Electrical Penetration Graph Recording of Russian Wheat Aphid (Hemiptera: Aphididae) Feeding on Aphid-Resistant Wheat and Barley.

Biotypes of Russian wheat aphid, Diuraphis noxia (Kurdjumov), have nullified D. noxia-resistant wheat. In this study, feeding of North American D. noxia was measured in aphids fed resistant and susceptible wheat and barley using electrical penetration graph (EPG) recordings. Interactions between barley genotypes and D. noxia biotypes were significant. EPG recordings of biotype 1 aphids fed on D. noxia-resistant IBRWAGP4-7 barley plants displayed significantly more non-phloem (pathway) phase movements and significantly less sieve element phase (SEP) feeding than on susceptible plants. EPG recordings of D. noxia biotype 2 feeding are the first ever recorded, but no differences between biotype 2-susceptible and -resistant barley plants were found for any EPG parameter in biotype 2 aphids fed barley. No wheat genotype-D. noxia biotype interactions were detected, but when responses were averaged across resistant and susceptible wheat genotypes, biotype 1 displayed a significantly longer pathway phase and significantly more SEP feeding than biotype 2, and biotype 2 engaged in significantly more xylem drinking than biotype 1. IBRWAGP4-7 barley resistance to biotype 1 appears to be controlled by both intercellular factors encountered during the pathway phase and intracellular factors ingested during SEP feeding. The lack of differences in EPG parameters displayed by biotype 2 feeding on barley suggests that biotype 2 resistance in IBRWAGP4-7 barley is based on tolerance to D. noxia feeding instead of altered feeding patterns. Resistance in 'KS94H871' wheat appears to be a function of phloem, non-phloem, and xylem factors that extend the duration of pathway feeding and limit SEP feeding.

Virulence and biotype analyses of Hessian fly (Diptera: Cecidomyiidae) populations from Texas, Louisiana, and Oklahoma.

Hessian fly, Mayetiola destructor (Say, 1817), is a major pest of wheat, and is controlled mainly through deploying fly-resistant wheat cultivars. The challenge for the plant resistance approach is that virulence of Hessian fly populations in the field is dynamic, and wheat cultivars may lose resistance within 6-8 yr. To ensure continuous success of host plant resistance, Hessian fly populations in the field need to be constantly monitored to determine which resistance genes remain effective in different geographic regions. This study investigated five Hessian fly populations collected from Texas, Louisiana, and Oklahoma, where infestation by Hessian fly has been high in recent years. Eight resistance genes, H12, H13, H17, H18, H22, H25, H26, and Hdic, were found to be highly effective against all tested Hessian fly populations in this region, conferring resistance to > or = 80% of plants containing one of these resistance genes. The frequencies ofbiotypes virulent to resistance genes H13 (biotype vH13), H18 (vH18), H21 (vH21), H25 (vH25), H26 (vH26), and Hdic (vHdic) were determined, and were found to vary from population to population, ranging from 0 to 45%. A logistic regression model was established to predict biotype frequencies based on the correlation between the percentages of susceptible plants obtained in a virulence test and the log-odds of virulent biotype frequencies determined by a traditional approach.

Molecular bases of plant resistance to arthropods.

Arthropod-resistant crops provide significant ecological and economic benefits to global agriculture. Incompatible interactions involving resistant plants and avirulent pest arthropods are mediated by constitutively produced and arthropod-induced plant proteins and defense allelochemicals synthesized by resistance gene products. Cloning and molecular mapping have identified the Mi-1.2 and Vat arthropod resistance genes as CC-NBS-LRR (coiled coil-nucleotide binding site-leucine rich repeat) subfamily NBS-LRR resistance proteins, as well as several resistance gene analogs. Genetic linkage mapping has identified more than 100 plant resistance gene loci and linked molecular markers used in cultivar development. Rice and sorghum arthropod-resistant cultivars and, to a lesser extent, raspberry and wheat cultivars are components of integrated pest management (IPM) programs in Asia, Australia, Europe, and North America. Nevertheless, arthropod resistance in most food and fiber crops has not been integrated due primarily to the application of synthetic insecticides. Plant and arthropod genomics provide many opportunities to more efficiently develop arthropod-resistant plants, but integration of resistant cultivars into IPM programs will succeed only through interdisciplinary collaboration.

