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.     

Differential expression of root-knot nematode resistance genes in tomato and pepper: evidence with Meloidogyne incognita virulent and avirulent near-isogenic lineages

Reproduction of artificially selected near isogenic Meloidogyne incognita lineages virulent and avirulent against the Mi resistance gene of tomato was assessed on host and resistant lines and cultivars of pepper. Egg mass production following inoculation of individual potted seedlings with second-stage juveniles was studied in experiments conducted in controlled environment. Artificially selected Mi-virulent nematode populations were unable to develop on resistant pepper lines PM 217 and PM 687. This suggests that the genetic systems governing resistance to root-knot nematodes are differently expressed in tomato and pepper, in spite of the very close phylogenetic relationships and structural genomic homologies occurring between these two vegetable crops. Moreover, these artificially selected nematode populations were also found unable to develop on the susceptible pepper cultivars California Wonder and Doux Long des Landes, while their pathogenicity was not significantly affected on susceptible tomatoes. Due to the existence of naturally virulent Meloidogyne populations, these results enhance the need for a better understanding of the mechanisms involved, in order to develop new forms of management of plant resistance to root-knot nematodes.     

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.     

Tracking changes in life‐history traits related to unnecessary virulence in a plant‐parasitic nematode

Evaluating trade‐offs in life‐history traits of plant pathogens is essential to understand the evolution and epidemiology of diseases. In particular, virulence costs when the corresponding host resistance gene is lacking play a major role in the adaptive biology of pathogens and contribute to the maintenance of their genetic diversity. Here, we investigated whether life‐history traits directly linked to the establishment of plant–nematode interactions, that is, ability to locate and move toward the roots of the host plant, and to invade roots and develop into mature females, are affected in Meloidogyne incognita lines virulent against the tomato Mi‐1.2 resistance gene. Virulent and avirulent near‐isogenic lines only differing in their capacity to reproduce or not on resistant tomatoes were compared in single inoculation or pairwise competition experiments. Data highlighted (1) a global lack of trade‐off in traits associated with unnecessary virulence with respect to the nematode ability to successfully infest plant roots and (2) variability in these traits when the genetic background of the nematode is considered irrespective of its (a)virulence status. These data suggest that the variation detected here is independent from the adaptation of M. incognita to host resistance, but rather reflects some genetic polymorphism in this asexual organism.     

Effects of antagonistic coevolution on parasite‐mediated host coexistence

Parasites can promote diversity by mediating coexistence between a poorer and superior competitor, if the superior competitor is more susceptible to parasitism. However, hosts and parasites frequently undergo antagonistic coevolution. This process may result in the accumulation of pleiotropic fitness costs associated with host resistance, and could breakdown coexistence. We experimentally investigated parasite‐mediated coexistence of two genotypes of the bacterium Pseudomonas fluorescens, where one genotype underwent coevolution with a parasite (a virulent bacteriophage), whereas the other genotype was resistant to the evolving phages at all time points, but a poorer competitor. In the absence of phages, the resistant genotype was rapidly driven extinct in all populations. In the presence of the phages, the resistant genotype persisted in four of six populations and eventually reached higher frequencies than the sensitive genotype. The coevolving genotype showed a reduction in the growth rate, consistent with a cost of resistance, which may be responsible for a decline in its relative fitness. These results demonstrate that the stability of parasite‐mediated coexistence of resistant and susceptible species or genotypes is likely to be affected if parasites and susceptible hosts coevolve.     

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.     

Reproduction of Virulent Isolates of Meloidogyne incognita on Susceptible and Mi-resistant Tomato

The reproductive potential of natural and laboratory-selected Meloidogyne incognita isolates virulent against the tomato Mi resistance gene, all derived from a single egg-mass, were compared when the nematodes were inoculated on susceptible and resistant tomato. Fewer second-stage juveniles (P = 0.01) of the two virulent populations selected under laboratory conditions matured to females on the resistant tomato compared to the susceptible cultivar. In contrast, no differences were found between the number of egg masses produced on the resistant versus the susceptible tomato by the two natural virulent isolates. No clear general trends concerning the fecundity of the females could be inferred from the comparative analysis of the numbers of eggs per egg mass x tomato cultivar combination. These observations suggested that the genetic changes induced under environmentally controlled nematode growth might be different from those occurring in natural Mi-resistance breaking biotypes grown without environmental control.     

