David L. Trudgill

Utility of nematode community analysis as an integrated measure of the functional state of soils: perspectives and challenges

Soil nematode communities have the potential to provide unique insights into many aspects of soil processes. Since most nematodes are active in soil throughout the year, they can potentially provide a holistic measure of the biotic and functional status of soils. In contrast to other soil microbial groups, representative samples of soil nematode communities are relatively easy to obtain. However, most current nematode ecological information has been survey-based or purely observational in nature, with a persistent focus on detailed taxonomic analysis of nematode communities. The development of a Maturity Index, MI, represents a significant advance in classifying communities and it continues to be refined and developed. But, to develop a wide capacity to use soil nematode information for diagnostic and predictive purposes, particularly for agricultural soils, we need a new, more robust approach, which does not require extensive taxonomic skill and includes more functional criteria. One of the key attributes of nematodes is the relationship between structural form (principally oesophagal feeding apparatus) and function (i.e. trophic group). Nematode form is readily determinable by direct observation of extracted nematodes and high-level taxonomic skills are not needed to assign the major community components to their different trophic and ecological groups. Consequently, the trophic structure of nematode communities is relatively easy to determine and can provide an integrated measure of the status of the other groups on which they feed. Similarly, population numbers and proportions of juveniles and adults can be readily determined, permitting calculation of relative biomass and dynamics of population growth. The size distribution of individuals within the community is likely also to be an indicator of the structural status of soils from a biotic standpoint. However, fundamental gaps remain in our understanding which limit our ability to relate differences in nematode communities to functional differences. There needs to be a greater emphasis on the development and experimental testing of hypotheses, a greater integration of nematology into soil-process related studies, and the development of a specific, soil-nematode related theoretical framework for understanding epidemiological and soil colonisation processes.

Immunocytochemical localization of neurosecretory amines and peptides in the free-living nematode,Goodeyus ulmi

SummaryMammalian antibodies to the neuroamines, serotonin and γ-amino-butyric acid (GABA) and to the neuropeptides, adrenocorticotrophic hormone (ACTH) and FMRF-amide evoked a response toGoodeyus ulmi, a free-living nematode. Serotonin-like immunoreactivity was found in cell bodies in the nerve ring and in the ventral nerve cord in all developmental stages. Neurons in the vulva, implicated in egg-laying, were immunoreactive to anti-serotonin inG. ulmi females, while in males serotonergic nerve fibres was found in the spicular region. Immunoreactivity to ACTH was also seen to differ depending on the developmental stage ofG. ulmi, being present only in the ventral cord from the late L3 stage. Anti-GABA immunoreactivity was localized in two cell bodies near the amphids in all life stages and FMRF-amide immunoreactivity was seen in the nerve ring in all developmental stages. No reactivity was found with antibodies to vasointestinal peptide and somatostatin-14.

Variation in root-knot nematode (Meloidogyne spp.) in Crete in relation to control with resistant tomato and pepper

A targeted survey of vegetable production areas in Crete, Greece, showed that the majority of infestations of rootknot nematodes (RKN) were Meloidogyne javanica. M. incognita appeared to be limited to areas growing pepper that is a host for M. incognita but not for M. javanica. Tests with resistant tomato carrying the Mi gene showed that the M. incognita and the majority of the M. javanica populations were avirulent, and that virulent nematodes could not be selected from them. However, three populations of M. javanica, including some previously identified, were virulent. An AFLP study of the DNA from 22 populations of M. javanica produced 264 scorable amplification products and gave a very high mean similarity (99.4%) between populations, indicating that both virulent and avirulent populations probably derived from the same founder population. Mi-resistant tomato and pepper appear to have considerable utility in the integrated management of RKN in Crete, provided care is taken to monitor the species and virulence or reproductive ability of the nematode populations. Die Variation bei Wurzelgallennematoden (Meloidogyne spp.) auf Kreta in Bezug auf ihre Bekampfung mit resistenten Tomaten und Paprika - Eine gezielte Untersuchung von Gemuseanbaugebieten auf Kreta Griechenland, ergab, dass die Mehrzahl der von Wurzelgallennematoden (RKN) befallenen Flachen mit Meloidogyne javanica verseucht war. M. incognita was offentsichtlich auf Gebiete des Paprikaanbaus beschrankt. Paprika ist ein Wirt fur M. incognita aber nicht fur M. javanica . Versuche mit resistenten Tomaten, die das Mi-Gen besassen, zeigten, dass M. incognita und die Mehrzahl der Populationen von M. javanica avirulent waren, und dass aus ihnen keine virulenten Nematoden selektiert werden konnten. Drei Populationen von M. javanica, darunter schon einige schon vorher identifizierte, waren virulent. Eine AFLP-Untersuchung der DNA von 22 Populationen von M. javanica ergab 264 unterscheidbare Amplifikationsprodukte und zeigte eine hohe mittlere Ahnlichkeit (99,4%) zwischen den Populationen. Dies deutete daraufhin, das virulente and avirulente Populationen wahrscheinlich von der gleichen Grunderpopulation abstammten. Mi-resistente Tomaten und Paprika sind offensichtlich von betrachtlichen Nutzen bei der integrierten Bekampfung von RKN auf Kreta. Voraussetzung ist, dass die Arten und die Virulenz oder die Vermehrungsfahigkeit der Populationen sorgfalting festgestell werden.

