutants were associated with surface loops. This and previous work indicates that the CP structural model has value in predicting the structural architecture of the virion.

A Chinese isolate of barley yellow dwarf virus-PAV represents a third distinct species within the PAV serotype

The complete nucleotide sequence of barley yellow dwarf virus (BYDV) PAV-CN genomic RNA was determined. This represents the seventh complete genome sequence of a BYDV-PAV serotype. The genome organization of PAV-CN was comparable to that of other BYDV-PAV serotypes, but the nucleotide sequence of full genome was only 76.9-80.3% similar. Sequence similarity of individual open reading frames and untranslated regions (UTR) between PAV-CN and other PAV isolates ranged from 37.9 to 98.2%. Overall, PAV-CN was most similar to BYDV-PAS, which belongs to one of two distinct species within the PAV serotype of BYDV, although the 5' UTR and ORF1 of PAV-CN was most similar to BYDV-GAV, another member of the genus Luteovirus that is not serologically related to BYDV-PAV. These data suggest that PAV-CN may have undergone a recombination event with GAV and that PAV-CN represents a third distinct species within the PAV serotype of BYDV.

Transmission of two viruses that cause Barley Yellow Dwarf is controlled by different loci in the aphid, Schizaphis graminum

Clonal populations of the aphid, Schizaphis graminum, have been separated into biotypes based on host preference and their ability to overcome resistance genes in wheat. Recently, several biotypes were found to differ in their ability to transmit one or more of the viruses that cause barley yellow dwarf disease in grain crops, and vector competence was linked to host preference. The genetics of host preference has been studied in S. graminum, but how this may relate to the transmission of plant viruses is unknown. Sexual morphs of a vector and nonvector S. graminum genotype were induced from parthenogenetic females and reciprocal crosses made. Eighty-nine hybrids were generated and maintained by parthenogenesis. Each hybrid was evaluated for its ability to transmit Barley yellow dwarf virus-PAV and Cereal yellow dwarf virus-RPV, and for its ability to colonize two wheat genotypes each expressing a different gene that confers resistance to S. graminum. The F1 genotypes were genetically variable for their ability to transmit virus and to colonize the aphid resistant wheat, but these traits were not genetically correlated. Individual F1 genotypes ranged in transmission efficiency from 0-100% for both viruses, although the overall mean transmission efficiency was similar to the transmission competent parent, indicating directional dominance. The direction of the cross did not significantly affect the vector competency for either virus, suggesting that maternally inherited cytoplasmic factors, or bacterial endosymbionts, did not contribute significantly to the inheritance of vector competency in S. graminum. Importantly, there was no genetic correlation between the ability to transmit Barley yellow dwarf virus and Cereal yellow dwarf virus-RPV in the F1 genotypes. These results taken together indicate that multiple loci are involved in the circulative transmission, and that the successful transmission of these closely related viruses is regulated by different sets of aphid genes.

Biometrical genetic analysis of luteovirus transmission in the aphid Schizaphis graminum

The aphid Schizaphis graminum is an important vector of the viruses that cause barley yellow dwarf disease. We studied the genetic architecture of virus transmission by crossing a vector and a non-vector genotype of S. graminum. F1 and F2 hybrids were generated, and a modified line-cross biometrical analysis was performed on transmission phenotype of two of the viruses that cause barley yellow dwarf: Cereal yellow dwarf virus (CYDV)-RPV and Barley yellow dwarf virus (BYDV)-SGV. Our aims were to (1) determine to what extent differences in transmission ability between vectors and non-vectors is due to net additive or non-additive gene action, (2) estimate the number of loci that determine transmission ability and (3) examine the nature of genetic correlations between transmission of CYDV-RPV and BYDV-SGV. Only additive effects contributed significantly to divergence in transmission of both CYDV-RPV and BYDV-SGV. For each luteovirus, Castle-Wright's estimator for the number of effective factors segregating for transmission phenotype was less than one. Transmission of CYDV-RPV and BYDV-SGV was significantly correlated in the F2 generation, suggesting that there is a partial genetic overlap for transmission of these luteoviruses. Yet, 63% of the F2 genotypes transmitted CYDV-RPV and BYDV-SGV at significantly different rates. Our data suggest that in S. graminum, the transmission efficiency of both CYDV-RPV and BYDV-SGV is regulated by a major gene or set of tightly linked genes, and the transmission efficiency of each virus is influenced by a unique set of minor genes.

Genetic Regulation of Polerovirus and Luteovirus Transmission in the Aphid Schizaphis graminum

ABSTRACT Sexual forms of two genotypes of the aphid Schizaphis graminum, one a vector, the other a nonvector of two viruses that cause barley yellow dwarf disease (Barley yellow dwarf virus [BYDV]-SGV, luteovirus and Cereal yellow dwarf virus-RPV, polerovirus), were mated to generate F1 and F2 populations. Segregation of the transmission phenotype for both viruses in the F1 and F2 populations indicated that the transmission phenotype is under genetic control and that the parents are heterozygous for genes involved in transmission. The ability to transmit both viruses was correlated within the F1 and F2 populations, suggesting that a major gene or linked genes regulate the transmission. However, individual hybrid genotypes differed significantly in their ability to transmit each virus, indicating that in addition to a major gene, minor genes can affect the transmission of each virus independently. Gut and salivary gland associated transmission barriers were identified in the nonvector parent and some progeny, while other progeny possessed only a gut barrier or a salivary gland barrier. Hemolymph factors do not appear to be involved in determining the transmission phenotype. These results provide direct evidence that aphid transmission of luteoviruses is genetically regulated in the insect and that the tissue-specific barriers to virus transmission are not genetically linked.