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.     

The Effects of Root-knot Nematode Infection and Mi-mediated Nematode Resistance in Tomato on Plant Fitness

The Mi-1.2 resistance gene in tomato (Solanum lycopersicum) confers resistance against several species of root-knot nematodes (Meloidogyne spp.). This study examined the impact of M. javanica on the reproductive fitness of near-isogenic tomato cultivars with and without Mi-1.2 under field and greenhouse conditions. Surprisingly, neither nematode inoculation or host plant resistance impacted the yield of mature fruits in field microplots (inoculum=8,000 eggs/plant), or fruit or seed production in a follow-up greenhouse bioassay conducted with a higher inoculum level (20,000 eggs/plant). However, under heavy nematode pressure (200,000 eggs/plant), greenhouse-grown plants carrying Mi-1.2 had more than ten-fold greater fruit production than susceptible plants and nearly forty-fold greater estimated lifetime seed production, confirming prior reports of the benefits of Mi-1.2. In all cases Mi-mediated resistance significantly reduced nematode reproduction. These results indicated that tomato can utilize tolerance mechanisms to compensate for moderate levels of nematode infection, but that the Mi-1.2 resistance gene confers a dramatic fitness benefit under heavy nematode pressure. No significant cost of resistance was detected in the absence of nematode infection.     

Feeding behaviour and performance of different populations of the black currant‐lettuce aphid,Nasonovia ribisnigri,on resistant and susceptible lettuce

When crops are bred for resistance to herbivores, these herbivores are under strong selection pressure to overcome this resistance, which may result in the emergence of virulent biotypes. This is a growing problem for crop species attacked by aphids. The Nr‐gene in lettuce confers near‐complete resistance against the black currant‐lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). Since 2007, populations of N. ribisnigri have been reported in several locations in Europe to infest resistant lettuce varieties that possess the Nr‐gene. The objective of this study was to analyse the behaviour and level of virulence of several N. ribisnigri populations observed to have colonized Nr‐locus‐containing lettuce lines. We analysed the stylet penetration and feeding behaviour, and the performance of these N. ribisnigri populations on resistant and susceptible lettuce lines. Large variation in the degree of virulence to the Nr‐locus‐containing lettuce lines was found among populations of the Nr:1 biotype. The German population was highly virulent on the Nr‐containing resistant lettuce lines, and showed similar feeding behaviour and performance on both the susceptible and resistant lettuces. The French population from Paris was the second most virulent, though reproduction on the resistant lines was reduced. The French population from Perpignan and a population from Belgium, however, showed reduced performance and feeding rate on the resistant compared to the susceptible lettuces. The lettuce background in which the Nr‐gene is expressed influences the level of resistance to the various Nr:1 aphid populations, because the performance and feeding behaviour differed between the aphids on the cultivars (romaine lettuce) compared to the near‐isogenic lines (butterhead/iceberg lettuce). This study also shows that being able to feed on a plant not automatically implies that a population can successfully develop on that plant, because aphids showed phloem ingestion during the 8‐h recording period on resistant lettuce, but were not able to survive and reproduce on the same lettuce line.     

<Emphasis Type="Italic">CaMi</Emphasis>, a root-knot nematode resistance gene from hot pepper (<Emphasis Type="Italic">Capsium annuum</Emphasis> L.) confers nematode resistance in tomato