Comparative analyses of transcriptional responses of Dectes texanus LeConte (Coleoptera: Cerambycidae) larvae fed on three different host plants and artificial diet.

Dectes texanus is an important coleopteran pest of soybeans and cultivated sunflowers in the Midwestern United States that causes yield losses by girdling stems of their host plants. Although sunflower and giant ragweed are primary hosts of D. texanus, they began colonizing soybeans approximately 50 years ago and no reliable management method has been established to prevent or reduce losses by this pest. To identify genes putatively involved when feeding soybean, we compared gene expression of D. texanus third-instar larvae fed soybean to those fed sunflower, giant ragweed, or artificial diet. Dectes texanus larvae differentially expressed 514 unigenes when fed on soybean compared to those fed the other diet treatments. Enrichment analyses of gene ontology terms from up-regulated unigenes in soybean-fed larvae compared to those fed both primary hosts highlighted unigenes involved in oxidoreductase and polygalacturonase activities. Cytochrome P450s, carboxylesterases, major facilitator superfamily transporters, lipocalins, apolipoproteins, glycoside hydrolases 1 and 28, and lytic monooxygenases were among the most commonly up-regulated unigenes in soybean-fed larvae compared to those fed their primary hosts. These results suggest that D. texanus larvae differentially expressed unigenes involved in biotransformation of allelochemicals, digestion of plant cell walls and transport of small solutes and lipids when feeding in soybean.

Global phylogenetics of Diuraphis noxia (Hemiptera: Aphididae), an invasive aphid species: evidence for multiple invasions into North America.

The Russian wheat aphid, Diruaphis noxia (Kudjumov) (Hemiptera: Aphididae), is globally one of the most devastating pests of bread wheat, Tritium aestivum L., durum wheat, Triticum turgidum L., and barley, Hordeum vulgare L. Several sources of D. noxia resistance have been incorporated in commercial wheat and barley genotypes, but up to eight virulent biotypes, defined based on their ability to damage different wheat and barley genotypes, now occur across the western United States since the first appearance of D. noxia in North America in 1986. Critical to the study of D. noxia and other invasive species is an understanding of the number and origin of invasions that have occurred, as well as the rate or potential of postinvasion adaptation and geographic range expansion. The goal of this study was to determine whether D. noxia biotypes are by-products of a single invasion or multiple invasions into North America. We used the genome-wide technique of amplified fragment length polymorphisms, in combination with 22 collections of D. noxia from around the world, to assess this question, as well as patterns of genetic divergence. We found multiple lines of evidence that there have been at least two D. noxia invasions of different origin into North America, each resulting in subsequent postinvasion diversification that has since yielded multiple biotypes.

Modeling Aceria tosichella biotype distribution over geographic space and time.

The wheat curl mite, Aceria tosichella Keifer, one of the most destructive arthropod pests of bread wheat worldwide, inflicts significant annual reductions in grain yields. Moreover, A. tosichella is the only vector for several economically important wheat viruses in the Americas, Australia and Europe. To date, mite-resistant wheat genotypes have proven to be one of the most effective methods of controlling the A. tosichella-virus complex. Thus, it is important to elucidate A. tosichella population genetic structure, in order to better predict improved mite and virus management. Two genetically distinct A. tosichella lineages occur as pests of wheat in Australia, Europe, North America, South America and the Middle East. These lineages are known as type 1 and type 2 in Australia and North America and in Europe and South America as MT-8 and MT-1, respectively. Type 1 and type 2 mites in Australia and North America are delineated by internal transcribed spacer 1 region (ITS1) and cytochrome oxidase I region (COI) sequence differences. In North America, two A. tosichella genotypes known as biotypes are recognized by their response to the Cmc3 mite resistance gene in wheat. Aceria tosichella biotype 1 is susceptible to Cmc3 and biotype 2 is virulent to Cmc3. In this study, ITS1 and COI sequence differences in 25 different populations of A. tosichella of known biotype 1 or biotype 2 composition were characterized for ITS1 and COI sequence differences and used to model spatio-temporal dynamics based on biotype prevalence. Results showed that the proportion of biotype 1 and 2 varies both spatially and temporally. Greater ranges of cropland and grassland within 5000m of the sample site, as well as higher mean monthly precipitation during the month prior to sampling appear to reduce the probability of occurrence of biotype 1 and increase the probability of occurrence of biotype 2. The results suggest that spatio-temporal modeling can effectively improve A. tosichella management. Continual integration of additional current and future precipitation and ground cover data into the existing model will further improve the accuracy of predicting the occurrence of A. tosichella in annual wheat crops, allowing producers to make informed decisions about the selection of varieties with different A. tosichella resistance genes.

