Vertical distribution of nematodes in arable soil under grass (Festuca pratensis) and barley (Hordeum distichum)

Summary The connection between faunal composition and soil factors is discussed in this study on vertical distribution of soil nematodes under grass and barley. The investigation was undertaken on the field site of a Swedish integrated research project Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling. Higher nematode number (7.6 × 10⁶ m^–2) and biomass (340 mg dry wt. m^–2) were found under a 4-year-old grass ley than under barley (5.0 × 10⁶ m^–2; biomass, 136 mg dry wt. m^–2). Plant feeders dominated under the grass ley (3.2 × 10⁶ m^–2 whereas under barley the bacterial feeders (2.4 × 10⁶ m^–2) were the most abundant feeding group. Number, biomass, mean individual size and various community parameters indicated a much better nutritive situation for the nematodes under grass than under barley. The vertical changes in the various parameters, including proportion of egg-carrying females, indicated an increasing food shortage for the nematode populations towards greater depths. In the top soil, predation could be an important factor in regulating nematode number.Dedicated to the late Prof. Dr. M.S. Ghilarov     

Chaotic or deterministic development of nematode populations in pine forest humus incubated in the laboratory

Humus material from an old stand of Scots pine and from an open area clearcut 13–15 years ago was incubated in the laboratory. The incubations were started in different years and the differences in nematode faunal development between the years were compared with site characteristics. In all incubations there was an increase in total nematode abundance. The total number and diversity of nematodes were higher in humus from the forest than from the clearing. The results indicated that biotic control mechanisms were much weaker in the humus from the clearcut area, where a strong dominance of Acrobeloides nanus occurred in most incubations. The differences in faunal development between years were rather large. A certain pattern of unpredictability indicated that the nematode fauna was a rather labile component of the community of soil organisms in a pine forest soil.     

Host status of 10 fungal isolates for two nematode species, Filenchus misellus and Aphelenchus avenae

The classification of nematodes in the family Tylenchidae into plant parasites, plant associates or fungal-feeders for community analyses, have been much discussed by nematode ecologists. For an appropriate classification, fungal-feeding habits in the family need to be studied. To evaluate the host status of 10 fungal isolates for Filenchus misellus (Tylenchidae) and Aphelenchus avenae (Aphelenchida, Aphelenchidae), population growth rates, body length and width and sex ratios of the nematodes were measured after 40-day culture on fungal colonies at 25 °C. For F. misellus, the fungi determined as good hosts were two Basidiomycota fungi (Agaricus bisporus, Coprinus cinereus), three Ascomycota fungi (Chaetomium cochlioides, Chaetomium funicola, Chaetomium globosum) and a plant-pathogenic fungus (Rhizoctonia solani) on the basis of nematode population growth rate and female body length. Interestingly Pleurotus ostreatus, known as a predaceous fungus for the other nematodes, was also a good host for F. misellus. While, for A. avenae, good hosts were four plant-pathogenic fungi (Fusarium oxysporum f. sp. conglutinans, F. oxysporum f. sp. cucumerinum, Pythium ultimum, R. solani) and A. bisporus. A. avenae was trapped and preyed upon by Pleurotus hyphae. In F. misellus, males were 7-21% of adults, but the ratio did not correlate significantly with the population growth rate. In A. avenae, no male occurred. Differences in habitat preference between Filenchus and Aphelenchus were explained on the basis of the host status and habitat preferences of the tested fungi.     

Diversity of soil nematodes across a Mediterranean ecotone

As an indicator of ecological maturity, we have analyzed the nematode community of an erosion-prone Mediterranean macchia and a pine stand within it. Species richness, abundance, diversity and maturity indexes have been measured from a number of standardized soil samples taken along parallel transects laid across the boundary between the pine stand and the macchia. Results of multifactor analysis on these data show three distinct nematode communities of different species composition. The intermediate nematode community (ecotone) is not a mixture of elements of adjacent communities (pine stand and macchia), thus suggesting the existence of an ecotone-specific community beyond the expected boundary effect.     