Several root-knot nematode (Meloidogyne spp.) resistance genes have been discovered in different pepper (Capsium annuum L.) lines; however, none of them has yet been cloned. In this study, a candidate root-knot nematode resistance gene (designated as CaMi) was isolated from the resistant pepper line PR 205 by degenerate PCR amplification combined with the RACE technique. Expression profiling analysis revealed that this gene was highly expressed in roots, leaves, and flowers and expressed at a lower level in stems and was not detectable in fruits. To verify the function of CaMi, a sense vector containing the genomic DNA spanning the full coding region of CaMi was constructed and transferred into root-knot nematode susceptible tomato plants. Sixteen transgenic plants carrying one to five copies of T-DNA inserts were generated from two nematode susceptible tomato cultivars. RT-PCR analysis revealed that the expression levels of CaMi gene varied in different transgenic plants. Nematode assays showed that the resistance to root-knot nematodes was significantly improved in some transgenic lines compared to untransformed susceptible plants, and that the resistance was inheritable. Ultrastructure analysis showed that nematodes led to the formation of galls or root knots in the susceptible lines while in the resistant transgenic plants, the CaMi gene triggered a hypersensitive response (HR) as well as many necrotic cells around nematodes. Rugang Chen and Hanxia Li are contributed equally to this work.     

Effects of the Mi-1 and the N root-knot nematode-resistance gene on infection and reproduction of Meloidogyne enterolobii on tomato and pepper cultivars

Meloidogyne enterolobii is widely considered to be an aggressive root-knot nematode species that is able to reproduce on root-knot nematode-resistant tomato and pepper cultivars. In greenhouse experiments, M. enterolobii isolates 1 and 2 from Switzerland were able to reproduce on tomato cultivars carrying the Mi-1 resistance gene as well as an N-carrying pepper cultivar. Reproduction factors (Rf) ranged between 12 and 109 depending on the plant cultivar, with M. enterolobii isolate 2 being more virulent when compared to isolate 1. In contrast, M. arenaria completely failed to reproduce on these resistant tomato and pepper cultivars. Although some variability in virulence and effectiveness of root-knot nematode-resistance genes was detected, none of the plant cultivars showed Rf values less than 1 or less than 10% of the reproduction observed on the susceptible cv. 'Moneymaker' (Rf = 23-44) used to characterize resistance. The ability of M. enterolobii to overcome the resistance of tomato and pepper carrying the Mi-1 and the N gene makes it difficult to manage this root-knot nematode species, particularly in organic farming systems where chemical control is not an option.     

A SNARE-Like Protein and Biotin Are Implicated in Soybean Cyst Nematode Virulence

Phytoparasitic nematodes that are able to infect and reproduce on plants that are considered resistant are referred to as virulent. The mechanism(s) that virulent nematodes employ to evade or suppress host plant defenses are not well understood. Here we report the use of a genetic strategy (allelic imbalance analysis) to associate single nucleotide polymorphisms (SNPs) with nematode virulence genes in Heterodera glycines, the soybean cyst nematode (SCN). To accomplish this analysis, a custom SCN SNP array was developed and used to genotype SCN F3-derived populations grown on resistant and susceptible soybean plants. Three SNPs reproducibly showed allele imbalances between nematodes grown on resistant and susceptible plants. Two candidate SCN virulence genes that were tightly linked to the SNPs were identified. One SCN gene encoded biotin synthase (HgBioB), and the other encoded a bacterial-like protein containing a putative SNARE domain (HgSLP-1). The two genes mapped to two different linkage groups. HgBioB contained sequence polymorphisms between avirulent and virulent nematodes. However, the gene encoding HgSLP-1 had reduced copy number in virulent nematode populations and appears to produce multiple forms of the protein via intron retention and alternative splicing. We show that HgSLP-1 is an esophageal-gland protein that is secreted by the nematode during plant parasitism. Furthermore, in bacterial co-expression experiments, HgSLP-1 co-purified with the SCN resistance protein Rhg1 α-SNAP, suggesting that these two proteins physically interact. Collectively our data suggest that multiple SCN genes are involved in SCN virulence, and that HgSLP-1 may function as an avirulence protein and when absent it helps SCN evade host defenses.     