Parasitism genes and host range disparities in biotrophic nematodes: the conundrum of polyphagy versus specialisation

This essay considers biotrophic cyst and root‐knot nematodes in relation to their biology, host–parasite interactions and molecular genetics. These nematodes have to face the biological consequences of the physical constraints imposed by the soil environment in which they live while their hosts inhabit both above and below ground environments. The two groups of nematodes appear to have adopted radically different solutions to these problems with the result that one group is a host specialist and reproduces sexually while the other has an enormous host range and reproduces by mitotic parthenogenesis. We consider what is known about the modes of parasitism used by these nematodes and how it relates to their host range, including the surprising finding that parasitism genes in both nematode groups have been recruited from bacteria. The nuclear and mitochondrial genomes of these two nematode groups are very different and we consider how these findings relate to the biology of the organisms. BioEssays 30:249–259, 2008.

Host status differences and their relevance to damage by Meloidogyne incognita

Four populations of Meloidogyne incognita were tested on 57 species of plants from temperate and tropical regions. Juveniles of one or more populations invaded, and induced galls on almost all plants. Except for the population from the West Indies which induced most galls on Nicotiana spp., there was no obvious relationship between the origins of populations and their ability to induce galls on plants from particular regions. Although most plants were hosts for all four populations, many produced only a few galls/egg masses with some of the populations. The differences in galling between populations, and the interaction with plant species were both significant (P < 0.001). Poor hosts often contained fewer juveniles and the rate of juvenile development was often decreased compared with good hosts. Solanum nigrum was an exception in that it was invaded by fewer juveniles than susceptible tomato, but rates of development were similar. The effect of host status differences in four plant species on damage by M. incognita was investigated but only the growth of susceptible tomato was decreased. The growth of the non-host Urtica dioica, of the poor host Datura metal, and of an intermediate host S. nigrum, was unaffected. Unterschiede im Wirtsstatus und ihre Bedeutung fur den durch Meloidogyne incognita verursachten Schaden - Vier Populationen von Meloidogyne incognita wurden an 57 Pflanzenarten aus gemassigten und tropischen Gebieten gepruft. Juvenile einer oder mehrerer Populationen wanderten in fast alle Pflanzen ein und induzierten Gallen. Mit Ausnahme der Population von den Westindischen Inseln, die an Nicotiana spp. die meisten Gallen induzierte, bestand keine deutliche Beziehung zwischen der Herkunft der Populationen und ihrer Fahigkeit, an Pflanzen bestimmter Gebiete Gallbildung zu induzieren. Obwohl die meisten Pflanzen Wirte fur alle vier Populationen waren, bildeten viele von ihnen mit einigen Populationen nur wenige Gallen/Eimassen. Die Unterschiede zwischen den Populationen in Bezug auf Gallbildung und auf Wechselwirkungen mit den Pflanzen waren beide significant (P < 0.001). Schlechte Wirte enthielten oft weniger Juvenile als gute Wirte, und die Entwicklungsrate der Juvenilen war oft vermindert im Vergleich zu guten Wirten. Solanum nigrum bildete insofern eine Ausnahme, als weniger Juvenile eindrangen als in anfallige Tomaten, die Entwicklungsraten aber ahnlich waren. Die Wirkung der Unterschiede im Wirtsstatus auf die Schadigung durch M. incognita wurde an vier Pflanzenarten untersucht. Nur das Wachstum von anfalligen Tomaten wurde vermindert. Das Wachstum des Nichtwirtes Urtica dioica, des schlechten Wirtes Datura metal und des massig guten Wirtes S. nigrum wurde nicht beeinflusst.