Big Genes, Small Effectors: Pea Aphid Cassette Effector Families Composed From Miniature Exons.

Aphids secrete proteins from their stylets that evidence indicates function similar to pathogen effectors for virulence. Here, we describe two small candidate effector gene families of the pea aphid, Acyrthosiphon pisum, that share highly conserved secretory signal peptide coding regions and divergent non-secretory coding sequences derived from miniature exons. The KQY candidate effector family contains eleven members with additional isoforms, generated by alternative splicing. Pairwise comparisons indicate possible four unique KQY families based on coding regions without the secretory signal region. KQY1a, a representative of the family, is encoded by a 968 bp mRNA and a gene that spans 45.7 kbp of the genome. The locus consists of 37 exons, 33 of which are 15 bp or smaller. Additional KQY members, as well as members of the KHI family, share similar features. Differential expression analyses indicate that the genes are expressed preferentially in salivary glands. Proteomic analysis on salivary glands and saliva revealed 11 KQY members in salivary proteins, and KQY1a was detected in an artificial diet solution after aphid feeding. A single KQY locus and two KHI loci were identified in Myzus persicae, the peach aphid. Of the genes that can be anchored to chromosomes, loci are mostly scattered throughout the genome, except a two-gene region (KQY4/KQY6). We propose that the KQY family expanded in A. pisum through combinatorial assemblies of a common secretory signal cassette and novel coding regions, followed by classical gene duplication and divergence.

Resistance to Russian wheat aphid biotype 2 in CIMMYT synthetic hexaploid wheat lines.

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), devastates wheat and barley production on all continents except Australia. Although D. noxia-resistant cultivars exist, virulent D. noxia populations exist in Asia, North America, and South America that have the ability to overcome resistance. In this study, synthetic hexaploid wheat genotypes created at the International Maize and Wheat Improvement Center (CIMMYT) were evaluated and characterized for resistance to North American biotype 2 (RWA2). Fewer RWA2 were produced on several genotypes, and D. noxia-induced leaf rolling and chlorosis were reduced on these genotypes as well. Aphid numbers were positively correlated with chlorosis and with leaf rolling. However, some genotypes were highly resistant to leaf rolling and chlorosis while supporting large RWA2 populations. There were negative correlations between leaf chlorosis and leaf dry weight (r = -0.267, df = 106, P = 0.006) and between aphid numbers and leaf dry weight (r = -0.297, df = 105, P = 0.002). These results indicate that chlorosis and aphid number individually explain at least 27% of the changes observed in leaf dry weights. Interestingly, there was no correlation between leaf rolling and leaf dry weight. The RWA2-resistant lines identified, which are also resistant to D. noxia populations in Mexico and to greenbug (Schizaphis graminum Rondani) biotype G, are strong candidates for use in improving the genetic diversity in bread wheat for resistance to different biotypes of both S. graminum and D. noxia.

Feeding behavior of Russian wheat aphid (Hemiptera: Aphididae) biotype 2 in response to wheat genotypes exhibiting antibiosis and tolerance resistance.