Identification of Small Grains Genotypes Resistant to Soilborne wheat mosaic virus

Soilborne wheat mosaic virus (SBWMV) was detected in New York in 1998 for the first time and has been associated with yield loss where identified. We assessed 115 regionally adapted small grains genotypes for resistance to SBWMV over four growing seasons. Resistance to SBWMV reduces the percentage of plants that develop detectable viral titer and symptoms. Logistic regression was used to analyze disease incidence data and was compared with a general linear model for categorizing relative resistance to SBWMV. Logistic regression facilitated assessment of the effects of small sample size, low disease incidence, and nonuniform disease distribution. By increasing sample size from 20 to 30 stems per replicate, the number of resistance categories was increased through improved resolution of intermediate resistance classes. In environments with low disease incidence, the number of genotypes categorized as susceptible decreased while intermediate genotypes appeared to be resistant in the analysis. Inclusion of disease distribution data as covariates in a spatially balanced experiment did not increase the power of the logistic analysis. No genotype assessed in multiple years was immune to infection. However, 41 of the regionally adapted genotypes tested repeatedly expressed strong resistance to SBWMV, providing growers a choice of cultivars resistant to SBWMV.

Identification of Small Grains Genotypes Resistant to Wheat spindle streak mosaic virus

Once Wheat spindle streak mosaic virus (WSSMV) becomes established in a field, the only available control strategy is the planting of resistant genotypes. In this study, we assessed 112 genotypes of winter wheat, rye, triticale, and barley for resistance to WSSMV in a 3-year trial in a field that had been used continuously for WSSMV evaluation for over 20 years. Because resistance to WSSMV reduces the percentage of plants that develop detectable virus titer and symptoms, we collected and analyzed disease incidence data. None of the genotypes was immune to infection. Sixty-two of the regionally adapted genotypes repeatedly expressed resistance to WSSMV, thus providing growers with a choice of cultivars resistant to WSSMV. Because of a significant interaction between genotypes and environment (year), genotypes should be assessed for incidence of symptomatic plants in multiple years, particularly when differentiating intermediate responses from highly susceptible and highly resistant responses.

Biological and Serological Properties of Potato virus Y Isolates in Northeastern United States Potato

A survey of six potato viruses, Potato virus A (PVA), Potato virus M (PVM), Potato virus S(PVS), Potato virus X (PVX), Potato virus Y (PVY), and Potato leafroll virus (PLRV), was conducted in New York and Maine during 2002 and 2003. Leaf samples were tested by enzyme-linked immunosorbent assay and PVY-positive samples were further tested to determine whether a necrotic strain of PVY (PVY^N) or a strain able to induce necrosis in tobacco and in potato tubers (PVY^NTN) were present. In both years, PVY and PVS were identified in a majority of the samples, and mixed infections predominated in 83% of the symptomatic leaves in 2002. Of the total 394 PVY-positive samples, 3 reacted with monoclonal antibody (MAb) 1F5 and caused veinal necrosis (VN) in tobacco. Two of these isolates caused tuber necrosis in the potato cv. Yukon Gold. Three PVY isolates reacted with MAb 1F5 but did not cause VN in tobacco, and two caused VN but did not react with MAb 1F5. None of these eight isolates were able to overcome the Ry resistance gene in the potato cultivar Eva, but several were able to overcome the Ny resistance gene found in Allegany. PVY^N isolates were not widespread in the northeastern United States; however, several PVY isolates differed from both PVY^N and the ordinary strain of PVY and may represent strain recombinants.

Testing alternative explanations of character shifts against ecological character displacement in brook sticklebacks (Culaea inconstans) that coexist with ninespine sticklebacks (Pungitius pungitius)

Ecological character displacement (ECD) provides opportunities to test how resource competition generates diversifying selection that results in adaptive divergence. We quantify an association between phenotypic and ecological divergence between two similar small fishes, brook (Culaea inconstans) and ninespine (Pungitius pungitius) sticklebacks, in replicate northern Ontario lakes, Canada. The two species partition resources and habitat, where they coexist, and brooks that coexist with ninespines are more benthically specialized in body form and diet than brooks from local allopatric populations. Here we test various explanations for this pattern. Chance is unlikely to have been the primary cause because divergence is replicated in three separate populations. Preliminary comparisons indicate that resource availability and a variety of abiotic ecological conditions are generally similar between sympatric and allopatric sites, and so do not readily account for the divergence. Biased colonization or extinction is less likely to account for the divergence because character values in sympatry tend to exceed those in allopatry, as expected if they have repeatedly evolved under diversifying selection. Recent studies have also demonstrated that these two species compete, and that competitive effects are more severe for allopatric compared to sympatric brook forms, as predicted if divergence reflects the ghost of competition past. Ongoing studies indicate heritable variation in this system. Our results suggest that even small amounts of character shifts can influence competition and hence relative fitness, further implicating a role for ECD in the evolution of biodiversity.

A surface loop of the potato leafroll virus coat protein is involved in virion assembly, systemic movement, and aphid transmission

Two acidic domains of the Potato leafroll virus (PLRV) coat protein, separated by 55 amino acids and predicted to be adjacent surface features on the virion, were the focus of a mutational analysis. Eleven site-directed mutants were generated from a cloned infectious cDNA of PLRV and delivered to plants by Agrobacterium-mediated mechanical inoculation. Alanine substitutions of any of the three amino acids of the sequence EWH (amino acids 170 to 172) or of D177 disrupted the ability of the coat protein to assemble stable particles and the ability of the viral RNA to move systemically in four host plant species. Alanine substitution of E109, D173, or E176 reduced the accumulation of virus in agrobacterium-infiltrated tissues, the efficiency of systemic infection, and the efficiency of aphid transmission relative to wild-type virus, but the mutations did not affect virion stability. A structural model of the PLRV capsid predicted that the amino acids critical for virion assembly were located within a depression at the center of a coat protein trimer. The other amino acids that affected plant infection and/or aphid transmission were predicted to be located around the perimeter of the depression. PLRV virions play key roles in phloem-limited virus movement in plant hosts as well as in transport and persistence in the aphid vectors. These results identified amino acid residues in a surface-oriented loop of the coat protein that are critical for virus assembly and stability, systemic infection of plants, and movement of virus through aphid vectors.

