W1186: Genetic Variability in the Cyst and Root-Knot Nematodes
Statement of Issues and Justification
The Need as Indicated by Stakeholders: Plant-parasitic nematodes cause an estimated 10-14% average annual yield loss among the world's major crops (Sasser and Freckman, 1987). Losses in United States major crops due to plant-parasitic nematodes are estimated to range from minimal in some localities to as high as 15% in other areas (Koenning et al., 1999; McSorley et al., 1987). Increasingly, scientific evidence and public awareness have heightened concerns about environment quality, food quality, and human health and safety relative to pest management in agricultural production. The need for alternative, integrated nematode management has been propelled by the actions triggered by the Montreal Protocol and the Food Quality Protection Act (FQPA) of the 1990s. Based on FQPA requirements, it is likely that several widely-used and efficacious nematicides will be unavailable in the future. For example, the nematicide 1,3-dichloropropene (Telone II) is a B2 carcinogen, and is being reviewed under FQPA. In addition, methyl bromide will become unavailable during the proposed project period due to its U.S. EPA phase-out of production and importation by 2005. Both locally and nationally, the agricultural production community (our stakeholders) is scrambling to find viable alternatives to chemical-based soil pathogen and nematode control. In addition, world travel and commerce have accelerated the dissemination of pest species, including plant-parasitic nematodes. The accurate identification of nematode species is beneficial and necessary for national and international regulatory and quarantine agencies relative to free trade and economics. The nematology community has repeatedly advocated the need for funding support focused on the basic and applied research required to advance alternative management approaches. The proposed project addresses these needs directly for the most important groups of plant parasitic nematodes, by building on advances made in the W-186 project over the last 10 years.Importance of, and Consequences Without, the Work: The cyst and root-knot species are the most important groups of plant-parasitic nematodes in the United States. The management of these nematodes in United States agriculture during the past four decades has been largely via the application of broadly efficacious nematicides. Nematicidal activity, especially of soil fumigants, is generally non-discriminating, even between nematode species and genera. Therefore, understanding the genetic variability among nematodes was not important for effective nematode control. In contrast, desirable alternative nematode management strategies involve combinations of rotation, host plant resistance, cultural manipulations and biological control, all of which may have specific genotype-level interactions with nematodes and are influenced by environmental conditions. Hence, the genetic variability in nematode populations must be considered to successfully develop and deploy alternative management strategies. This regional project was initiated because the membership recognized the increasing importance of characterizing the genetic variation in nematode populations and its influence on success of alternative nematode management strategies. An example highlights the value of this project: Years of research went into the development of cyst-nematode resistant soybeans but the potential benefits of the resistance were limited due to the rapid selection of resistance-breaking nematode isolates.
A dramatic shift in nematode-management strategies is occurring, from almost exclusive reliance on soil-applied nematicides, to the use of combinations of alternative strategies such as crop rotation, host plant resistance, cultural manipulations and biological control (Ferris et al., 1992). An important difference from nematicides is that the alternatives are influenced directly by genetic variability existing in target nematode field populations. Hence, the successful use of alternatives requires more information to implement than nematicide-based strategies. Herein lies the logic and raison djtre of W-186 and its proposed revision: assessment and characterization of genetic variability extant in nematode populations will assist in the successful application of alternative management approaches, in addition to guiding initial development and deployment of new strategies. For example, knowing the frequency of virulence genes in a nematode population will allow deployment of corresponding resistance genes so that the utility of the resistance genes in new cultivars is maintained over time.
Except in the clearly demonstrable instances where resistance-breaking nematode populations are detected, the subtle influence of genetic variability in nematode populations has been considered only to a limited extent. However, the research conducted under the current W-186 multistate project has provided considerable evidence that this variability is important. We hypothesize that genetic variability in nematode populations is responsible for the aberrant results of many experiments assessing resistance, crop rotations, host ranges, cover/trap cropping, and biological control. The plasticity of nematode responses to abiotic environmental factors such as temperature, moisture, and host nutrient status stems from genetic variability, and such responses are poorly characterized. Greater understanding of nematode genetic response and adaptation to abiotic factors will be important in optimizing the design of cultural management tactics such as manipulations of planting and harvest times, wet or dry fallow, and soil solarization.
Without the proposed work continuing in a coordinated manner, the participants believe that effective nematode management alternatives will be developed more slowly, with success coming more on an ad hoc basis and with economic inefficiencies and a high likelihood of short-term failure of new products or management approaches. Knowledge gained from our main focus on cyst and root-knot nematodes also will be applied and tested on other important nematode groups within the revised project (see Table 1 matrix in Attachment). This will strengthen the overall scientific scope of the research activities and will broaden the impact of the findings to benefit agriculture in multiple states.