In this study, wheat, Triticum aestivum L. (em Thell), genotypes containing the Dnx, Dn7, Dn6, and Dn4 genes for resistance to the Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), along with Dn0, a susceptible control, were assessed to determine the categories of D. noxia biotype 2 (RWA2) resistance in each genotype and RWA2 feeding behaviors on Dnx and Dn0 plants by using the electronic penetration graph technique. At 14 d postinfestation, Dn0 plants exhibited intense chlorosis and leaf rolling, and all test genotypes expressed some degree of chlorosis and leaf rolling, except Dn7, which was not damaged. Both Dn7 and Dnx expressed antibiosis effects, significantly reducing the numbers of aphids on plants and the intrinsic rate of aphid increase. Dn6 plants seemed to contain tolerance, exhibiting tolerance index measurements for leaf and root dry weight and plant height that were significantly lower than those of the susceptible Dn0 plants. Principal component analyses indicated that antibiosis and leaf rolling data explained 80% of the variance among genotypes. Electronic penetration graph analysis demonstrated contrasting results between RWA1 and RWA2 phloem sieve element phase feeding events, but results indicated that Dnx resistance factors are present in the sieve element cells or phloem sap. Plants containing Dnx exhibit antibiosis resistance to D. noxia RWA2 similar to that in plants containing the Secale cereale L. (rye)-based Dn7 gene without the negative baking quality traits associated with Dn7.

Resistance of Select Winter Wheat (Triticum aestivum) Cultivars to Rhopalosiphum padi (Hemiptera: Aphididae).

The bird cherry-oat aphid (Rhopalosiphum padi L.) is a global pest of wheat and vectors some of the most damaging strains of barley yellow dwarf virus (BYDV). In years of heavy R. padi infestation, R. padi and BYDV together reduce wheat yields by 30-40% in Kansas and other states of the U.S. Great Plains wheat production area. Cultivation of wheat cultivars resistant to R. padi can greatly reduce production costs and mitigate R. padi-BYDV yield losses, and increase producer profits. This study identified cultivars of hard red and soft white winter wheat with R. padi resistance that suppress R. padi populations or tolerate the effects of R. padi feeding damage. 'Pioneer (S) 25R40,' 'MFA (S) 2248,' 'Pioneer (S) 25R77,' and 'Limagrain LCS Mint' significantly reduced R. padi populations. MFA (S) 2248, Pioneer (S) 25R40, and 'Limagrain LS Wizard' exhibited tolerance expressed as significantly greater aboveground biomass. These findings are significant in that they have identified wheat cultivars currently available to producers, enabling the immediate improvement of tactics to manage R. padi and BYDV in heavily infested areas. Secondarily, these results identify cultivars that are good candidates for use in breeding and genetic analyses of arthropod resistance genes in wheat.

Wheat Genotypes With Combined Resistance to Wheat Curl Mite, Wheat Streak Mosaic Virus, Wheat Mosaic Virus, and Triticum Mosaic Virus.

The wheat curl mite, Aceria tosichella Keifer, (WCM) is a global pest of bread wheat that reduces yields significantly. In addition, WCM carries Wheat streak mosaic virus (WSMV, family Potyviridae, genus Tritimovirus), the most significant wheat virus in North America; High Plains wheat mosaic virus (HPWMoV, genus Emaravirus, formerly High plains virus); and Triticum mosaic virus (TriMV, family Potyviridae, genus Poacevirus). Viruses carried by WCM have reduced wheat yields throughout the U.S. Great Plains for >50 yr, with average yield losses of 2-3% and occasional yield losses of 7-10%. Acaricides are ineffective against WCM, and delayed planting of winter wheat is not feasible. Five wheat breeding lines containing Cmc4, a WCM resistance gene from Aegilops tauschii, and Wsm2, a WSMV resistance gene from wheat germplasm CO960293-2 were selected from the breeding process and assessed for phenotypic reaction to WCM feeding, population increase, and the degree of WSMV, HPWMoV, and TriMV infection. Experiments determined that all five lines are resistant to WCM biotype 1 feeding and population increase, and that two breeding lines contain resistance to WSMV, HPWMoV, and TriMV infection as well. These WCM-, WSMV-, HPWMoV-, and TriMV-resistant genotypes can be used improve management of wheat yield losses from WCM-virus complexes.

Categories and inheritance of resistance to Russian wheat aphid (Homoptera: Aphididae) biotype 2 in a selection from wheat cereal introduction 2401.