An examination of fitness costs of glyphosate resistance in the common morning glory,Ipomoea purpurea

Fitness costs are frequently invoked to explain the presence of genetic variation underlying plant defense across many types of damaging agents. Despite the expectation that costs of resistance are prevalent, however, they have been difficult to detect in nature. To examine the potential that resistance confers a fitness cost, we examined the survival and fitness of genetic lines of the common morning glory, Ipomoea purpurea, that diverged in the level of resistance to the herbicide glyphosate. We planted a large field experiment and assessed survival following herbicide application as well as fitness of the divergent selection lines in the absence of the herbicide. We found that genetic lines selected for increased resistance exhibited lower death compared to control and susceptible lines in the presence of the herbicide, but no evidence that resistant lines produced fewer seeds in the absence of herbicide. However, susceptible lines produced more viable seeds than resistant or control lines, providing some evidence of a fitness cost in terms of seed germination, and thus potential empirical support for the expectation of trait trade‐offs as a consequence of adaptation to novel environments.     

Plants expressing the Pto disease resistance gene confer resistance to recombinant PVX containing the avirulence gene AvrPto

Elicitation of hypersensitive cell death and induction of plant disease resistance by Pseudomonas syringae pv. tomato (Pst) is dependent on activity of the Pst Hrp secretion system and the gene-for-gene interaction between the tomato resistance gene Pto and the bacterial avirulence gene avrPto. AvrPto was expressed transiently in resistant or susceptible plant lines via a potato virus X (PVX) vector. We found that while PVX is normally virulent on tomato, a PVX derivative expressing avrPto was only capable of infecting plants lacking a functional Pto resistance pathway. Mutations in either the Pto or Prf genes allowed systemic spread of the recombinant virus. These results indicate that recognition of AvrPto by Pto in resistant plant lines triggers a plant defense response that can confer resistance to a viral as well as a bacterial pathogen.     

Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem

Abstract Virulence is thought to be a driving force in host–pathogen coevolution. Theoretical models suggest that virulence is an unavoidable consequence of pathogens evolving towards a high rate of intrahost reproduction. These models predict a positive correlation between the reproductive fitness of a pathogen and its level of virulence. Theoretical models also suggest that the demography and genetic structure of a host population can influence the evolution of virulence. If evolution occurs faster in pathogen populations than in host populations, the predicted result is local adaptation of the pathogen population. In our studies, we used a combination of molecular and physiological markers to test these hypotheses in an agricultural system. We isolated five strains of the fungal pathogen Mycosphaerella graminicola from each of two wheat cultivars that differed in their level of resistance to this pathogen. Each of the 10 fungal strains had distinct genotypes as indicated by different DNA fingerprints. These fungal strains were re‐inoculated onto the same two host cultivars in a field experiment and their genotype frequencies were monitored over several generations of asexual reproduction. We also measured the virulence of these 10 fungal strains and correlated it to the reproductive fitness of each fungal strain. We found that host genotypes had a strong impact on the dynamics of the pathogen populations. The pathogen population collected from the moderately resistant cultivar Madsen showed greater stability, higher genotype diversity, and smaller selection coefficients than the pathogen populations collected from the susceptible cultivar Stephens or a mixture of the two host cultivars. The pathogen collection from the mixed host population was midway between the two pure lines for most parameters measured. Our results also revealed that the measures of reproductive fitness and virulence of a pathogen strain were not always correlated. The pathogen strains varied in their patterns of local adaptation, ranging from locally adapted to locally maladapted.     

Gain of virulence by Soybean mosaic virus on Rsv4‐genotype soybeans is associated with a relative fitness loss in a susceptible host

‘Gene‐for‐gene’ theory predicts that gain of virulence by an avirulent pathogen on plants expressing resistance (R) genes is associated with fitness loss in susceptible hosts. However, the validity of this prediction has been studied in only a few plant viral pathosystems. In this study, the Soybean mosaic virus (SMV)–Rsv4 pathosystem was exploited to test this prediction. In Rsv4‐genotype soybeans, P3 of avirulent SMV strains provokes an as yet uncharacterized resistance mechanism that restricts the invading virus to the inoculated leaves. A single amino acid substitution in P3 functionally converts an avirulent to a virulent strain, suggesting that the genetic composition of P3 plays a crucial role in virulence on Rsv4‐genotype soybeans. In this study, we examined the impact of gain of virulence mutation(s) on the fitness of virulent variants derived from three avirulent SMV strains in a soybean genotype lacking the Rsv4 gene. Our data demonstrate that gain of virulence mutation(s) by all avirulent viruses on Rsv4‐genotype soybean is associated with a relative fitness loss in a susceptible host. The implications of this finding on the durable deployment of the Rsv4 gene in soybean are discussed.     