Spatial patterns of variation in the composition and structure of nematode communities in relation to different microhabitats: a case study of Quercus dalechampii Ten. forest

Variability in the spatial distribution of nematode communities in relation to the structural heterogeneity of the environment was studied in nine different microhabitats within a relatively small area of a natural oak forest in Bulgaria. Maturity and diversity indices, trophic structure and the distribution of colonizer-persister groups were applied to analyze the quality of substrate and ecological processes involved from a functional point of view. Two main groups of nematode communities, below- and above-ground, were distinguished in terms of the location of the microhabitats. Our results indicated a higher percentage similarity between nematode communities inhabiting microhabitats with a higher resemblance in substrate structure, and abiotic and biotic conditions than between microhabitats with more dissimilar microenvironmental conditions. The application of Detrended Correspondence Analysis helped to reveal two ecological gradients. The first one was from microhabitats characterized by smaller fluctuations in microclimatic conditions and nutrient supply to microhabitats with more adverse abiotic conditions and dynamics of food resources. Along this gradient from below- to above-ground microhabitats, the proportion of general opportunists (cp 2 taxa) increased, whereas the diversity, MI and the proportions of persisters (cp (3-5) taxa), decreased. Along the second gradient a gradual decrease in the decomposition rate within above-ground microhabitats was revealed, which was indicated by the proportion of enrichment opportunists (cp 1 taxa). The nematode communities of decaying wood had the most specific cp groups' distribution characterized by a high proportion of enrichment opportunists (colonizers). Each microhabitat has developed nematode communities with a characteristic trophic structure that was related to the relative importance of primary production and decomposition processes occurring within the microhabitat. The nematode communities of mosses growing on soil, stones and tree trunks had similar trophic structure dominated by bacterial-feeding nematode taxa. Our results supported the role of nematode communities as potential indicators of environmental conditions.     

New sources of resistance to race Ro1 of the Golden nematode (Globodera rostochiensis Woll.) among reputed duplicate germplasm accessions of Solanum tuberosum L. subsp. andigena (Juz. et Buk.) Hawkes in the VIR (Russian) and US Potato Genebanks

Cultivated Solanum tuberosum L. subsp. andigena is well known as a rich source of valuable traits for potato breeding, especially for resistance to diseases and pests. The golden potato cyst nematode, Globodera rostochiensis Woll., is considered to be one of todays most serious hindrances to potato production in Europe and North America. Thus, the breeding of new cultivars that have resistance to PCN is of great importance. The USPG (USA) and VIR (Russian) potato genebanks, as well as others, maintain many samples of primitive cultivated and wild potato species originating from Latin America. Many of these samples are assumed to be genetically duplicated because the material in both genebanks came from the same original source. A joint investigation of new genotypes of subsp. andigena forms resistant to potato cyst nematode (PCN) was carried out on samples of subsp. andigena at VIR with reputed duplicate samples at USPG. After careful screening, 14 samples which possessed resistance to PCN were identified. A high level of this resistance was transmitted to sexual progeny at a high frequency for all of the selections. Eleven of the accessions found to be resistant have reputed duplicates in USPG that were not previously known to be resistant. Thus, this research not only broadens the choice of parents available for resistance breeding, but identifies model materials for future research to test the parity of PCN resistance among reputed duplicate samples in the two genebanks.     

Assessing risks of disease transmission between wildlife and livestock: The Saiga antelope as a case study

Disease transmission between wildlife and livestock can undermine conservation efforts, either by challenging the viability of threatened populations, or by eroding public tolerance of actual or potential wildlife disease reservoirs. This paper describes the use of transmission models to assess the risk of disease transfer across the wildlife-livestock boundary, and to target control strategies appropriately. We focus on pathogens of the Saiga antelope (Saiga tatarica) and domestic ruminants in Central Asia. For both foot and mouth disease and gastrointestinal nematodes, the main risk is associated with infection of saigas from livestock, and subsequent geographical dissemination of infection through saiga migration. The chance of this occurring for foot and mouth disease is predicted to be highly dependent on saiga population size and on the time of viral introduction. For nematodes, the level of risk and predicted direction of transmission are affected by key parasite life history traits, such that prolonged off-host survival of Marshallagia in autumn enables infection of saigas and transfer northwards in spring. Field estimates of parasite abundance provide qualitative support for model predictions. The application of models as tools for the early evaluation of disease transmission between wildlife and livestock is discussed.     

