Variation In Disease Response Research Articles

Photosymbiont Density Is Correlated with Constitutive And Induced Immunity in The Facultatively Symbiotic Coral, Astrangia Poculata.

Scleractinian corals, essential ecosystem engineers that form the base of coral reef ecosystems, have faced unprecedented mortality in recent decades due to climate-change related stressors, including disease outbreaks. Despite this emergent threat to corals, many questions still remain regarding mechanisms underlying observed variation in disease susceptibility. Recent data suggests at least some degree of variation in disease response may be linked to variability in the relationship between host corals and their algal photosymbionts (Family Symbiodiniaceae). Still, the nuances of connections between symbiosis and immunity in cnidarians, including scleractinian corals, remain poorly understood. Here we leveraged an emergent model species, the facultatively symbiotic, temperate, scleractinian coral Astrangia poculata, to investigate associations between symbiont density and both constitutive and induced immunity. We used a combination of controlled immune challenges with heat-inactivated pathogens and transcriptomic analyses. Our results demonstrate that A. poculata mounts a robust initial response to pathogenic stimuli that is highly similar to responses documented in tropical corals. We document positive associations between symbiont density and both constitutive and induced immune responses, in agreement with recent preliminary studies in A. poculata. A suite of immune genes, including those coding for antioxidant peroxiredoxin biosynthesis, are positively associated with symbiont density in A. poculata under constitutive conditions. Furthermore, variation in symbiont density is associated with distinct patterns of immune response; low symbiont density corals induce preventative immune mechanisms whereas high symbiont density corals mobilize energetic resources to fuel humoral immune responses. In summary, our study reveals the need for more nuanced study of symbiosis-immune interplay across diverse scleractinian corals, preferably including quantitative energy budget analysis for full disentanglement of these complex associations and their effects on pathogen susceptibility.

The Genetic Basis of Natural Variation in Drosophila melanogaster Immune Defense against Enterococcus faecalis.

Dissecting the genetic basis of natural variation in disease response in hosts provides insights into the coevolutionary dynamics of host-pathogen interactions. Here, a genome-wide association study of Drosophila melanogaster survival after infection with the Gram-positive entomopathogenic bacterium Enterococcus faecalis is reported. There was considerable variation in defense against E. faecalis infection among inbred lines of the Drosophila Genetics Reference Panel. We identified single nucleotide polymorphisms associated with six genes with a significant (p < 10−08, corresponding to a false discovery rate of 2.4%) association with survival, none of which were canonical immune genes. To validate the role of these genes in immune defense, their expression was knocked-down using RNAi and survival of infected hosts was followed, which confirmed a role for the genes krishah and S6k in immune defense. We further identified a putative role for the Bomanin gene BomBc1 (also known as IM23), in E. faecalis infection response. This study adds to the growing set of association studies for infection in Drosophila melanogaster and suggests that the genetic causes of variation in immune defense differ for different pathogens.

SNP-based linkage mapping for validation of adult plant stripe rust resistance QTL in common wheat cultivar Chakwal 86

Wheat crops in China are constantly challenged by stripe rust. Deployment of cultivars with diverse resistances is the best strategy to control the disease. A recombinant inbred line (RIL) population derived from a cross between the resistant cultivar Chakwal 86 and susceptible landrace Mingxian 169 was studied in multiple environments to examine the underlying genetics and to identify quantitative trait loci (QTL) for stripe rust resistance. One hundred and twenty-eight RILs were genotyped with wheat 35K SNP array and a genome-wide linkage map with 1480 polymorphic SNP markers, or bins, was constructed. Two major QTL on chromosomes 1BL and 3BS, and one minor QTL on 6BS had significant effects in reducing stripe rust severity. The QTL were validated using composite interval mapping (CIM) and inclusive composite interval mapping (ICIM). These methods explained 59.0%–74.1% of the phenotype variation in disease response. The QTL on chromosome 1BL was confirmed to be Yr29/Lr46 and the one on 3BS was the resistance allele identified in CIMMYT germplasm but was not Yr30/Sr2. The QTL on 6BS probably corresponded to previously known QTL. RILs with combined QTL were more resistant than those with single or no QTL. Kompetitive allele-specific PCR (KASP) assays for the QTL with largest effect QTL on chromosome 3BS were performed on a subset of RILs and 150 unrelated wheat lines. The QTL on 3BS with its linked KASP markers can be used in marker-assisted selection to improve stripe rust resistance in breeding programs.