Variation of Virulence, in Terms of Quantitative Reproduction of Globodera Pallida Populations, From Europe and South America, in Relation To Resistance From Solanum Vernei and S. Tuberosum Ssp. Andigena Cpc 2802

Forty populations of Globodera pallida originating from either South America or field sites in Europe were tested for their reproductive ability on a susceptible potato cultivar and five genotypes with quantitative resistance from either Solanum vernei or S. tuberosum ssp. andigena CPC 2802 in a glasshouse pot experiment. The results showed that there was a wide and continuous range of virulence (85% to 4% reproduction) to all the resistant genotypes. This range was as broad within the European populations as within those from S. America. There was a significant host clone x nematode population interaction, largely accounted for by grouping the clones according to the source of resistance and the populations by their continent of origin. Populations from Europe were relatively more virulent on genotypes derived from S. vernei. In contrast, populations from S. America were relatively more virulent on genotypes derived from S. tuberosum ssp. andigena CPC 2802. Examination of the data from the European populations also showed a significant host x population interaction tending to separate British from mainland European populations.

Influence of temperature on the development and survival of Meloidogyne javanica

Responses to temperature of a population of Meloidogyne javanica from Tanzania were examined. The minimum time for development from J2 to first J2 of the next generation was determined over a temperature range from 18-30°C. The rate of development increased linearly between 18°C and 27°C and, on a thermal time basis, the threshold or base temperature for development was estimated by extrapolation to be 12.9° C and the requirement for development c. 350°C days above base. These values are compared with those for M. hapla, a temperate species, and their effects on development time in relation to temperature are discussed. The effect of higher temperatures on egg viability was also tested. Short exposures to 70°C were lethal and exposure to 45°C and above killed a progressively increasing proportion of eggs. Exposure in the field in Tanzania of egg masses buried at different depths to solarization under clear polythene sheeting showed that at 15 cm depth all eggs were dead after 2-3 weeks with an average and a maximum soil temperature at 16.00 h of 43°C and 45°C respectively for the covered and 40°C and 42°C for the uncovered plots. Irrigating the plots before covering slightly decreased mean soil temperatures compared with the covered, dry plots and greatly increased egg survival.

Effects of rates of a nematicide and of fertiliser on the growth and yield of cultivars of potato which differ in their tolerance of damage by potato cyst nematodes (Globodera rostochiensis andG. pallida)

Potato plants growing in soil heavily infested with potato cyst nematode (PCN) contained less N, P and K in their leaf dry matter than plants growing in the same soil treated with a nematicide. These differences were less in tolerant than intolerant cultivars. Applying additonal fertiliser increased the growth of untreated plants more than that of nematicide-treated plants and nematicides increased growth most in plots receiving the lowest rate of fertiliser. Overall, the results are consistent with the hypothesis that damage by invading juveniles of PCN decreases the effectiveness of the potato root system leading to a chronic deficiency of one or more nutrients and a consequential reduction in the rate of top growth.

Genetic Variation in British Populations of Globodera Pallida as Revealed By Isozyme and Dna Analyses

The current pathotype scheme for Globodera rostochiensis and G. pallida differentiates some pathotypes on the basis of qualitative differences in relation to major resistance genes and others on the basis of quantitative differences in reproduction of populations on differential hosts with polygenic resistance. The former group can be differentiated genotypically whilst the latter are differentiated phenotypically and probably consist of populations which are heterogeneous for virulence genes. This paper is concerned with this latter group. The potato cyst nematode was introduced into Europe from South America and knowledge of the characteristics of each separate introduction might form the basis for classifying the various populations and virulence groups. Several mainly British G. pallida Pa2/3 populations were studied to see which, if any, natural groups could be identified by the use of isozyme and RFLP analysis. Both methods indicated similar results namely that there were no clear and consistent patterns either in terms of virulence or geographic location with one exception from Scotland. This population shows a marked difference in its virulence on the partially resistant cv. Morag.

Concepts of Resistance, Tolerance and Susceptibility in Relation to Cyst Nematodes

Where one organism lives on or within and is dependent upon another there may be a range of relationships from symbiosis (mutualism) to predator/prey. Between are the parasite/host and, if the parasite damages its host, the pathogen/host relationship. Cyst nematodes cause damage and are therefore pathogens, but Koch’s postulates cannot readily be applied to them; in part this is because they multiply and spread much less rapidly than many other plant pathogens and, in part, because the damage they cause is usually proportional to their population density at planting.