Site-directed mutagenesis and generation of chimeric viruses by homologous recombination in yeast to facilitate analysis of plant-virus interactions

A yeast homologous recombination system was used to generate mutants and chimeras in the genome of Potato leafroll virus (PLRV). A yeast-bacteria shuttle vector was developed that allows mutants and chimeras generated in yeast to be transformed into Escherichia coli for confirmation of the mutations and transformed into Agrobacterium tumefaciens to facilitate agroinfection of plants by the mutant PLRV genomes. The advantages of the system include the high frequency of recovered mutants generated by yeast homologous recombination, the ability to generate over 20 mutants and chimeras using only two restriction endonuclease sites, the ability to introduce multiple additional sequences using three and four DNA fragments, and the mobilization of the same plasmid from yeast to E. coli, A. tumefaciens, and plants. The wild-type PLRV genome showed no loss of virulence after sequential propagation in yeast, E. coli, and A. tumefaciens. Moreover, many PLRV clones with mutations generated in the capsid protein and readthrough domain of the capsid protein replicated and moved throughout plants. This approach will facilitate the analysis of plant-virus interactions of in vivo-generated mutants for many plant viruses, especially those not transmissible mechanically to plants.

Using air pressure cells to evaluate the effect of soil environment on the transmission of soilborne viruses of wheat

ABSTRACT An air pressure cell, a laboratory tool that precisely controls soil matric potential, was utilized in a novel approach to investigate the epidemiology and management of soilborne disease. Matric potentials of -1, -5, -20, and -40 kPa were established in cores of field soil infested with Wheat soilborne mosaic virus (WSBMV) and its presumed vector Polymyxa graminis. Equilibrated soil cores were planted to wheat (Triticum aestivum), and after intervals of growth under controlled environment, virus transmission was assessed by serological detection of the virus in washed roots. Transmission occurred at all but the driest soil matric potential tested, -40 kPa, in which only pores with a diameter of 7.4 mum or less were water-filled, possibly obstructing movement of P. graminis zoospores. By starting plants at -40 kPa for 10.5 days and then watering them to conducive matric potential, we found that WSBMV transmission occurred between 12 to 24 h at 15 degrees C, and within 36 h at 20 degrees C. No significant transmission occurred within 96 h at 6.5 degrees C. In contrast, transmission of Wheat spindle streak mosaic virus (WSSMV) did not occur at 15 degrees C (the only transmission temperature tested), suggesting either that WSSMV is unable to establish infection at 15 degrees C or that a different vector is involved. The air pressure cell is a novel tool with many potential applications in research on the epidemiology and management of soilborne pathogens. Applications of the precise environmental control attained through the use of air pressure cells range from assessing the effects of cultural practices on soilborne inoculum to standardized virulence assays for soilborne pathogens to preliminary screens of host resistance and pesticide efficacy.

Bipolar depression: relationship between episode length and antidepressant treatment

BACKGROUND

The role of antidepressant medications in bipolar depression remains controversial, mainly due to a lack of research in this area. In this study the authors examined the episode length in bipolar depression and the relationship between antidepressant therapy and episode length.

METHOD

A retrospective chart review of 165 subjects identified 50 (30%) with bipolar illness who experienced a major depressive episode between 1 January 1998 and 15 December 2000. Data gathered utilized a structured instrument completed by the clinician at each visit. This instrument includes modified SCID mood modules as well as continuous ratings for each associated symptom of depression and mood elevation. Survival analysis was employed to calculate the median length of the depressive episodes for the entire group. Further survival analysis compared the episode length for subjects treated with antidepressants during the depression (N = 33) with those who did not receive antidepressants (N = 17). The rate of switch into elevated mood states was compared for the two groups.

RESULTS

The survival analysis for the entire sample demonstrated 25%, 50% and 75% probability of recovery at 33 (S.E. 8.7), 66 (S.E. 17.9) and 215 (S.E. 109.9) days, respectively. Comparing those who received (N = 33) and those who did not receive (N = 17) antidepressants during the episode did not reveal any difference in the length of the depressive episode. Switch rates were not significantly different between those receiving antidepressants and those not taking these medications (15.2% v. 17.6%, respectively).

CONCLUSIONS

Over the past 20 years little progress has been made in reducing the length of depressive episodes in those with bipolar illness. This is despite increasing pharmacological options available for treating depression. Clinicians treating bipolar depression should discuss with their patients the likelihood that the episode will last between 2-3 months. Our results also suggest that antidepressant treatment may not reduce the length of depressive episodes, neither did it appear to contribute to affective switch in our sample.

Host-dependent requirement for the Potato leafroll virus 17-kda protein in virus movement

The requirement for the 17-kDa protein (P17) of Potato leafroll virus (PLRV) in virus movement was investigated in four plant species: potato (Solanum tuberosum), Physalis floridana, Nicotiana benthamiana, and N. clevelandii. Two PLRV P17 mutants were characterized, one that does not translate the P17 and another that expresses a P17 missing the first four amino acids. The P17 mutants were able to replicate and accumulate in agroinoculated leaves of potato and P. floridana, but they were unable to move into vascular tissues and initiate a systemic infection in these plants. In contrast, the P17 mutants were able to spread systemically from inoculated leaves in both Nicotiana spp., although the efficiency of infection was reduced relative to wild-type PLRV. Examination of virus distribution in N. benthamiana plants using tissue immunoblotting techniques revealed that the wild-type PLRV and P17 mutants followed a similar movement pathway out of the inoculated leaves. Virus first moved upward to the apical tissues and then downward. The P17 mutants, however, infected fewer phloem-associated cells, were slower than wild-type PLRV in moving out of the inoculated tissue and into apical tissues, and were unable to infect any mature leaves present on the plant at the time of inoculation.

