Fitness of Virulent Meloidogyne incognita Isolates on Susceptible and Resistant Cowpea

A study of life-history traits was made to determine factors associated with the fitness of Meloidogyne incognita isolates virulent to resistance gene Rk in cowpea. Egg hatch, root penetration, egg mass production, and fecundity (eggs per egg mass) of avirulent and virulent phenotypes were compared among M. incognita isolates, isofemale lines, and single descent lines over multiple generations on resistant and susceptible cowpea. Variation (P ≤ 0.05) in both hatch and root penetration rates was found among isolates at a given generation. However, this variation was not consistent within nematode lines among generations, and there was no correlation with level of virulence, except for penetration and virulence on resistant cowpea at generation 20. Resistant and susceptible cowpea roots were penetrated at similar levels. Differences in reproductive factors on resistant plants were correlated with levels of virulence expression. In some isofemale lines, single descent lines, and isolates, lower (P ≤ 0.05) rates of egg mass production and fecundity on susceptible cowpea were associated with virulence to Rk, indicating a trade-off between reproductive fitness and virulence. Other virulent nematode lines from the same isolates did not have reduced reproductive ability on susceptible cowpea over 27 generations. Thus, virulent lineages varied in reproductive ability on susceptible cowpea, contributing to adaptation and maintenance of virulence within M. incognita populations under stabilizing selection.     

Impact of Host Resistance on the Variability in Virulence of Xanthomonas campestris pv. oryzae

Twenty isolates of Xanthomonas campestris pv. oryzae (Ishiyama) Dye each from susceptible and resistant rice cultivars, were inoculated on the susceptible genotype IR-8 and resistant M Sungsong to measure the impact of host selection pressure on virulence change. Isolates virulent on resistant genotype tended to be less adaptive on susceptible cultivars. While isolates from susceptible genotype showed 2.2 times more general virulence (GV) than specific virulence (SV), isolates of resistant host origin had only 1.3 times GV, indicating that the resistant host plant displays considerable increase in virulence. The SV value increased 1.64 times after one cropping with resistants indicated the potential of the pathogen to change to be slow.     

Host resistance and pathogen virulence across a plant hybrid zone

Hybridization between locally adapted plant populations has been postulated to have significant evolutionary consequences, and, in particular, may influence host-pathogen interactions with respect to resistance and virulence structure. This study investigated patterns of resistance and virulence in a hybrid zone between ”bog” and ”hill” ecotypes of the native Australian flax, Linum marginale, where the host is subject to attack by the rust pathogen, Melampsora lini. Analysis of the resistance structure of adjoining bog, hill and hybrid populations found that bog plants were generally susceptible to pathogen isolates taken from all these sites, but that hybrids exhibited resistance levels similar to the more resistant hill plants. Similarly, the virulence structure of rust isolates collected from the hybrid population was more similar to that of the hill isolates than the bog. Controlled crosses between bog and hill plants showed that crosses in one direction (bog females×hill males) were much more successful than the other. A multi-year reciprocal transplant study further indicated that bog plants had significantly higher survivorship than hill plants, regardless of site. It is suggested that likelihood of differential gene flow and survivorship for bog and hill plants may at least partially explain the maintenance of a relatively narrow hybrid zone. Received: 5 October 1999 / Accepted: 1 July 1999     

Isofemale Line Analysis of Meloidogyne incognita Virulence to Cowpea Resistance Gene Rk

Isofemale lines (IFL) from single egg masses were studied for genetic variation in Meloidogyne incognita isolates avirulent and virulent to the resistance gene Rk in cowpea (Vigna unguiculata). In parental isolates cultured on susceptible and resistant cowpea, the virulent isolate contained 100% and the avirulent isolate 7% virulent lineages. Virulence was selected from the avirulent isolate within eight generations on resistant cowpea (lineage selection). In addition, virulence was selected from avirulent females (individual selection). Virulence differed (P ≤ 0.05) both within and between cohorts of IFL cultured for up to 27 generations on susceptible or resistant cowpea. Distinct virulence profiles were observed among IFL. Some remained avirulent on susceptible plants and became extinct on resistant plants; some remained virulent on resistant and susceptible plants; some changed from avirulent to virulent on resistant plants; and others changed from virulent to avirulent on susceptible plants. Also, some IFL increased in virulence on susceptible plants. Single descent lines from IFL showed similar patterns of virulence for up to six generations. These results revealed considerable genetic variation in virulence in a mitotic parthenogenetic nematode population. The frequencies of lineages with stable or changeable virulence and avirulence phenotypes determined the overall virulence potential of the population.     