Development of a protocol to phenotype sweet cherry (Prunus avium L.) for resistance to bacterial canker

Suppression of bacterial canker disease of sweet cherry caused by Pseudomonas syringae pv. syringae (Pss), utilizing resistant scion and rootstock varieties holds promise as a cost-effective management strategy. However, a reproducible and rapid method for screening large breeding populations for resistance to Pss poses a challenge. Sweet cherry cultivars Bing, Sweetheart, Regina, Moreau, Emperor Francis and Rainier were used to examine the effects of Pss isolate, inoculum concentration (1 × 102 to 1 × 108 cfu/ml), leaf age (collected from the tip, middle or base of the shoot) and inoculation assay method (attached versus detached leaf) on disease development. Disease severity was influenced significantly (P ≤ 0.05) by inoculum concentration and the virulence of the Pss isolate. An inoculum concentration of 1 × 108 cfu/ml provided the best disease response in both leaf assays and is recommended for disease screening. Also, a significant Pss isolate x cultivar effect was observed suggesting that proper selection of Pss isolate for disease screening is critical. Disease severity was significantly (P ≤ 0.05) greater for newly expanding leaves in detached assays than for leaves classified as young or old. Disease response among cultivars for attached and detached leaf assays was significantly correlated (r = 0.53, P = 0.002), but the detached leaf assay provided better separation in disease severity among cultivars. We conclude that the genotypic variation in disease response among sweet cherry germplasm can be differentiated based on a detached assay using new leaves, a highly virulent Pss isolate, and an inoculum concentration of 1 × 108 cfu/ml.

Inferring infection hazard in wildlife populations by linking data across individual and population scales.

Our ability to infer unobservable disease‐dynamic processes such as force of infection (infection hazard for susceptible hosts) has transformed our understanding of disease transmission mechanisms and capacity to predict disease dynamics. Conventional methods for inferring FOI estimate a time‐averaged value and are based on population‐level processes. Because many pathogens exhibit epidemic cycling and FOI is the result of processes acting across the scales of individuals and populations, a flexible framework that extends to epidemic dynamics and links within‐host processes to FOI is needed. Specifically, within‐host antibody kinetics in wildlife hosts can be short‐lived and produce patterns that are repeatable across individuals, suggesting individual‐level antibody concentrations could be used to infer time since infection and hence FOI. Using simulations and case studies (influenza A in lesser snow geese and Yersinia pestis in coyotes), we argue that with careful experimental and surveillance design, the population‐level FOI signal can be recovered from individual‐level antibody kinetics, despite substantial individual‐level variation. In addition to improving inference, the cross‐scale quantitative antibody approach we describe can reveal insights into drivers of individual‐based variation in disease response, and the role of poorly understood processes such as secondary infections, in population‐level dynamics of disease.

Differences in the responses of melon accessions to fusarium root and stem rot and their colonization by Fusarium oxysporum f. sp. radicis‐cucumerinum

Fusarium root and stem rot caused by the fungus Fusarium oxysporum f. sp. radicis‐cucumerinum is a major disease in greenhouse cucumbers. Over the past decade, the disease has been documented in melon greenhouses in Greece, and recently it has been sporadically recorded in greenhouse melons in Israel. Variations in disease response were found among 41 melon accessions artificially inoculated with the pathogen: 10 accessions were highly susceptible (90–100% mortality), 23 exhibited an intermediate response (20–86%) and eight were resistant (0–4%). Two melon accessions – HEM (highly resistant) and TAD (partially resistant) – were crossed with the susceptible accession DUL. The responses of the three accessions and F1 crosses between the resistant and susceptible parents were evaluated. HEM contributed higher resistance to the F1 hybrid than TAD. Roots of susceptible and resistant accessions were 100 and 79% colonized, respectively, following artificial inoculation. However, only susceptible plants showed colonization of the upper plant tissues. Microscopic evaluation of cross sections taken from the crown region of the susceptible DUL revealed profuse fungal growth in the intercellular spaces of the parenchyma and in xylem vessels. In the resistant cultivar HEM, very little fungal growth was detected in the intercellular spaces of the parenchyma, and none in the xylem or any other vascular tissue. Finding resistant accessions may create an opportunity to study the genetics of resistance inheritance and to develop molecular markers that will facilitate breeding resistant melon cultivars.