Virus Transmission Phenotype Is Correlated with Host Adaptation Among Genetically Diverse Populations of the Aphid Schizaphis graminum

ABSTRACT Schizaphis graminum is an important insect pest of several grain crops and an efficient vector of cereal-infecting luteoviruses and poleroviruses. We examined the virus transmission characteristics of several distinct populations and various developmental stages of the aphid. Seven well-characterized S. graminum biotypes maintained at the USDA-ARS laboratory in Stillwater, OK, and two biotypes maintained in New York (one collected in Wisconsin and the other collected in South Carolina) were tested for their ability to transmit five viruses that cause barley yellow dwarf disease (BYD). Four of the Oklahoma biotypes, which do not commonly colonize agronomic crops, and the Wisconsin biotype, were efficient vectors of several viruses. The three other Oklahoma biotypes, which do colonize agronomic crops, and the South Carolina biotype, were poor vectors of all five viruses. Thus, the vector specificity long associated with viruses causing BYD is not limited to the level of aphid species; it clearly extends to populations within a single species. S. graminum nymphs are reported to be more efficient vectors of Barley yellow dwarf virus (BYDV-SGV) than are adults. This was confirmed only for the Wisconsin biotype, but not for the other eight S. graminum biotypes. Thus, there does not appear to be a generalized developmentally regulated barrier to the transmission of BYDV-SGV in S. graminum. Furthermore, the developmentally regulated vector competency observed in the Wisconsin biotype did not extend to other viruses. BYDV-PAV and Cereal yellow dwarf virus-RPV were transmitted with similar efficiency by all S. graminum biotypes when acquired by nymphs or adults.

A New York isolate of Soil-borne wheat mosaic virus differs considerably from the Nebraska type strain in the nucleotide sequences of various coding regions but not in the deduced amino acid sequences

A wheat-infecting furovirus found in Tompkins County, New York, U.S.A. was identified as a strain of Soil-borne wheat mosaic virus (SBWMV) by means of sequence analyses of portions of its RNA 1 and 2. The nucleotide sequences of several of its genes differed by c. 9 to 12% from those of the corresponding genome regions of the Nebraska type strain of SBWMV. The deduced amino acid sequences of the putative translation products, however, suggested much closer relationships. Thus, the amino acid sequences of the coat proteins of the two virus strains were 100% identical despite the fact that their coding regions differed in as many as 68 nucleotide positions. The New York (NY) strain of SBWMV is possibly closely related to an isolate from Illinois for which so far only the nucleotide sequences of its coat protein gene and the 5' untranslated region of its RNA 2 are known.

Suicide prevention: increasing education and awareness

Suicide is a serious and complex public health problem. Health care providers, including both psychiatrists and primary care physicians, are just beginning to understand the intricacies involved in suicide and its prevention. Suicide rates continue to rise, making the education of the public and physicians regarding awareness and prevention, recognition of a wide range of risk factors, and research into suicide prevention strategies very important.

Psychosocial approaches to suicide prevention: applications to patients with bipolar disorder

Hopelessness, dysfunctional attitudes, and poor problem-solving abilities are psychosocial risk factors that have been identified as predictors of suicide. These psychosocial risk factors may help clinicians apply specific therapies and treatments to patients with bipolar disorder at risk for suicide. A search of the literature on suicide prevention revealed 17 randomized, controlled studies, which the authors reviewed to determine the efficacy of strategies aimed at eliminating psychosocial risk factors for suicide. Three strategies emerged as efficacious: (1) applying interventions to elicit emergency care by patients at times of distress; (2) training in problem-solving strategies; and (3) combining comprehensive interventions that include problem solving with intensive rehearsal of cognitive, social, emotional-labeling, and distress-tolerance skills. On the basis of their review of the literature, the authors make recommendations for suicide prevention for patients with bipolar disorder.

Mood disorders and suicide

The mood disorders unipolar major depression and bipolar disorder increase the risk of suicidal ideation, attempted suicide, and death by completed suicide. This article reviews the epidemiologic data on the relationship between mood disorders and suicide, with an emphasis on the substantial risk of suicide, while reassessing older data that may no longer apply. Widespread underdiagnosis and undertreatment of major depression and bipolar disorder contribute to an unacceptable risk of suicide, a preventable tragedy.

Variation in Transmission Efficiency Among Barley yellow dwarf virus-RMV Isolates and Clones of the Normally Inefficient Aphid Vector, Rhopalosiphum padi

The RMV strain of Barley yellow dwarf virus (BYDV-RMV) is an unassigned member of the Luteoviridae that causes barley yellow dwarf in various cereal crops. The virus is most efficiently vectored by the aphid Rhopalosiphum maidis, but can also be vectored with varying efficiency by R. padi and Schizaphis graminum. Field collections of alate aphids migrating into the emerging winter wheat crop in the fall of 1994 in central New York identified a high proportion of R. padi transmitting BYDV-RMV. This prompted a comparison of the BYDV-RMV isolates and the R. padi populations found in the field with type virus and aphid species maintained in the laboratory. A majority of the field isolates of BYDV-RMV were similar to each other and to the type BYDV-RMV isolate in disease severity on oat and in transmission by the laboratory-maintained population of R. maidis and a field-collected population of R. maidis. However, several field populations of R. padi differed in their ability to transmit the various BYDV-RMV isolates. The transmission efficiency of the R. padi clones was increased if acquisition and inoculation feeding periods were allowed at higher temperatures. In addition, the transmission efficiency of BYDV-RMV was significantly influenced by the aphid that inoculated the virus source tissue. In general, BYDV-RMV transmission by R. padi was higher when R. padi was the aphid that inoculated the source tissue than when R. maidis was the inoculating aphid. The magnitude of the change varied among virus isolates and R. padi clones. These results indicate that, under certain environmental conditions, R. padi can play a significant role in the epidemiology of BYDV-RMV. This may be especially significant in regions where corn is a major source of virus and of aphids that can carry virus into a fall-planted wheat crop.

Vector specificity of barley yellow dwarf virus (BYDV) transmission: identification of potential cellular receptors binding BYDV-MAV in the aphid, Sitobion avenae

Two proteins (SaM35 and SaM50) isolated from head tissues of the aphid vector, Sitobion avenae, were identified as potential receptors for barley yellow dwarf virus MAV isolate (Luteoviridae) based on MAV virus overlay assays and immunoblots of urea SDS 2-D gels. An anti-idiotypic antibody (MAV4 anti-ID) that mimics an epitope on MAV virions and competes with MAV in antibody binding assays also bound to SaM50 and SaM35 and to six additional proteins including a GroEL homolog. No MAV-binding proteins were detected from the nonvector aphid, Rhopalosiphum maidis, although MAV4 anti-ID did react with four proteins from R. maidis. It is hypothesized that SaM35 and SaM50 may be MAV receptors involved in MAV transmission based on their high affinity for MAV and their unique association with the vector, S. avenae. The additional aphid proteins binding the MAV4 anti-ID may represent less specific virus-binding proteins facilitating transmission through different aphid tissues.