Interactive effects between diet and genotypes of host and pathogen define the severity of infection

Host resistance and parasite virulence are influenced by multiple interacting factors in complex natural communities. Yet, these interactive effects are seldom studied concurrently, resulting in poor understanding of host‐pathogen‐environment dynamics. Here, we investigated how the level of opportunist pathogen virulence, strength of host immunity and the host condition manipulated via diet affect the survival of wood tiger moth Parasemia plantaginis (Arctidae). Larvae from “low cuticular melanin” and “high cuticular melanin” (considered as low and high pathogen resistance, respectively) selection lines were infected with moderately and highly virulent bacteria strains of Serratia marcescens, while simultaneously manipulating host diet (with or without antibacterial compounds). We measured host survival and food preference before and after infection to test whether the larvae “self‐medicate” by choosing an anti‐infection diet (Plantago major, i.e., plantain leaf) over lettuce (Lactuca sativa). “High melanin” larvae were more resistant than “low melanin” larvae to the less virulent strain that had slower growth and colonization rate compared with the more virulent strain. Cuticular melanin did not enhance survival when the larvae were infected with the highly virulent strain. Anti‐infection diet enhanced survival of the “high melanin” but not the “low melanin” hosts. Survival was dependent on family origin even within the melanin selection lines. Despite the intrinsic preference for lettuce, no evidence of self‐medication was found. These results demonstrate that the relative benefit of host cuticular melanin depends on both diet and pathogen virulence: plantain diet only boosted the immunity of already resistant “high melanin” hosts, and cuticular melanin increased host survival only when infected with moderately virulent pathogen. Moreover, there was considerable variation in host survival between families within both melanin lines suggesting genetic basis for resistance. These results indicate that although melanin is an important predictor of insect immunity, its effect on disease outcomes greatly depends on other interacting factors.     

葡萄生轴霜霉菌孢子囊的长期保存

为了解决葡萄生轴霜霉菌的活体保存问题,本文开展了多种保藏方法的研究。采用二甲基亚砜和甘油分别于不同低温条件下对孢子囊进行了保存,取出后进行接种试验。结果表明,孢子囊用不同保护剂处理后保存于-76℃中5d,10%二甲基亚砜处理的孢子囊经接种后发病面积百分率为68.3%,其致病性明显强于25%甘油处理;孢子囊用不同浓度的二甲基亚砜保存于-76℃中30d,5%和10%二甲基亚砜处理后致病性差异不显著,但明显高于15%、20%、25%二甲基亚砜处理的孢子囊;5%和10%二甲基亚砜处理的孢子囊,在-76℃中保存5d和15d后,致病性明显高于-20℃的处理;孢子囊经-76℃保存240d后,10%二甲基亚砜处理的孢子囊无致病性,而5%二甲基亚砜处理的发病面积百分率为56.3%,与对照无显著差异,说明孢子囊用含5%的二甲基亚砜处理能在-76℃中长期保存。     

Necrotic Lesions as Unusual Symptoms of Ring Rot in the Potato Leaves

Test-tube plants and suspension cell cultures of two cultivars of the potato (Solanum tuberosum L.) differing in their resistance to ring rot caused by Clavibacter michiganensis subsp. sepedonicus and six strains of this bacterium were used to test the relationship between the virulence, the leaf ability to adsorb bacteria, and the symptoms of the disease. In addition to chlorosis and drying, heavy inoculation with virulent strains caused unusual symptoms, such as leaf necrotic lesions. In the resistant cultivar, the necrotic lesions were predominantly local, whereas in the susceptible cultivar, they expanded. Unlike the susceptible cultivar, suspension cells of the resistant cultivar weakly adhered bacteria of the tested strains. Bacteria entered the plants through the leaf stomata. The sorption and penetration were much more pronounced in the susceptible cultivar. It was concluded that strain virulence varies depending on the conditions of inoculation, and uncharacteristic symptoms (necrotic lesions) arise. The local necrotic lesions are considered a hypersensitive response, and exopolysaccharides of the pathogen as the factors of virulence.     