Sustainable yield: The contribution of modern breeding and biotechnology in helping farmers increase yield and reduce environmental impacts in Asia

Yellow mosaic disease (YMD) of black gram is an important production constraint in India. Black gram germplasm, consisting of 344 accessions, originally collected from different geographic regions of India and conserved in National Gene Bank at National Bureau of Plant Genetic Resources, India, were evaluated for their response against YMD. None of the accessions tested was found to be immune, however, considerable variation in disease response was observed among them. Of the 344 accessions, 32 showed resistance in the field during the rainy season (June–September) 2010 as assessed by coefficient of infection (CI) and area under disease progress curve (AUDPC). These accessions were re-evaluated during rainy season (June–September) 2011 and 2012 for their disease resistance along with yield-associated characters. Eight accessions that exhibited consistently highly resistant (HR) or resistant (R) reaction (CI ≤ 9) along with lower AUDPC value (≤650), were further evaluated following inoculation with whiteflies or via agroinoculation. Along with the resistant trait, these eight accessions were also found either similar or even better in their yield potential as compared to check. After artificial inoculation using viruliferous whiteflies under glasshouse, of the eight field-resistant accessions, response of two (IC144901 and IC001572) were categorized as HR as <10% plants exhibited minute yellow specks after 17–22 dpi. Two accessions viz. IC011613 and IC485638 showed <20% incidence with bright yellow specks or spots after 15–20 dpi and were grouped into R category. The resistance of these four accessions was further confirmed through agroinoculation of the infectious cloned viral DNA under controlled conditions. In resistant accessions low accumulation of viral DNA was observed over a period of time. These four accessions could be used in future breeding programmes to develop black gram cultivars resistant to YMD or could be used directly as varieties to manage YMD after adaption to various agro-climatic regions.

Evaluation of an Indian collection of black gram germplasm and identification of resistance sources to Mungbean yellow mosaic virus

Yellow mosaic disease (YMD) of black gram is an important production constraint in India. Black gram germplasm, consisting of 344 accessions, originally collected from different geographic regions of India and conserved in National Gene Bank at National Bureau of Plant Genetic Resources, India, were evaluated for their response against YMD. None of the accessions tested was found to be immune, however, considerable variation in disease response was observed among them. Of the 344 accessions, 32 showed resistance in the field during the rainy season (June–September) 2010 as assessed by coefficient of infection (CI) and area under disease progress curve (AUDPC). These accessions were re-evaluated during rainy season (June–September) 2011 and 2012 for their disease resistance along with yield-associated characters. Eight accessions that exhibited consistently highly resistant (HR) or resistant (R) reaction (CI ≤ 9) along with lower AUDPC value (≤650), were further evaluated following inoculation with whiteflies or via agroinoculation. Along with the resistant trait, these eight accessions were also found either similar or even better in their yield potential as compared to check. After artificial inoculation using viruliferous whiteflies under glasshouse, of the eight field-resistant accessions, response of two (IC144901 and IC001572) were categorized as HR as <10% plants exhibited minute yellow specks after 17–22 dpi. Two accessions viz. IC011613 and IC485638 showed <20% incidence with bright yellow specks or spots after 15–20 dpi and were grouped into R category. The resistance of these four accessions was further confirmed through agroinoculation of the infectious cloned viral DNA under controlled conditions. In resistant accessions low accumulation of viral DNA was observed over a period of time. These four accessions could be used in future breeding programmes to develop black gram cultivars resistant to YMD or could be used directly as varieties to manage YMD after adaption to various agro-climatic regions.