Technical Feasibility of the Research: New molecular and genetic methodologies and knowledge will facilitate the study of nematode genetic variability at much greater resolution than has been possible. Development of interactive database programs that provide information and advisory information on combinations of nematode management tactics will facilitate the adoption of integrated nematode management. Such on-line databases and knowledge-based systems will assist in information transfer to user groups in the relevant agricultural communities.
The root-knot and cyst nematodes are distributed throughout the United States and are damaging pathogens, parasitizing a wide range of important crops. Three groups of nematodes are the primary focus for this project: Group I - The warm-temperature root-knot species (Meloidogyne incognita, M. javanica, M. arenaria); Group II - The temperate root-knot species (M. chitwoodi and M. hapla); Group III - The cyst species (Heterodera schachtii, H. cruciferae and H. glycines). These nematodes are the subject of research efforts in the designated participating states. Current research is addressing many areas of management for these three groups, including: development and deployment of nematode-resistant plants; rotation to reduce population densities of these pathogens; cover crops and trap crops to reduce population densities; characterization of resistance genes and resistance responses; and the development of biochemical and molecular diagnostics for nematode identification. Thus the project participants share a strong common interest that will provide the central focus for the project members and other collaborators. In addition, parallel studies will be made by some participants on other endoparasitic nematodes, including reniform nematode (Rotylenchulus reniformis) and lesion nematodes (Pratylenchus spp.), and important ectoparasitic nematodes, including stubby root (Paratrichodorus spp.) and dagger (Xiphinema spp.) nematodes. This will maximize both the scientific scope of the project and its multi-state impact in agriculture.
Characterizing genetic variability requisite for novel management strategies: The unifying theme of this proposal is that genetic variability is a critical biological feature of species and populations of the highly specialized plant parasitic nematodes. W-186 participants and others have begun to document the extent of genetic variability within populations, and the factors that influence it. New developments in molecular biology techniques and their application through this project will continue to increase our understanding of the genetic processes involved. As understanding of genetic variability in nematode populations increases, it has become clear that the race concept and other means of characterizing nematode population differences is inadequate. Failure of current nematode management, such as breakdown of resistance, and successful development of novel approaches can be resolved through greater understanding of the underlying genetic and biological processes in parasitic nematode populations vis a vis management. For example, the importance of mutation compared to maintained variability in field populations is unclear, and this is a research area that will be pursued.
Genetic variability can impact both the effectiveness and longevity of alternative nematode-management strategies based on host plant resistance, crop rotation, cultural manipulations and biological control. As a consequence, greater understanding of genetic variability should provide rational guidance for the design and development of management strategies. The W-186 project has focused on understanding nematode genetic variability, such that it can be identified, characterized, and managed or manipulated to benefit agricultural production systems. The underlying principles to this focus require research on the phenotypic and genotypic characterization of variability and gene frequencies, including aspects of stability and adaptability, of host range, response to resistance, response to environmental conditions, biological processes (e.g. fecundity) and morphology. This approach is being complemented and aided by development of markers to identify variability by molecular, biochemical, histochemical, and morphological polymorphisms. The development of molecular techniques with greater efficiency, predictability and ease of use will expedite the nematode genetic analyses and design of management systems. Current and previous work under W-186 has allowed participants to make advances on these research goals. However, much remains to be accomplished in this rapidly evolving research area. Accordingly, the research initiated under W-186 cannot be considered complete. Relative to our objectives, it is exciting that the arsenal of new tools used to address our applied research questions is increasing rapidly (e.g., Atkinson et al., 2001; Cai et al., 1997; Grosberg et al., 1996; Ibrahim et al., 1997; Karl and Avise, 1993; Lynch, 1996; Ouedraogo et al., 2002; Powers et al., 2001; Sambrook and Russell, 2001; Vos et al., 1995; Williams et al., 1990).
Four key considerations based on nematode genetic variability are central to development and deployment of alternative management strategies as proposed under this multistate project:
Host plant resistance The genetic composition of nematode populations is changed by the selection pressure imposed by growing resistant cultivars. The changes include shifts in species composition, and shifts in presence and frequency of nematode virulence alleles matching specific resistance genes in crop cultivars. Similar potential shifts may occur in response to nematode-resistant trap crops. Little is known of the existing frequency of virulence alleles, the frequency with which new alleles are generated, or the underlying mechanisms that regulate changes in genetic variability in root-knot and cyst nematode populations. As more sources of resistance are bred into cultivars, knowledge of gene frequency and stability effects assumes greater importance in determining the direction and requirements of breeding for nematode resistance, and the effective long-term deployment of available resistant cultivars (Starr et al., 2002).
Host range for rotations and cover-cropping - The host ranges of important nematode species have been defined within general limits, but the extent of variability in host range among populations within species is not well-characterized. Thus, although most cyst nematodes have narrow host ranges and are amenable to control by nonhost rotation programs, little is known about the extent of reported hosts outside the typical host taxa, or the likelihood of shifts in host range. For example, although sugarbeet cyst nematode hosts are found almost entirely within the Brassicaceae and Chenopodiaceae, tomato (Solanaceae) has been reported to host this nematode in California and Utah, with potentially serious consequences for rotation planning in western sugarbeet production areas. Evidence for genetic adaptations and modification of nematode host range has also been presented whereby local nematode populations are better adapted to local weed populations. The processes involved in these interactions are poorly understood.