Fitness costs associated with Cry1F resistance in the European corn borer

Crops producing insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are widely planted to manage insect pests. Bt crops can provide an effective tool for pest management; however, the evolution of Bt resistance can diminish this benefit. The European corn borer, Ostrinia nubilalis Hübner, is a significant pest of maize and is widely managed with Bt maize in the Midwest of the United States. When Bt crops are grown in conjunction with non‐Bt refuges, fitness costs of Bt resistance can delay the evolution of resistance. Importantly, fitness costs often vary with ecological factors, including host‐plant genotype and diapause. In this study, we examined fitness costs associated with Cry1F resistance in O. nubilalis when insects were reared on three maize lines. Fitness costs were tested in two experiments. One experiment assessed the fitness costs when Cry1F‐resistant and Cry1F‐susceptible insects were reared on plants as larvae and experienced diapause. The second experiment tested resistant, susceptible and F1 heterozygotes that were reared on plants but did not experience diapause. Despite some evidence of greater adult longevity for Cry1F‐resistant insects, these insects produced fewer fertile eggs than Cry1F‐susceptible insects, and this occurred independent of diapause. Reduced fecundity was not detected among heterozygous individuals, which indicated that this fitness cost was recessive. Additionally, maize lines did not affect the magnitude of this fitness cost. The lower fitness of Cry1F‐resistant O. nubilalis may contribute to the maintenance of Cry1F susceptibility in field populations more than a decade after Cry1F maize was commercialized.     

Field experiments with spring barley resistant to cereal cyst nematode, 1965–1968

Two bulk populations of spring barley lines differing in respect of a single dominant gene for cereal cyst nematode (Heterodera avenae) resistance were used in trials over 4 years to assess the effect of the nematode on grain yield. On an infested site the resistant lines consistently and significantly out-yielded the susceptible lines by an average of 9 % over the 4 years. On non-infested sites, there was no difference in yield between the resistant and susceptible lines. To measure changes in the cereal cyst nematode population under continuous barley cultivation, the resistant and susceptible lines were each sown at the infested site on the same plots for 4 consecutive years. The cereal cyst nematode population declined under both susceptible and resistant barley, but more rapidly under the latter. Migratory nematodes, mostly Pratylenchus minyus, were latterly prevalent on all plots. There was no detectable change in the pathogenicity of the cereal cyst nematode population after 3 years of growing resistant barley.     

Genetic variation in natural populations of the cereal cyst nematode (Heterodera avenae Woll.) submitted to resistant and susceptible cultivars of cereals

The purpose of the present work was to study the genetic characteristics of cereal cyst nematode (Heterodera avenae) populations re-established after the long-term use of resistant oat cultivars in field conditions. Population features were analyzed through fitness components and variation in enzymatic polymorphism (esterase and malate dehydrogenase loci). The longest (6 year) use of the same resistance genes (oat cv Panema) at high frequency (Rotation IB) led to the selection of a resistance-breaking pathotype and to a decrease in viability which suggested either a founder effect or a lower reproductive potential for the new pathotype. Analysis of esterase allelic frequencies led to the conclusions that: (1) the genetic constitution of this pathotype was different from the reference population maintained on the susceptible host (oat cv Peniarth), and (2) that the esterase locus may develop a disequilibrium linkage with loci involved in virulence. Random mating was recorded at the whole-field level but not always at the single-plant level, suggesting that cultivation practices such as annual ploughing could play a major role in homogenizing subpopulations developed in the vicinity of a plant. These phenomena showed that the long-term use of highly effective resistance could provok marked genetic modifications in populations. These risks should be taken into account when deciding strategies for optimal use of resistance genes in nematode management programs.

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