Resistance to Fusarium oxysporum f. sp. fragariae and Predicted Breeding Values in Strawberry

Fusarium wilt of strawberry, incited by Fusarium oxysporum f. sp. fragariae (Fof), is a major disease of the cultivated strawberry (Fragaria ×ananassa) worldwide. An increase in disease outbreaks of the pathogen in Western Australia and Queensland plus the search for alternative disease management strategies place emphasis on the development of resistant cultivars. In response, a partial incomplete diallel cross involving four parents was performed for use in glasshouse resistance screenings. The resulting progeny were evaluated for their susceptibility to Fof. Best-performing progeny and suitability of progenies as parents were determined using data from disease severity ratings and analyzed using a linear mixed model incorporating a pedigree to produce best linear unbiased predictions of breeding values. Variation in disease response, ranging from highly susceptible to resistant, indicates a quantitative effect. The estimate of the narrow-sense heritability was 0.49 ± 0.04 (se), suggesting the population should be responsive to phenotypic recurrent selection. Several progeny genotypes have predicted breeding values higher than any of the parents. Knowledge of Fof resistance derived from this study can help select best parents for future crosses for the development of new strawberry cultivars with Fof resistance.

Response of Sorghum Accessions from Four African Countries against &amp;lt;i&amp;gt;Colletotrichum sublineolum&amp;lt;/i&amp;gt;, Causal Agent of Sorghum Anthracnose

Seventy-two sorghum accessions were randomly selected from the Ethiopia, Mali, Sudan, and Uganda germplasm collections maintained by the US National Plant Germplasm System to evaluate variation in anthracnose resistance. The accessions were planted in a randomized complete block design in College Station, Texas during the 2007 and 2008 growing seasons. Twenty-six accessions exhibited a resistant response across growing seasons with 8 accessions showing a susceptible response. Twenty-nine accessions showed variation in disease response within and between experiments. Seven accessions were rated as resistant in 2007 but showed a susceptible reaction in 2008. The frequency of resistant germplasm varied based on country of origin with 80% of the accessions from Mali, 48% of the accessions from Uganda, 24% of the accessions from Sudan, and 7% of the accessions from Ethiopia exhibiting a resistance response. When the same accessions were evaluated in Isabela, Puerto Rico, 100% of the accessions from Mali, 43% of the accessions from Uganda, and 28% of the accessions from Sudan exhibited a resistant response. All the accessions from Ethiopia were susceptible to anthracnose when evaluated in Isabela, Puerto Rico. In both locations, 22 accessions exhibited a resistant response. Four accessions rated as resistant in Texas were found to be susceptible in Puerto Rico; whereas, five accessions rated as resistant in Puerto Rico showed a susceptible response in Texas. These results indicated that the Mali, Sudan, and Uganda sorghum collections may be an important source of anthracnose resistance. However, the identification of anthracnose resistant germplasm from many diverse regions could result in the identification of new sources of genetic variation for resistance. Also, greater genetic variation for resistance could be present in regions with a high frequency of resistant germplasm.

Rapid phenotyping for adult‐plant resistance to stripe rust in wheat

With 6 figures and 4 tablesAbstractStripe or yellow rust (YR) is a significant problem in wheat crops worldwide. The deployment of adult‐plant resistance (APR) genes in wheat cultivars is considered a sustainable management strategy, as these genes confer partial resistance that is usually non‐race specific. Screening for APR typically involves assessment of adult plants in the field, where expression may be influenced by environmental factors. We report a high‐throughput screening method for YR APR that can be used to assess fixed lines or segregating populations grown under controlled environmental conditions (CEC). Inoculation of 3‐week‐old wheat plants from lines with known APR responses to YR, when grown under constant light and temperature, provided disease responses typical of adult plants. Two F2 populations (‘H45’ × ‘ST93’ and ‘Wyalkatchem’ × ‘ST93’) segregating for APR were assessed under both CEC and field conditions. These populations showed similar variation in disease response and lines assessed in both environments attained similar rankings. Phenotypic screening using CEC and continuous light provides an opportunity to accelerate the development of new wheat cultivars with durable resistance.