Heterozygosis and pathogenicity of Cryptococcus neoformans AD-hybrid isolates

Nineteen Cryptococcus neoformans AD-hybrid isolates were investigated to assess whether hybrid genomic background could affect virulence in a mouse model. The level of heterozygosity of each strain was analyzed using primers specific for allele A and D of 15 polymorphic genes. Virulence was tested in a mouse model of systemic infection by measuring time of survival. In addition, the putative virulence attributes, melanin, phospholipase, and capsule production, as well as growth at 39°C and UV sensitivity were investigated. Eight strains showed to be heterozygous in up to 70% of loci, other eight strains were heterozygous in less than 60% of loci, while the remaining three strains were homozygous at all tested loci. Mice infected with hybrids with a high percentage of heterozygosis showed significantly (P?相似文献   

Virulence and fitness of the fungal pathogen Entomophaga maimaiga in its host Lymantria dispar, for pathogen and host strains originating from Asia, Europe, and North America

In this study, we tested (1) whether non-North American gypsy moth strains are susceptible to North American isolates of Entomophaga maimaiga and (2) the potential for erosion in the efficacy of E. maimaiga in controlling gypsy moth. We used bioassays to assess the variability in virulence (measured as time to death) as well as fitness of the pathogen (measured as spore production) in four gypsy strains challenged with six E. maimaiga isolates, using host and pathogen strains originating from Asia, Europe, and North America. We found that all E. maimaiga isolates tested were pathogenic to all strains of Lymantria dispar, regardless of the geographical origin of the fungal isolate, with at least 86% mortality for all combinations of fungal isolate and gypsy moth strain. We therefore conclude that Asian gypsy moths are susceptible to North American strains of E. maimaiga. No significant interactions between fungal isolates and gypsy moth strains with regard to time to death were found, indicating that each fungal isolate had the same overall effect on all the gypsy moth strains tested. However, fungal isolates differed significantly with regard to virulence, with a Russian isolate being the slowest to kill gypsy moth (5.1+/-0.1 days) and a Japanese isolate being the overall fastest to kill its host (4.0+/-0.1 days). Fungal isolates also differed in fitness, with variability in types of spores produced. These differences in virulence and fitness were, however, not correlated with geographical origin of the fungal isolate. Gypsy moth strains had no or only little effect on fungal virulence and fitness. Based on our studies with laboratory-reared gypsy moth strains, erosion of successful control of gypsy moth by E. maimaiga seems unlikely.     

The host guides morphogenesis and stomatal targeting in the grapevine pathogen <Emphasis Type="Italic">Plasmopara viticola</Emphasis>

The oomycete grape downy mildew (Plasmopara viticola Berk. & Curt. Ex de Bary) is a serious pathogen of grapevine and spreads by extremely efficient cycles of asexual propagation. The high efficiency must involve efficient sensing of the host. We therefore analyzed the time course and morphology of the early development of this pathogen in a host system, by infection of leaf discs of grapevine (Vitis vinifera L. cv. Müller-Thurgau), and in a host-free system. Host factors were demonstrated to influence pathogen development in the following ways: (i) the release of zoospores from mature sporangia was accelerated, (ii) the morphogenesis of the germ tube was coordinated, and (iii) the zoospores were targeted to the stomata by factors that depended on stomata closure. The findings show that the early development of P. viticola is regulated, specifically and coordinately, by factors originating from the host plant.     

Changes in Peroxidase Activity during Potato Ring Rot Infection

The effects of the ring rot causal agent Clavibacter michiganensis subsp. sepedonicus (a virulent strain 5369) on the peroxidase activity of various tissues of potato plants grown under axenic conditions were studied. Root infection enhanced peroxidase activity in all plant tissues (roots, leaves, and stems). In the resistant cv. Lugovskoi, peroxidase activity was much higher than in the susceptible cv. Luk'yanovskii. Co-culturing of the suspension cells of these potato cultivars with the bacterial pathogen also activated peroxidase in the cells of the resistant cultivar; in the cells of the susceptible cultivar, peroxidase activation was less pronounced. Treating suspension cell with exopolysaccharides secreted by the pathogen enhanced the activity of extra- and intracellular peroxidases, and the degree of this enhancement differed in the two potato cultivars.     

Appartenance Raciale et Pathogenie Comparée de Quelques Isolats de Verticillium dabliae (Kleb.) Obtenus a Partir de Tomateś Résistantes au Maroc

Pathogenecity and race classification of some isolates of V. dahliae from resistant tomato in Morocco In Morocco, the cv. H 204, resistant to Verticillium dahliae race 1 is widely used under plastic tunnels. Use of this cultivar controlled the disease for many years but recently a high incidence of Verticillium on this hybird was obsereved. V. dahliae was isolated from 90 % of the wilted plants.The other 10 % were infected by F. oxysporum f. sp. lycopersici alone (7 %) or in association with V dabliae (3 %). The inoculation of the Verticillium susceptible cv. Vemone and of the race 1 resistant cv. H 204 by 120 isolates obtained from the resistant hybird, showed that (69 %) of the isolates are race 1 and 31 % are race 2. There are different virulence levels of V. dahliae race 2 both on the susceptible cv. Vemone and on the race 1 resistant cv. H 204. Race 2 on the average was less virulenton the susceptible than on the resistant cultivar.     