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), is one of the most devastating insect pests of wheat (Triticum spp.) and barley (Hordeum spp.) in the world. Yield losses and control costs are valued at several hundred million dollars each year. The use of D. noxia-resistant cultivars is an ecologically, economically, and biologically sound method of managing this pest. Several D. noxia resistance (Dn) genes from wheat have been used to develop cultivars resistant to D. noxia. However, a new U.S. D. noxia biotype (biotype 2) in Colorado is virulent to all known Dn genes except the Dn7 gene from rye (Secale spp.). Hence, there is an immediate need to identify and characterize unique sources of D. noxia resistance to biotypes. In this article, we report resistance to D. noxia biotype 2, identified in a selection from wheat cereal introduction (CItr) 2401, that is controlled by two dominant genes. CItr2401 has a strong antibiosis effect that is exhibited as a reduced intrinsic rate of increase of D. noxia biotype 2. CItr2401 plants also exhibit tolerance to leaf rolling and chlorosis. No antixenosis was detected in CItr2401.

Molecular basis of plant gene expression during aphid invasion: wheat Pto- and Pti-like sequences are involved in interactions between wheat and Russian wheat aphid (Homoptera: Aphididae).

The Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), is a major pest of bread wheat, Triticum aestivum L. (em Thell), in most wheat-growing areas worldwide. Aphid-resistant cultivars are used to combat this pest, but very little is known about the molecular basis of resistance. In this study, differential gene expression in D. noxia biotype 1-resistant wheat plants containing the Dnx gene and D. noxia biotype 1 feeding on Dnx plants was investigated using suppressive subtraction hybridization. The derived subtracted cDNA library includes sequences similar to Pto and Pti1, genes involved in gene-for-gene recognition of and resistance to bacterial speck disease in tomato, Lycopersicon esculentum (L.). Pto- and Pti1-like sequences contain an activation domain with conserved amino acid residues crucial for avr protein recognition and binding by Pto, and avr-Pto phosphorylation of Pti1. Wheat defense signaling is represented by sequences putatively involved in producing sterols, jasmonates, Ca2+, and abscisic and gibberellic acids. We suggest that reductions in populations of D. noxia fed Dnx plants are related to the expression of sequences involved in defensive chemical production, cellular transport, and exocytosis. Dnx plant tolerance of D. noxia feeding is proposed to be based on the expression of sequences putatively involved in self-defense against reactive oxygen species and toxins, and proteolysis; DNA, RNA, and protein synthesis; chloroplast and mitochondrial function; carbohydrate metabolism; and maintenance of cell homeostasis. D. noxia unsuccessfully counter Dnx by expressing sequences putatively involved in detoxification; proteolysis; DNA, RNA, protein, and lipid synthesis; carbohydrate metabolism; and mitochondrial function.

Virulent Diuraphis noxia Aphids Over-Express Calcium Signaling Proteins to Overcome Defenses of Aphid-Resistant Wheat Plants.

The Russian wheat aphid, Diuraphis noxia, an invasive phytotoxic pest of wheat, Triticum aestivum, and barley, Hordeum vulgare, causes huge economic losses in Africa, South America, and North America. Most acceptable and ecologically beneficial aphid management strategies include selection and breeding of D. noxia-resistant varieties, and numerous D. noxia resistance genes have been identified in T. aestivum and H. vulgare. North American D. noxia biotype 1 is avirulent to T. aestivum varieties possessing Dn4 or Dn7 genes, while biotype 2 is virulent to Dn4 and avirulent to Dn7. The current investigation utilized next-generation RNAseq technology to reveal that biotype 2 over expresses proteins involved in calcium signaling, which activates phosphoinositide (PI) metabolism. Calcium signaling proteins comprised 36% of all transcripts identified in the two D. noxia biotypes. Depending on plant resistance gene-aphid biotype interaction, additional transcript groups included those involved in tissue growth; defense and stress response; zinc ion and related cofactor binding; and apoptosis. Activation of enzymes involved in PI metabolism by D. noxia biotype 2 aphids allows depletion of plant calcium that normally blocks aphid feeding sites in phloem sieve elements and enables successful, continuous feeding on plants resistant to avirulent biotype 1. Inhibition of the key enzyme phospholipase C significantly reduced biotype 2 salivation into phloem and phloem sap ingestion.

First-principles analysis of the effects of alloying Pd with Ag for the catalytic hydrogenation of acetylene-ethylene mixtures.