Seasonal abundance of aphids (Homoptera: Aphididae) in wheat and their role as barley yellow dwarf virus vectors in the South Carolina coastal plain

Aphid (Homoptera: Aphididae) seasonal flight activity and abundance in wheat, Triticum aestivum L., and the significance of aphid species as vectors of barley yellow dwarf virus were studied over a nine-year period in the South Carolina coastal plain. Four aphid species colonized wheat in a consistent seasonal pattern. Greenbug, Schizaphis graminum (Rondani), and rice root aphid, Rhopalosiphum rufiabdominalis (Sasaki), colonized seedlingwheat immediately after crop emergence, with apterous colonies usually peaking in December or January and then declining for the remainder of the season. These two aphid species are unlikely to cause economic loss on wheat in South Carolina, thus crop managers should not have to sample for the subterranean R. rufiabdominalis colonies. Bird cherry-oat aphid, Rhopalosiphum padi (L.), was the second most abundant species and the most economically important. Rhopalosiphum padi colonies usually remained below 10/row-meter until peaking in February or March. Barley yellow dwarf incidence and wheat yield loss were significantly correlated with R. padi peak abundance and aphid-day accumulation on the crop. Based on transmission assays, R. padi was primarily responsible for vectoring the predominant virus serotype (PAV) we found in wheat. Pest management efforts should focus on sampling for and suppressing this aphid species. December planting reduced aphid-day accumulation and barley yellow dwarf incidence, but delayed planting is not a practical management option. English grain aphid, Sitobion avenae (F.), was the last species to colonize wheat each season, and the most abundant. Sitobion avenae was responsible for late-season virus transmission and caused direct yield loss by feeding on heads and flag leaves during an outbreak year.

Tagging potato leafroll virus with the jellyfish green fluorescent protein gene

A full-length cDNA corresponding to the RNA genome of Potato leafroll virus (PLRV) was modified by inserting cDNA that encoded the jellyfish green fluorescent protein (GFP) into the P5 gene near its 3' end. Nicotiana benthamiana protoplasts electroporated with plasmid DNA containing this cDNA behind the 35S RNA promoter of Cauliflower mosaic virus became infected with the recombinant virus (PLRV-GFP). Up to 5% of transfected protoplasts showed GFP-specific fluorescence. Progeny virus particles were morphologically indistinguishable from those of wild-type PLRV but, unlike PLRV particles, they bound to grids coated with antibodies to GFP. Aphids fed on extracts of these protoplasts transmitted PLRV-GFP to test plants, as shown by specific fluorescence in some vascular tissue and epidermal cells and subsequent systemic infection. In plants agroinfected with PLRV-GFP cDNA in pBIN19, some cells became fluorescent and systemic infections developed. However, after either type of inoculation, fluorescence was mostly restricted to single cells and the only PLRV genome detected in systemically infected tissues lacked some or all of the inserted GFP cDNA, apparently because of naturally occurring deletions. Thus, intact PLRV-GFP was unable to move from cell to cell. Nevertheless, PLRV-GFP has novel potential for exploring the initial stages of PLRV infection.

Expression of hexokinase 1 and hexokinase 2 in mammary tissue of nonlactating and lactating rats: evaluation by RT-PCR

Because the initial step in the metabolism of glucose involves phosphorylation by hexokinase (HK), we tested the hypothesis that the expression of the isozymes, hexokinase type 1 (HK1) and hexokinase type 2 (HK2), would be different in rat mammary tissue during pregnancy and lactation. RNA was extracted from mammary tissue dissected from timed pregnant rats (from gestional days 10 to 21) and nursing rat mothers (up to postnatal day 5) for mRNA examination by reverse transcriptase and polymerase chain reaction (RT-PCR) using isozyme specific oligonucleotide primers to the HK1 and HK2 cDNAs. The HK1 mRNA was expressed in both the nonlactating and lactating mammary gland tissue, but HK2 mRNA was found only during lactation. We speculate that the pattern of HK expression might affect human milk production and quality.

Application of random effects models and other methods to the analysis of multidimensional quality of life data in an AIDS clinical trial

BACKGROUND

Current analytic methods applied to multidimensional health-related quality of life (HRQOL) data do not borrow strength across analyses and do not produce summary estimates of effect.

OBJECTIVES

To compare a random effects modelling approach for the analysis of multidimensional HRQOL data to the following: (1) separate analyses for each dimension; (2) O'Brien's global test procedure; and (3) multivariate analysis of variance (MANOVA).

RESEARCH DESIGN

Randomized clinical trial comparing 3 treatments (Trimethoprim-Sulfamethoxazole [TS], Dapsone-Trimethoprim [DT], and Clindamycin-Primaquine [CP] for Pneumocystis carinii pneumonia [PCP]).

SUBJECTS

Patients with PCP enrolled in AIDS Clinical Trials Group Protocol 108.

MEASURES

A 33-item battery assessing 7 dimensions of HRQOL: physical functioning, pain, energy, general health perceptions, disability, pulmonary symptoms, and constitutional symptoms.

RESULTS

Analyses focused on changes in score from baseline to Day 7 (n = 145). Separate analyses for each dimension suggested a trend favoring CP versus TS, but using a Bonferroni correction no differences were statistically significant. O'Brien's global procedure for a test of no-treatment effect versus superiority of one treatment yielded P = 0.07. MANOVA did not reveal significant differences among treatment groups. A random effects model using fixed treatment and dimension effects and separate random effects for each person showed a significant overall treatment effect (P = 0.02); changes in scores for CP averaged 10 points greater than for TS.

CONCLUSIONS

Random-effects models provide a flexible class of models for analyzing multidimensional quality of life data and estimating treatment effects because they borrow strength across dimensions.