Adaptation to resistant hosts increases fitness on susceptible hosts in the plant parasitic nematode Globodera pallida

Trade‐offs between virulence (defined as the ability to infect a resistant host) and life‐history traits are of particular interest in plant pathogens for durable management of plant resistances. Adaptation to plant resistances (i.e., virulence acquisition) is indeed expected to be associated with a fitness cost on susceptible hosts. Here, we investigated whether life‐history traits involved in the fitness of the potato cyst nematode Globodera pallida are affected in a virulent lineage compared to an avirulent one. Both lineages were obtained from the same natural population through experimental evolution on resistant and susceptible hosts, respectively. Unexpectedly, we found that virulent lineages were more fit than avirulent lineages on susceptible hosts: they produced bigger cysts, containing more larvae and hatching faster. We thus discuss possible reasons explaining why virulence did not spread into natural G. pallida populations.     

Rearing history affects behaviour and performance of two virulent Nasonovia ribisnigri populations on two lettuce cultivars

Many aphid species have become virulent to host‐plant resistance, which limits the sustainability of insect resistance breeding. However, when this adaptation to resistant plants is associated with fitness costs for the aphids, virulence can be lost in the absence of resistant plants. For two populations of the lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae), we evaluated whether virulence to Nr‐gene‐based resistance was lost on a susceptible lettuce, Lactuca sativa L. (Asteraceae), and assessed possible costs of virulence. The feeding behaviour and performance of these aphids, reared and tested on susceptible and resistant lettuce, were investigated. The rearing plant affected feeding behaviour and performance of the aphids. Temporary reduction and long‐term loss of virulence were found. The total duration of phloem intake was shorter after being reared on susceptible lettuce and tested on resistant lettuce. In addition, one population had a lower survival on resistant lettuce after being reared on susceptible lettuce. There were also indications of fitness costs of the virulence in both populations.     

Epidemiology and disease-control under gene-for-gene plant-pathogen interaction

An introduction of disease-resistant variety of a crop plant often leads to the development of a virulent race in pathogen species that restores the pathogenicity to the resistant crop. This often makes disease control of crop plants extremely difficult. In this paper, we theoretically explore the optimal 'multiline' control, which makes use of several different resistant varieties, that minimizes the expected degree of crop damages caused by epidemic outbreaks of the pathogen. We examine both single-locus and two-locus gene-for-gene (GFG) systems for the compatibility relationship between host genotypes and pathogen genotypes, in which host haplotype has either susceptible or resistant allele in each resistance locus, and the pathogen haplotype has either avirulent or virulent allele in the corresponding virulence locus. We then study the optimal planting strategy of host resistant genotypes based on standard epidemiological dynamics with pathogen spore stages. The most striking result of our single-locus GFG model is that there exists an intermediate optimum mixing ratio for the susceptible and resistant crops that maximizes the final yield, in spite of the fact that the susceptible crop has no use to fight against either avirulent or virulent race of the pathogen. The intermediate mixture is optimum except when the initial pathogen spore population in the season consists exclusively of the virulent race. The optimal proportion of resistant crops is approximately 1/R(0), where R(0) is the basic reproductive ratio of pathogen--the rest (the vast majority if R(0) is large) of crops should be the susceptible genotype. By mixing susceptible and resistant crops, we can force the pathogen races to compete with each other for their available hosts. This competition between avirulent and virulent races prevents the fatal outbreak of the virulent race (the super-race) that can infect all the host genotypes. In the two-locus GFG control, there again exists the optimal mixing ratio for the fraction of universally susceptible genotype and the total fraction of various resistant genotypes, with the ratio close to 1/R(0).     

Genotype variation and capsular serotypes of Porphyromonas gingivalis from chronic periodontitis and periodontal abscesses

Porphyromonas gingivalis is considered an important pathogen in periodontal disease. While this organism expresses a number of virulence factors, no study combining different virulence polymorphisms has, so far, been conducted. The occurrence of combined virulence (Cv) genotypes in 62 isolates of P. gingivalis was investigated from subjects displaying either chronic periodontitis or periodontal abscess. The Cv genotypes, based on gene variation of fimbriae (fimA), Lys-specific cystein proteinase (kgp) and Arg-specific cystein proteinase (prpR1/rgpA), were evaluated by PCR. The isolates were also subjected to capsular polysaccharide K-serotyping. A total of 18 Cv genotype variants based on fimA: kgp: rgpA were identified, of which II:I:A and II:II:A Cv genotypes (53.3%) were the two most frequently detected combinations. Moreover, 36% of the isolates were K-typeable, with the K6 serotype being the most prevalent (23%). Two isolates had the same genotype as the virulent strain W83. The results indicate that chronic periodontitis is not associated with a particularly virulent clonal type. A highly virulent genotype (e.g. strain W83) of P. gingivalis can be found in certain periodontitis patients.     