Targeted comparative genome analysis and qualitative mapping of a major partial-resistance gene to the soybean cyst nematode

A major partial-resistance locus to the soybean cyst nematode (Heterodera glycines Ichinohe; SCN) was identified on linkage group 'G' of soybean [Glycine max (L.) Merr.] using restriction fragment length polymorphisms (RFLPs). This locus explained 51.4% (LOD=10.35) of the total phenotypic variation in disease response in soybean Plant Introduction (PI) 209332, 52.7% (LOD=15.58) in PI 90763, 40.0% (LOD=10.50) in PI 88788, and 28.1% (LOD=6.94) in 'Peking'. Initially, the region around this major resistance locus was poorly populated with DNA markers. To increase marker density in this genomic region, first random, and later targeted, comparative mapping with RFLPs from mungbean [Vigna radiata (L.) R. Wilcz.] and common bean (Phaseolus vulgaris L.) was performed, eventually leading to one RFLP marker every 2.6 centimorgans (cM). Even with this marker density, the inability to resolve SCN disease response into discrete Mendelian categories posed a major limitation to mapping. Thus, qualitative scoring of SCN disease response was carried out in an F5∶6 recombinant inbred population derived from 'Evans'xPI 209332 using a 30% disease index cut-off for resistance. Using the computer program JoinMap, an integrated map of the region of interest was created, placing the SCN resistance locus 4.6 cM from RFLP marker B53 and 2.8 cM from Bng30. This study demonstrates how a combination of molecularmapping strategies, including comparative genome analysis, join mapping, and qualitative scoring of a quantitative trait, potentially provide the necessary tools for high-resolution mapping around a quantitative-trait locus.

Disease-Induced Natural Selection in a Diploid Host

Using for each genotype an SIR-type model of disease transmission dynamics, we describe natural selection in a continuously breeding diploid host whose disease susceptibility and resistance are carried at one locus with two alleles. The system is transformed into variables that for each disease class describe the number of individuals, the gene frequency, and the deviation from Hardy-Weinberg proportions as measured by Wright′s fixation index. An assumption of small variation in disease response among genotypes (slow selection) separates the system to first-order into three blocks. One block describes the population-wide disease dynamics, the second considers the fixation index in each class, and the third block provides the change in gene frequencies. The first two blocks settle to equilibrium at a rate determined by the population turnover time while the last block after a while is dominated by a slowly changing variable, the average gene frequency. The dynamics of the gene frequency take the usual form for a continuous time slow selection model, and this provides explicit, epidemiologically justified expressions for the genotypic fitnesses. We apply the method to other disease transmission patterns (SEI and SIS) and discuss how suitable time averages extend our results to diseases with temporally varying incidence.

Variation in Pathogenicity among Isolates of Diaporthe phaseolorum f. sp. caulivora

Monoascosporic isolates of Diaporthe phaseolorum f. sp. caulivora (DPC) from Maryland were found to exhibit a dimorphism associated with their degree of pathogenicity to soybean seedlings. Isolates that were “fast-growing” on acidified potato dextrose agar (APDA) were largely non-pathogenic to seedlings of the soybean cultivars Blackhawk and Zane, whereas “slow-growing” isolates were largely pathogenic. Pathogenicity was often accompanied by a variation in disease response among seedlings inoculated with a given isolate. Both “fast-growing” and “slow-growing” isolates recovered from inoculated seedlings often exhibited a marked change in pathogenicity when they were re-inoculated into other seedlings of the same cultivars from which they had originally been isolated. In contrast to the Maryland isolates, Alabama and Florida isolates did not exhibit a sharply defined separation into two groups based on their culture morphology. The clear association of pathogenicity with colony diameter seen in the Maryland isolates was not as evident in the Alabama isolates. The variable response observed in seedlings inoculated with the Maryland isolates, however, was also observed in seedlings inoculated with the Alabama isolates. Inoculation of Blackhawk seedlings with Alabama isolates failed to produce cankers whereas inoculation of seedlings of this cultivar with Maryland isolates produced many cankers.