The effect of alloying Pd with Ag on the hydrogenation of acetylene is examined by analyzing the chemisorption of all potential C(1) (atomic carbon, CH, methylene, and methyl) and C(2) (acetylene, vinyl, ethylene, ethyl, ethane, ethylidene, ethylidyne, and vinylidene) surface intermediates and atomic hydrogen along with the reaction energies for the elementary steps that produce these intermediates over Pd(111), Pd(75%)Ag(25%)/Pd(111), Pd(50%)Ag(50%)/Pd(111), and Ag(111) surfaces by using first-principle density functional theoretical (DFT) calculations. All of the calculations reported herein were performed at 25% surface coverage. The adsorption energies for all of the C(1) and C(2) intermediates decreased upon increasing the composition of Ag in the surface. Both geometric as well as electronic factors are responsible for the decreased adsorption strength. The modes of adsorption as well as the strengths of adsorption over the alloy surfaces in a number of cases were characteristically different than those found over pure Pd (111) and Ag (111). Adsorbates tend to minimize their interaction with the Ag atoms in the alloy surface. An electronic analysis of these surfaces shows that there is, in general, a shift in the occupied d-band states away from the Fermi level when Pd is alloyed with Ag. The s and p states also appear to contribute and may be responsible for small deviations from the Hammer-Nørskov model. The effect of alloying is more pronounced on the calculated reaction energies for different possible surface elementary reactions. Alloying Pd with Ag reduces the exothermicity (increases endothermicity) for bond-breaking reactions. This is consistent with experimental results that show a decrease in the decomposition products in moving from pure Pd to Pd-Ag alloys.(2-5) In addition, alloying increases the exothermicity of bond-forming reactions. Alloying therefore not only helps to suppress the unfavorable decomposition (bond-breaking) reaction rates but also helps to enhance the favorable hydrogenation (bond-forming) reaction rates.

Molecular mapping of sorghum genes expressing tolerance to damage by greenbug (Homoptera: Aphididae).

Genetic linkage maps are fundamental for the localization of genes conferring tolerance to greenbug, Schizaphis graminum (Rondani), feeding damage in sorghum, Sorghum bicolor (L.) Moench. Thirteen linkage groups (LGs) containing 60 simple sequence repeat (SSR) loci were mapped by using a set of sorghum recombinant inbred lines (RILs) obtained from the cross '96-4121' (greenbug-tolerant parent) x Redlan (greenbug-susceptible parent). The LG spanned a distance of 603.5 cM, with the number of loci per LG varying from 2 to 14. Seventeen additional SSR loci were unlinked at a log of odds value of 3.0. Based on chlorophyll loss occurring after greenbug feeding, visual damage ratings, and soil plant analysis development (SPAD), chlorophyll-loss indices were recorded for each RIL and for the parents used in the cross. Composite-interval mapping identified three quantitative trait loci (QTLs) associated with biotype I and five QTLs associated with biotype K. The amount of phenotypic variation explained by these QTLs ranged from 9 to 19.6%. The identification of QTLs that influence greenbug tolerance will not only facilitate the use of marker-assisted selection in sorghum breeding programs but also will provide a solid foundation for detailed characterization of individual loci implicated in greenbug tolerance in sorghum.

Resistance to greenbug (Heteroptera: Aphididae) biotype I in Aegilops tauschii synthetic wheats.

The greenbug, Schizaphis graminum (Rondani), is a major pest of wheat in North America, reducing U.S. wheat production by 60 to 100 million dollars each year. In this research, 149 wheat lines containing genes from Aegilops tauschii (Coss.) Schmal. were evaluated for resistance to greenbug biotype I. More than 50% of the lines sustained moderate foliar chlorosis from greenbug feeding, and approximately one third of all the lines were highly resistant. All lines with chlorosis scores similar to the resistant control 'Largo' expressed high levels of antibiosis, producing greenbug populations with mean weights ranging from 0.05 to 11.8 mg. There was no significant difference between greenbug weights on these lines and those reared on 'Largo', but the mean weight of individuals reared on the susceptible control 'Thunderbird' was significantly greater than those reared on 'Largo' or any of the test lines. The mean population size of greenbugs produced on plants of each line was significantly correlated with mean greenbug weight. Tolerance was not evident in any of the lines examined, but was unexpectedly apparent in 'Thunderbird' at a level similar to that in the tolerant control cultivar 'Largo'.