Mechanisms of arthropod transmission of plant and animal viruses

A majority of the plant-infecting viruses and many of the animal-infecting viruses are dependent upon arthropod vectors for transmission between hosts and/or as alternative hosts. The viruses have evolved specific associations with their vectors, and we are beginning to understand the underlying mechanisms that regulate the virus transmission process. A majority of plant viruses are carried on the cuticle lining of a vector's mouthparts or foregut. This initially appeared to be simple mechanical contamination, but it is now known to be a biologically complex interaction between specific virus proteins and as yet unidentified vector cuticle-associated compounds. Numerous other plant viruses and the majority of animal viruses are carried within the body of the vector. These viruses have evolved specific mechanisms to enable them to be transported through multiple tissues and to evade vector defenses. In response, vector species have evolved so that not all individuals within a species are susceptible to virus infection or can serve as a competent vector. Not only are the virus components of the transmission process being identified, but also the genetic and physiological components of the vectors which determine their ability to be used successfully by the virus are being elucidated. The mechanisms of arthropod-virus associations are many and complex, but common themes are beginning to emerge which may allow the development of novel strategies to ultimately control epidemics caused by arthropod-borne viruses.

Barley Yellow Dwarf Luteoviruses and Their Predominant Aphid Vectors in Winter Wheat Grown in South Carolina

Barley yellow dwarf is recognized as an important disease problem in winter wheat production in the southeastern United States, but there is relatively little known about the ecology and epidemiology of barley yellow dwarf virus (BYDV) in this region. From 1991 to 1993, and in 1996 and 1997, winter wheat was sampled for BYDV throughout the principal wheat production areas in South Carolina. In addition, in 1997, a small number of samples were collected from fields in North Carolina and Kentucky. Plant samples were assayed to determine the BYDV serotype and, subsequently, coat protein sequences of isolates within the same serotype were compared using restriction fragment length polymorphisms. Representative BYDV isolates from South Carolina and type isolates from New York were compared in aphid transmission experiments using aphid species collected from South Carolina and laboratory colonies maintained in New York. The predominant BYDV serotype in South Carolina (in all years) was PAV, accounting for 94% of the total BYDV-infected samples analyzed. The RPV serotypes were more abundant in samples collected from western North Carolina and Kentucky. PAV isolates from all regions were identical to the New York BYDV-PAV in terms of serology and restriction fragment patterns. Furthermore, the aphid transmission phenotypes were similar for South Carolina and New York BYDV isolates. The predominant aphids colonizing winter wheat in South Carolina included Schizaphis graminum, Rhopalosiphum rufiabdominalis, R. padi, and Sitobion avenea. The South Carolina clones of R. padi and S. avenae were similar to the New York laboratory clones in their abilities to transmit various BYDV isolates from New York and South Carolina. In contrast to the New York clone of Schizaphis graminum that can vector SGV, PAV, and RPV, the S. graminum clone from South Carolina was not a vector of any BYDV serotype tested. R. rufiabdominalis was found to be an efficient vector of PAV, RPV, and RMV isolates, but did not transmit MAV or SGV.

Estimating a treatment effect from multidimensional longitudinal data

Multidimensional longitudinal data result when researchers measure an outcome through time that is quantified by many different response variables. These response variables are often defined on different numerical scales. The objective of this paper is to present a method to summarize and estimate an overall treatment effect from this type of longitudinal data. A regression model is proposed that assumes the treatment effect can be parameterized as an acceleration or deceleration of the time scale of each response variable's trajectory. Generalized estimating equations are used to estimate the model parameters. Cognitive and functional ability data from Alzheimer's disease patients and quality of life data from an AIDS clinical trial are used to illustrate the model.

Characterization of wheatgrass-derived barley yellow dwarf virus resistance in a wheat alien chromosome substitution line

ABSTRACT Wheatgrass (Thinopyrum intermedium) possesses a high level of resistance to barley yellow dwarf virus (BYDV) subgroup I and subgroup II strains. A wheat line (P29), in which the 7D chromosome has been substituted with a group 7 chromosome from T. intermedium, was examined for the level of resistance to two subgroup I and two subgroup II BYDV strains. In P29 plants inoculated with the subgroup I PAV strains, the titer of virus in leaf and stem tissue was typically reduced 42 to 52% when compared with the BYDV-susceptible cv. Abe. P29 and 'Abe' had the same content of PAV in roots. These results and the absence of detectable virus in inoculated T. intermedium plants indicate that the complete resistance to subgroup I possessed by the wheatgrass has not been introgressed into P29. In contrast, P29 was completely resistant throughout the plant to the subgroup II strains, NY-RPV and NY-RMV, demonstrating that the complete resistance to subgroup II in T. intermedium was incorporated into P29. Further analysis of this resistance to NY-RPV showed that NY-RPV can replicate in mesophyll protoplasts of P29 and 'Abe', suggesting that this resistance is not operating at the single-cell level. Molecular marker analysis confirmed that the T. intermedium chromosome present in P29 is a different group 7 wheatgrass chromosome than that present in L1, a wheat line with BYDV resistance properties similar to those of P29.

Developmental expression of hexokinase 1 and 3 in rats

Mammalian hexokinase types one and three (HK1 and HK3) are 100 kDa isozymes that phosphorylate glucose to glucose-6-phosphate. HK1 is present in most tissues but is especially prominent in brain and kidney. HK3 is less well studied, but may be most prominent in the spleen and lymphocytes. In this study, we determined the ontogeny of the expression of these isoforms in the rat. Using immunohistochemistry, we identified HK1 and HK3 immunoreactivity in the brain, heart, kidney, liver, skeletal muscle and spleen from gestational day 14 (E14) to 45 days after birth (P45). With the exception of the liver and spleen, we observed a similar age- and cell-dependent staining pattern for both isoforms in all organs studied. The brain and spleen were analyzed in more detail to identify specific regions of immunoreactivity during maturation. A transient expression of HK1 and HK3 was noted in the cell bodies of mature neurons, including layers V and VI of the cerebral cortex and the cerebellar Purkinje cells followed by localization to the white matter of the cerebrum and cerebellum. In the spleen, HK3 immunoreactivity was detected postnatally and appeared to track with the infiltration of B cells. Our demonstration of changing patterns of immunoreactivity for HK1 and HK3 in fetal and postnatal organs suggests that these HK isoforms are involved the process of development. We speculate that HK1 and HK3 share a complex interaction during development of these organs and regulate glucose metabolism at multiple levels during development.