Solar UV-B influences microfaunal community composition in a Tierra del Fuego peatland

The peatlands of Tierra del Fuego are subject to increased solar ultraviolet-B radiation (UV-B) due to the influence of the Antarctic ‘ozone hole’. Research into the effects of climate change and ozone depletion on peatlands has predominantly focused on the higher plant community and neglected other organisms. In the second 3-year portion of a 6-year experiment, we intensified our investigations of the response of the peatland surface microfaunal community to current and attenuated solar UV-B, and assessed possible links to changes in the microenvironment. Near-ambient UV-B and reduced UV-B treatments were realised by stretching plastic film filters that differentially attenuate UV-B over peatland sample plots. We extracted the microfauna and analysed the dissolved nutrients held within Sphagnum capitula removed from the top 1-cm of the peatland. In line with previous findings in this system, testate amoebae were more abundant under near-ambient UV-B than under reduced UV-B. Populations of the most common genus, Assulina, and other less prominent amoebae species of Heleopera and Euglypha, were consistently increased under near-ambient UV-B. Overall diversity of testate amoebae was also higher under near-ambient UV-B than under reduced UV-B, whereas rotifers, nematodes and mites were less abundant under near-ambient UV-B. Concentrations of DOC and P were generally higher under near-ambient UV-B than under reduced UV-B. These changes, combined with the changes previously reported in the plant and fungal communities, have the potential to influence peatland C storage, and surface nutrient availability. The peatland microfaunal community under near-ambient solar UV-B may be regulated by the plant community through the leaching of nutrients from leaf cells, and changes in Sphagnum morphology that affect the capitulum microenvironment.     

Tolerance of chickpea (Cicer arietinum L.) lines to root-knot nematode, Meloidogyne javanica (Treub) Chitwood

The root-knot nematode, Meloidogyne javanica (Treub) Chitwood is an important parasite of chickpea (Cicer arietinum L.). Four chickpea genotypes were evaluated for tolerance to M. javanica in naturally infested fields at three locations. Each genotype was evaluated for number of galls, gall size, root area covered with galls and number of egg masses produced. All the cultivars were susceptible or highly susceptible. Seed yield, weight of 100 undamaged seeds, total dry matter and plant height were compared with checks. Chickpea cultivar Annigeri and a local check were used as nematode susceptible checks in all locations. The four promising nematode tolerant genotypes produced significantly greater yield and total dry matter than the checks in fields naturally infested with M. javanica at three locations. These M. javanica tolerant lines represent new germplasm and they are available in the chickpea genebank at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) bearing the identification numbers ICC 8932, ICC 11152, ICCV 90043 and ICCC 42.     

Nematode assemblages in the rhizosphere of spring barley (Hordeum vulgare L.) depended on fertilisation and plant growth phase

Nematodes from rhizosphere soil of barley grown at three fertiliser treatments (control (0), NK and NPK) were studied in a field experiment. Sampling was done twice, during vegetative growth and flowering, respectively, to determine how fertiliser effects on nematode assemblages depended on plant growth phase. At the growth stage the proportion of fungal feeding nematodes (dominated by Aphelenchoides spp. and Aphelenchus sp.) was highest in NK. During flowering, the abundance and proportion of fungal feeders in the 0 and NPK plots had increased and reached a level similar to the NK plot. Overall densities of bacterial feeders (mainly Cephalobidae and Rhabditidae) were similar, but opportunistic bacterial feeders constituted a higher proportion in the fertilised plots compared to the unfertilised. Ectoparasitic plant feeders (Tylenchorhynchus sp.) were more numerous in NK and NPK than in the control at both sampling dates. Endoparasite (Pratylenchus spp.) numbers were lower in the NPK plot at the growth stage. Numbers of Tylenchidae increased between samplings. The classification of Tylenchidae as epidermal cell and root hair feeders as opposed to hyphal feeders is discussed. Results thus indicate that: (i) bacterial and especially fungal feeding nematodes are stimulated by unbalanced fertilisation; (ii) ectoparasitic plant feeders are stimulated by N-fertilisation, while migratory endoparasites are inhibited at high and balanced fertilisation; (iii) nutrient effects diminish after plants reach the flowering stage.