Identification of Russian wheat aphid (Homoptera: Aphididae) populations virulent to the Dn4 resistance gene.

The Russian wheat aphid, Diuraphis noxia (Mordvilko), is a serious worldwide pest of wheat and barley. Russian wheat aphid populations from Hungary, Russia, and Syria have previously been identified as virulent to D. noxia (Dn) 4, the gene in all Russian wheat aphid-resistant cultivars produced in Colorado. However, the virulence of Russian wheat aphid populations from central Europe, North Africa, and South America to existing Dn genes has not been assessed. Experiments with plants containing several different Dn genes demonstrated that populations from Chile, the Czech Republic, and Ethiopia are also virulent to Dn4. The Czech population was also virulent to plants containing the Dnx gene in wheat plant introduction PI220127. The Ethiopian population was also virulent to plants containing the Dny gene in the Russian wheat aphid-resistant 'Stanton' produced in Kansas. The Chilean and Ethiopian populations were unaffected by the antibiosis resistance in Dn4 plants. There were significantly more nymphs of the Chilean population on plants of Dn4 than on Dn6 plants at both 18 and 23 d postinfestation, and the Ethiopian population attained a significantly greater weight on Dn4 plants than on plants containing Dn5 or Dn6. These newly characterized virulent Russian wheat aphid populations pose a distinct threat to existing or proposed wheat cultivars possessing Dn4.

Assessment of Aegilops tauschii for resistance to biotypes of wheat curl mite (Acari: Eriophyidae).

Aegilops tauschii, the wild diploid D-genome progenitor of wheat, Triticum aestivum L., is an important source of resistance to several arthropod pests and pathogens. A total of 108 Ae. tauschii accessions from different geographic regions were evaluated for resistance to biotypes of the wheat curl mite, Aceria tosichella Keifer, from Kansas, Nebraska, and Montana. The wheat curl mite is the only vector known to transmit wheat streak mosaic virus. Wheat curl mite resistance was detected in germplasm from all the geographic locations represented. The highest percentage of resistant accessions originated from Turkey, followed by Afghanistan and the Caspian Sea region of Iran. Sixty-seven percent of the accessions exhibited resistance to at least one wheat curl mite biotype and 19% were resistant to all the three biotyopes. Resistance to the accessions tested occurred more frequently in the Nebraska and Kansas biotypes (69% and 64%, respectively) than did resistance to the Montana biotype (42%), although the frequency of resistance was not significant. The differential reactions of accessions to the different wheat curl mite biotypes suggests that Ae. tauschii has at least five different genes for resistance to mite colonization. Ae. tauschii continues to be a very useful source for wheat curl mite resistance genes for bread wheat improvement.

Plant resistance to aphid feeding: behavioral, physiological, genetic and molecular cues regulate aphid host selection and feeding.

Aphids damage major world food and fiber crops through direct feeding and transmission of plant viruses. Fortunately, the development of many aphid-resistant crop plants has provided both ecological and economic benefits to food production. Plant characters governing aphid host selection often dictate eventual plant resistance or susceptibility to aphid herbivory, and these phenotypic characters have been successfully used to map aphid resistance genes. Aphid resistance is often inherited as a dominant trait, but is also polygenic and inherited as recessive or incompletely dominant traits. Most aphid-resistant cultivars exhibit constitutively expressed defenses, but some cultivars exhibit dramatic aphid-induced responses, resulting in the overexpression of large ensembles of putative aphid resistance genes. Two aphid resistance genes have been cloned. Mi-1.2, an NBS-LRR gene from wild tomato, confers resistance to potato aphid and three Meloidogyne root-knot nematode species, and Vat, an NBS-LRR gene from melon, controls resistance to the cotton/melon aphid and to some viruses. Virulence to aphid resistance genes of plants occurs in 17 aphid species--more than half of all arthropod biotypes demonstrating virulence. The continual appearance of aphid virulence underscores the need to identify new sources of resistance of diverse sequence and function in order to delay or prevent biotype development.