In contrast to cyst nematodes, root-knot nematode species have broad host ranges of more than 2000 plant species from diverse plant families. Much of this host range information has been compiled from numerous tests and observations based on non-standardized host testing procedures, and in most cases with only one or a few isolates of a root-knot species. Standardized conditions are needed to determine whether differences are due to variability in nematode populations or to differences in susceptibility in the plant lines used. Resolving the true levels and stability of host range relationships will be critical to development and implementation of non-host crops in rotation and cover-cropping programs, and for determining the role that host weed species play in maintaining nematode population levels.
Cultural controls - Most are based on manipulating abiotic effects on nematode populations to suppress nematode activity or infection. Examples include wet or dry fallowing so nematodes starve while active (wet fallow) or die from extreme moisture stress (dry fallow). Soil solarization involves natural heating of soil under plastic cover to attain the thermal death point of nematodes. Avoidance may include changes in planting and harvest dates, such as delaying planting in the fall to avoid infection activity, and early planting or late harvest of crops to avoid additional nematode generations. Genetic variability in nematodes for response to temperature and moisture has been demonstrated, but it is not known how quickly or how stable such adaptive changes are, nor their frequency.
Biological controls - Some potential biological control agents of cyst and root-knot nematodes are known to have specific host ranges among target nematode species, such as the host specificity of the bacterium Pasteuria penetrans among root-knot nematode species and populations. Such specificity may be controlled genetically, through surface protein binding and recognition between bacterium and nematode, suggesting that genetic variability in Meloidogyne may influence the potential of the bacterium and similar organisms as useful biological control agents.
Advantages of a Multistate Effort: During the previous life of this project, the membership effectively initiated research to apply emrging methodologies to obtain knowledge of the genetic variability in nematode populations. The W-186 membership proposes to continue and extend our efforts in this regard, to identify and characterize the genetic variability in the most important cyst and root-knot nematode groups and other important nematodes. The participants share research interests on primary nematode pathogens and bring complementary expertise to the project. In this iteration of the project, we will address gene frequencies, genetic stability, and adaptation and fitness, such that genetic variability can be managed and manipulated in agricultural production systems by appropriate alternative management strategies. The cyst and root-knot species are of primary importance as major nematode pathogens in most agricultural production areas and cropping systems throughout the United States. This is reflected in the proposed contributions from participating states across the country. The diversity in cropping systems and rank of importance of nematode groups among participating states clearly provides opportunities for conducting meaningful collaborative research on major nematode pathogens exposed to similarities and variations in crop (host), resistance, environmental and agroecological conditions. Via this project, the participants utilize the opportunity to collaborate in ways that enhance the benefits accrued from the research, as opposed to what might be gleaned if the researchers were to simply pursue individual projects within limited geographic boundaries. For example, the cool climate root-knot species will be studied by participants from nine of the 11 participating institutions in 10 states (see Attachment Table 1 Attachment) a group effort that will pay large dividends in understanding nematode population genetics relative to management.
We believe that the similarities in the target nematode groups and the problems for nematode management imposed by genetic variability can be researched most efficiently through this coordinated multistate project. This team approach enables a pooling of scientific expertise and resources to maximize the amount and quality of the information that can be generated. The resources available to researchers working within the Agricultural Experiment Stations have been continually declining in recent years, and are particularly limited for nematology programs at this time. Conversely, the demands and expectations for new, environmentally friendly management tactics have never been greater. Accordingly, the membership has experienced a do more with less environment. This multistate project can provide some relief, as a necessary forum for rapid scientific advancement in aspects of both basic and applied research directed toward development of alternative management strategies. For example, it is unlikely that all participating states will have programs devoted to molecular research on nematodes, and yet the need for molecular-level approaches to assess genetic variability is required to address significant problems. This project has ongoing molecular-based programs in a few states (e.g. California, Hawaii, Nebraska, New Mexico) that can act to facilitate research by other participant states. In turn, those states focusing on phenotypic differences in nematode populations can provide nematode populations and isolates for the molecular scientists. This coordinated approach minimizes unnecessary duplication of research programs, and provides fertile opportunities for a seamless, interactive approach to development of integrated nematode management. Most importantly, the project also enhances the quality and applicability of the research findings across geographic locations and agricultural production systems. Likely Impacts of Work: The application of the project findings should expedite the development of new, environmentally benign management strategies to minimize economic losses from nematodes. This in turn should help boost the international competitiveness of our agricultural production systems, at a time when competitive advantage is being eroded by the loss, and potential further loss, of nematicides.
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