Two distinct mechanisms regulate luteovirus transmission efficiency and specificity at the aphid salivary gland

Barley yellow dwarf luteovirus (BYDV) particles are transmitted by aphids in a species-specific manner. Transmission to plants requires that the virus particles be transported across the basal lamina and plasmalemma of the accessory salivary gland (ASG). To characterize the role of the ASG basal lamina in regulating BYDV transmission, five aphid species were microinjected with purified New York isolates BYDV-PAV or -RPV. Both viruses associated specifically only with the ASG basal lamina. The ability of virions to penetrate the basal lamina was separate from the ability to penetrate the plasmalemma. When the salivary glands of vector, Sitobion avenae, or non-vector, Rhopalosiphum maidis, aphids were incubated in vitro with New York isolate BYDV-MAV, virions only attached to the ASG basal lamina of S. avenae. When anionic and cationic ferritin were microinjected into aphids, only cationic ferritin aggregated on the surface of the ASG basal lamina and at openings of plasmalemma invaginations into the cytoplasm, suggesting that these sites had a net negative charge. In vitro studies of anionic and cationic gold penetration of ASG basal laminae indicated a macromolecular size exclusion limit of approximately 20 nm that depended on charge. Anionic gold particles did not accumulate in the basal lamina as densely as the 25 nm BYDV particles, suggesting that the virus particles have a greater affinity for the ASG basal lamina. These results indicate that both the ASG basal lamina and plasmalemma contain specific components independently involved in the recognition and transmission of luteoviruses.

Plant virus proteins involved in natural vector transmission

Plant viruses transmitted by invertebrate vectors either reversibly bind to vector mouthparts or are internalized by the vector and later secreted. Viral proteins mediate the binding of plant viruses to vector mouthparts and the transport of virus across vector-cell membranes. Both mechanisms probably involve conformational changes of virus proteins during their association with the vector.

Aphid transmission and systemic plant infection determinants of barley yellow dwarf luteovirus-PAV are contained in the coat protein readthrough domain and 17-kDa protein, respectively

Proteins encoded by open reading frames (ORF) 3, 4, and 5 of the barley yellow dwarf luteovirus genome are translated from a single subgenomic RNA. The structural proteins are encoded by ORF 3 (coat protein) and ORF 5 (readthrough domain) and contain undefined domains that regulate the movement of virus through aphid vectors. The biological function of the nonstructural 17-kDa protein encoded by ORF 4 is unknown. A complementation method was employed to test the ability of barley yellow dwarf virions carrying mutations within the readthrough domain and the 17-kDa protein to be transmitted by aphids and to cause systemic infections in plants. We show that the readthrough domain is required for aphid transmission; however, it is not required for virus to be taken up by aphid hindgut cells and released into the hemocoel. The circulative pathway of luteoviruses in aphid vectors requires that virus be actively transported from the hemolymph into the salivary system. Thus, it appears that the readthrough domain is required for transport of virus through membranes of the aphid salivary glands. Furthermore, the readthrough domain was not required for systemic infection of plants, but did influence the accumulation of virus in infected plants. The 17-kDa protein is required for the systemic infection of plants.

Readthrough protein associated with virions of barley yellow dwarf luteovirus and its potential role in regulating the efficiency of aphid transmission

Purified particles of barley yellow dwarf luteovirus (BYDV) contain a major 22-kDa protein and a minor protein of approximately 58 kDa. The 22-kDa capsid protein is encoded by open reading frame (ORF) 3. ORF 5 is immediately downstream and in frame with ORF 3 and a 72-kDa protein can be translated via a readthrough suppression of the ORF 3 termination codon. Antibodies were produced against two Escherichia coli expressed polypeptides that represent the amino- and carboxyl-terminal halves of a putative 50-kDa protein encoded by ORF 5. Immunological analyses indicated that the 58-kDa protein associated with purified virions contained sequences encoded by ORF 3 and ORF 5. The carboxyl terminal portion of the full-length (72 kDa) readthrough protein was absent from the 58-kDa protein. The full-length readthrough protein was detected in infected oat protoplasts and plant tissue, but was not associated with virus particles purified from plants. The carboxyl-terminal portion of the 72-kDa readthrough protein was not required for aphid transmission; however, virus was transmitted more efficiently from protoplast extracts containing virions and soluble 72-kDa readthrough protein than from mock-inoculated protoplast extracts to which plant purified virus was added. The full-length readthrough protein, although not required for transmission, may increase the transmission efficiency of BYDV by aphids.

Hexokinase binding in ischemic and reperfused piglet brain

Hexokinase catalyzes the first step in cerebral glucose utilization and is a rate-limiting enzyme in glycolysis. Glucose utilization is tightly coupled to cerebral blood flow so that during ischemia the brain has a decreased supply of glucose, as well as oxygen. We studied hexokinase enzymatic activity in a newborn piglet model of ischemia-reperfusion to determine if any changes in the activity or mitochondrial binding of the enzyme occurred. We observed that mitochondrial binding of cortical HK increased from 55 to 71% with ischemia and returned toward control levels, but did not completely recover, after 2 h of reperfusion.

Success of audit in reducing the time taken to administer thrombolysis and aspirin in patients with acute myocardial infarction

We assessed whether audit might reduce the time taken to give thrombolysis and aspirin in patients with acute myocardial infarction (N = 116). A retrospective analysis was performed of the sources of delay in giving the drugs (N-60) and the data were presented to clinical staff accompanied by guidelines aimed at eliminating delays. A prospective survey was undertaken (N = 56) after these interventions. Audit resulted in an overall 31% reduction (P = 0.013) in the time to administer thrombolysis (median 55 minutes [range 21-148] v 38 [15-155]): there was a 57% fall (P < 0.0001) in the time to record an electrocardiogram (14 minutes [4-34] v 6 [1-19]) and a 33% decrease (P = 0.047) in the time taken to begin thrombolysis in the coronary care unit (15 minutes [0-110) v 10 [5-70]). The time taken to give aspirin was also reduced (P = 0.001) from 58 minutes (15-400) to 15 (3-235). The time taken to administer thrombolysis and aspirin to patients admitted with acute myocardial infarction can be reduced by audit.