Selection and parasite evolution: a reproductive fitness cost associated with virulence in the parthenogenetic nematode <Emphasis Type="Italic">Meloidogyne incognita</Emphasis>

Selection in plant parasites for virulence on resistant hosts and the resulting effects on parasite fitness may be considered as a driving force in host-parasite coevolution. In the present study, we tested the hypothesis that a fitness cost may be associated with nematode virulence, using the interaction between the parthenogenetic species Meloidogyne incognita and tomato as a model system. The reproductive parameters of near-isogenic lines of the nematode, selected for avirulence or virulence against the tomato Mi resistance gene, were analysed and combined into a reproductive index that was taken as a measure of fitness. The lower fitness of the virulent lines on the susceptible tomato cultivar showed for the first time that a measurable fitness cost is associated with unnecessary virulence in the nematode. Although parthenogenesis should theoretically lead to little genetic variability, such cost may impose a direct constraint on the coevolution between the plant and the nematode populations, and suggests an adaptive significance of trade-offs between selected characters and fitness-related traits. These results indicate that, although plant resistance can be broken, it might prove durable in some conditions if the virulent nematodes are counterselected in susceptible plants, which could have important consequences for the management of resistant cultivars in the field.     

Serial infection of diverse host (Mus) genotypes rapidly impedes pathogen fitness and virulence

Reduced genetic variation among hosts may favour the emergence of virulent infectious diseases by enhancing pathogen replication and its associated virulence due to adaptation to a limited set of host genotypes. Here, we test this hypothesis using experimental evolution of a mouse-specific retroviral pathogen, Friend virus (FV) complex. We demonstrate rapid fitness (i.e. viral titre) and virulence increases when FV complex serially infects a series of inbred mice representing the same genotype, but not when infecting a diverse array of inbred mouse strains modelling the diversity in natural host populations. Additionally, a single infection of a different host genotype was sufficient to constrain the emergence of a high fitness/high virulence FV complex phenotype in these experiments. The potent inhibition of viral fitness and virulence was associated with an observed loss of the defective retroviral genome (spleen focus-forming virus), whose presence exacerbates infection and drives disease in susceptible mice. Results from our experiments provide an important first step in understanding how genetic variation among vertebrate hosts influences pathogen evolution and suggests that serial exposure to different genotypes within a single host species may act as a constraint on pathogen adaptation that prohibits the emergence of more virulent infections. From a practical perspective, these results have implications for low-diversity host populations such as endangered species and domestic animals.     

Carbohydrates and resistance to <Emphasis Type="Italic">Phytophthora infestans</Emphasis> in potato plants

Plants generally deal with biotic or abiotic stresses by altering components as for example cell wall constituents and metabolites. Infection by Phytophthora infestans, the causal agent of late blight, constitutes a stress condition for the plants and they react to it with changes arising in their metabolism depending on the resistance level of the plants. The present work compares two potato hybrids differing in their level of horizontal resistance to late blight. Carbohydrate content in stems and leaves of infected and uninfected plants was determined by HPLC. Some carbohydrates accumulated in the stems of the resistant hybrid infected by P. infestans, whereas they remained unchanged in the susceptible hybrid. On the other hand, in the leaves, these carbohydrates accumulated only in the infected susceptible hybrid.     

Resistance of Polish lines and hybrids of watermelon [Citrullus lanatus (Thunb.) Matsum et Nakai] to Fusarium oxysporum at the seedling stage

Watermelon is a species cultivated in the hot climate or in the greenhouse. Since recently it has also started to be grown in the open in the Polish climate. This species is frequently at risk of Fusarium oxysporum infection. Between 1996 and 1997 ten inbred lines and nine hybrids of Polish origin were tested for resistance to this pathogen. The test was conducted with the use of four isolates of F. oxysporum: three from Polish infected plants (formae speciales not determined), while the fourth from U.K. (F. oxysporum f. sp. niveum). In the three series of tests the control plants were Pannonia F(1) and Sugar Baby. No inbred line or hybrid was found to be highly resistant to the pathogen. However, it was possible to identify four lines and five hybrids showing a higher level of resistance as compared with the control. The level of hybrid resistance was determined by comparison with the parental genotypes.