Localization of a surface domain of the capsid protein of barley yellow dwarf virus

We describe a method for localizing protein domains situated at the surface of virus particles. A cDNA clone of the New York PAV isolate (NY-PAV) of barley yellow dwarf virus (BYDV) containing the capsid protein gene was generated and sequenced. A defined set of overlapping cDNA fragments specific to the capsid protein ORF of NY-PAV was subcloned into the pGEX expression vectors. Cells of Escherichia coli carrying these plasmids synthesize recombinant glutathione S-transferase/capsid proteins. These proteins were used in immunoblot experiments to localize the epitopes of three PAV-BYDV-directed monoclonal antibodies to a 20 amino acid-long segment of the NY-PAV capsid protein. All three monoclonal antibodies reacted with NY-PAV virions in a triple antibody sandwich enzyme-linked immunosorbent assay, indicating that their epitopes are located at the virion surface.

Use of group-specific primers and the polymerase chain reaction for the detection and identification of luteoviruses

A general diagnostic assay for a number of distinct luteoviruses was developed using the polymerase chain reaction (PCR) and restriction enzyme analysis. Two minimally degenerate, group-specific primers were derived from previously published RNA sequences of three luteoviruses. This primer pair generated specific PCR fragments of about 530 bp from extracts of plants infected with potato leafroll virus, beet western yellows virus, or New York barley yellow dwarf virus (BYDV) serotypes MAV, PAV, RMV, RPV and SGV, which span much of the respective viral coat protein gene. Each virus was easily distinguished from the others by restriction enzyme analysis of the amplified DNA products. Samples from BYDV-infected oat and wheat collected in Nebraska were identified as containing PAV-like serotypes; micro-heterogeneity was detected in several samples. This method provides a rapid, sensitive and relatively inexpensive means of luteovirus detection and identification. It is the first test capable of simultaneously detecting all five BYDV serotypes.

The mechanism and control of rabbit oviduct fluid formation

The formation of rabbit oviduct fluid was monitored continuously by using an in situ vascular perfusion technique. Oviduct fluid was secreted linearly for at least 3 h at a mean rate of 20.8 +/- 1.5 microliter/h in estrous does. The rate more than doubled on Day 1 following mating, was similar to the value at estrus on Day 2, and dropped to 8.3 microliter on Day 3. Dibutyryl cyclic adenosine 3',5'-monophosphate (cAMP, 1 mM) added to the vascular medium abolished fluid secretion. The same response was obtained, after a lag period, following the addition of cholera toxin (1 mM), forskolin (1 mM), theophylline (1 mM), phorbol dibutyrate (40 microM), A23187 (2 micrograms/ml), 4-acetamido-4'-isothiocyonatostilbene-2,2'-disulphonic acid (SITS, 1 mM), and bumetanide (10 microM) to the vascular medium. N-ethylmaleimide (1 mM), which inhibits adenylate cyclase, stimulated oviduct fluid formation. The transmural potential difference (p.d.) across the oviduct was 5.46 +/- 1.01 mV. This was increased after cAMP addition to 8.7 +/- 1.22 mV. The p.d. in oviducts taken 3 days post-ovulation was 7.6 +/- 1.75 mV, and was increased by cAMP to 12.7 +/- 0.53 mV. Exposure to cholera toxin and forskolin almost doubled the cAMP content of the oviduct. The undirectional flux of chloride ions from the vascular compartment into the lumen was reduced by about 75% after the addition of cAMP, SITS, and bumetanide. A tentative model to account for the formation and regulation of rabbit oviduct fluid in terms of ion fluxes and cAMP and calcium ion concentrations is presented.

Vascular perfusion: a novel means of studying oviduct function

An in situ preparation for the combined vascular and luminal perfusion of the rabbit oviduct has been developed. Medium 199, gassed with 5% CO2 in O2 and supplemented with heparin, antibiotics, and 2.5% wt/vol dialyzed bovine serum albumin was infused into the ovarian artery at a rate of 1 ml/min. Krebs Ringer bicarbonate medium was recirculated through the lumen at a rate of 50 microliter/min. The ovary was perfused together with the oviduct, and the preparation is viable for up to 3 h. Equal concentrations of pyruvate, lactate, glucose, and sucrose added to the vascular medium were transported at different rates into the lumen, as was a physiological mixture of amino acids. A proportion of the lactate entering the lumen was synthesized within the oviduct from vascular glucose. When glucose and pyruvate were omitted from the vascular medium, their appearance and that of lactate in the lumen was barely detectable, suggesting that these oviduct fluid components are mainly derived from the blood. The oviduct maintained a steady transmural potential difference of 5.9 mV (lumen negative). With vascular perfusion alone, oviduct fluid entered the oviduct lumen at a rate of 16.8 microliter/h. In oviducts taken from rabbits 3 days postovulation, there was a general decrease in the vascular to lumen flux of all nutrients measured. Preliminary work has shown that the preparation may be used to study ovulation, ovum pickup and transport, and fertilization.

Comparison of separation methods in the 125I-radioimmunoassay of serum cortisol

Solid-phase first antibody (SP), liquid-phase double-antibody (DA), and pre-precipitated double-antibody (PPT) separation methods have been compared in a radioimmunoassay (RIA) for cortisol in unextracted serum. Both double-antibody methods gave values for pools close to target values assigned by gas chromatography-mass spectrometry (GCMS) whereas the SP assay had a significant negative bias, p less than 0.001 (mean biases: SP -8%, DA -2%, PPT -3%). The SP and DA assays gave average values on patients' samples 12% and 4% lower than values by PPT. These differences could be attributed to different recoveries in the three methods. Between-assay precision of the DA and PPT systems was better than SP (mean CV:SP 11%, DA 7%, PPT 5%). This allowed sensitivity in the PPT assay comparable to that of SP and DA to be achieved, despite the less steep slope of the calibration curve. The DA and PPT assays have several practical advantages over SP. In addition, the PPT assay requires only a 1-hour incubation. It is concluded that the PPT separation system is the method of choice in terms of precision and practical convenience.