Search

A growing number of studies point at the involvement of root herbivores in influencing plant performance, community composition and succession. However, little is known about the factors that control root herbivore abundance and the role of local variation in the effectiveness of these factors. Here, we performed a full factorial experiment with plants, root-feeding nematodes and rhizosphere microbial communities from two dune sites, to test the hypothesis that the outcome of belowground multitrophic interactions depends on local differences between the interacting organisms. The organisms included the marram grass Ammophila arenaria, the cyst nematode Heterodera arenaria, microbial plant pathogens and natural enemies of the nematodes from two coastal foredune systems, one in The Netherlands and one in Wales. The two plant populations differed at the molecular and phenotypic level, and the microbial communities from the two dune sites differed in the composition of the dominant soil fungi but not of the dominant bacteria. Plants were negatively affected by the rhizosphere microorganisms from one of the sites. Nevertheless, nematode performance was not affected by the origin of both the host plants and the microbial communities. The reproductive output of the cyst nematode depended on the presence of microorganisms, as well as on inter-population variability in the response of the nematode to these natural enemies. In the absence of microorganisms, the two nematode populations differed in the number and size of the produced cysts, although maternal effects cannot be excluded. Inter-population differences in the host plant were a secondary factor in the nematode–microorganisms interactions, and did not influence bottom–up control of the cyst nematodes. Our results did not reveal strong signals of coevolution in belowground multitrophic interactions of plants, cyst nematodes and soil microbial communities. We conclude that the interactions between the studied organisms do not necessarily depend on their local vs. non-local origin. Nevertheless, we were able to show that local variation in soil organism community composition can be an important factor in determining the outcome of interactions in belowground multitrophic systems.

Root-feeding nematodes may play an important role in generating spatial and temporal variation in natural plant communities, but little is known about the performance of the nematodes in the plant root zone. We studied the emergence, survival and reproductive success of the cyst nematode Heterodera arenaria , a root feeding-specialist that occurs in mobile dunes. The host plants of H. arenaria are buried regularly by windblown sand to which the plants respond by upward clonal expansion. As a consequence, the nematodes have to migrate upwards in the soil profile to find new roots for feeding and reproduction, however, not all juveniles migrate. We tested the hypothesis that the offspring from migrated individuals would perform better than from individuals that remained behind and discuss the advantage of this dualistic behavior. The individual performance of the cyst nematodes was better when their juveniles migrated to the new root layer. In the field, in the new root layer the cysts had more eggs and juveniles than cysts collected from the 1-year-old root layer. Under controlled conditions, cysts from the new root layer released their first juveniles faster than cysts from the 1-year-old root layer. However, the juveniles that do not migrate might be crucial for the persistence of the population. In the past decade in two extremely dry years, we could not find cysts in the new root layer. Apparently, summer drought constrains cyst formation, so that the cysts produced in the older root layers serve as a reservoir for the population. Such reservoirs could become more important when climate change will result in increased incidence of summer droughts.

In coastal foredunes, the cyst nematode Heterodera arenaria has been supposed to play a role in degeneration of the pioneer grass Ammophila arenaria (marram grass). However, recent field surveys and field inoculation experiments suggested that the abundance of this cyst nematode is controlled by the host plant. Here, we test pathogenicity of H. arenaria by inoculating a range of densities of second-stage juveniles (J2) onto A. arenaria in sterilised soil. Plant root biomass was reduced after 6 weeks of growth, but not after 13 weeks, when cyst formation was completed. Shoot biomass was not influenced. In the field, H. arenaria occurred in sand and on roots of Elymus farctus and A. arenaria in mobile dunes, but not on A. arenaria in stable dunes. Although we observed Heterodera cysts and juveniles in later succession stages, this cyst nematode appeared to be H. hordecalis. In a host suitability test, H. arenaria produced cysts on E. farctus and A. arenaria, while there were almost no cysts produced when J2 had been added to the later succession species Festuca rubra ssp. arenaria, Carex arenaria, Elymus athericus and Calamagrostis epigejos. Heterodera arenaria did not negatively affect its natural host plants. However, H. arenaria reduced root growth of one later succession plant species, E. athericus, which was a poor host for H. arenaria. The effects of H. arenaria inoculation on plant biomass production did not differ when J2 were added to plants growing in non-sterilised soil. We conclude that H. arenaria is specialised on the pioneer grasses E. farctus and A. arenaria, and also on mobile dunes as a habitat. Our results confirm recent suggestions that H. arenaria may cause little direct growth reduction to its natural host plants. [KEYWORDS: AMMOPHILA ARENARIA ; BIOTROPHIC PARASITE ; COASTAL FOREDUNE ; HETERODERA HORDECALIS ; HOST SPECIFICITY]

Summary 1 The feedback between individual plants and their soil communities is a major driver of plant community processes. We analyse the rate at which plant–soil feedback develops in the root zone of the clonal dune grass Ammophila arenaria. 2 Ammophila arenaria grows vigorously when it can form new roots in newly deposited windblown beach sand. The colonization zone is hypothesized to provide an enemy-free space, with root pathogens and parasites possibly contributing to degeneration of A. arenaria when deposition stops. 3 We quantified root biomass and plant parasitic nematode densities in new and 1-year-old root zones, as well as in degenerate stands, in the field at monthly intervals. Each month, we studied the biomass production of Ammophila seedlings in controlled conditions in sterilized and non-sterilized soil from the different sampling sites, with and without nematicides. 4 Colonization of newly deposited sand by the endoparasitic nematodes Heterodera arenaria and Pratylenchus spp. within 1 month of root formation coincided with a negative feedback in the bioassays. Nematicides counteracted growth reduction significantly, but their effectiveness decreased in soil samples collected later in the growing season. 5 In older layers, roots and nematodes were already present before the sampling started. Growth reduction in unsterilized sand was observed at all sampling events, but was not counteracted by nematicides. 6 In the field, root biomass of A. arenaria in the newly colonized sand increased throughout the growing season, despite the development of a negative plant-soil feedback. 7 We conclude that the development of negative plant–soil feedback in the root zone of A. arenaria closely follows the colonization of newly deposited sand by roots and then by the endoparasitic nematode species Heterodera arenaria. There seems to be a shift in organisms causing this feedback during the first growing season, but root biomass nevertheless increases. However, in older root layers and in the degenerate stand, ongoing negative plant–soil feedback may contribute to a gradual decrease in root biomass.

Canonical discriminant analysis of four morphometric characters of juveniles and restriction enzymes analysis of ribosomal DNA sequences were used to distinguish Heterodera arenaria, H. aucklandica, H. avenae, H. filipjevi, H. hordecalis, H. iri, H. latipons, H. litoralis, H. schachtii and an undescribed species from grasslands. The results of unweighted pair group cluster analysis showed that H. avenae populations formed three groups and H. filipjevi two groups at the 80% level of similarity. Intraspecific polymorphism was revealed by rDNA-RFLP studies and two types of ITS regions within H. avenae populations can be distinguished. The pattern of restriction bands obtained with BsuRI, PstI and TaqI clearly distinguished populations of H. filipjevi from other species of the H. avenae group. Further enzymes and their combinations distinguished the other species. There are no enzymes which differentiate European populations of H. avenae from H. arenaria. Morphometrics, restriction endonuclease cleavage maps of ITS regions and a dendrogram of putative phylogenetic relations of several cyst-forming nematode species are given. Identification des especes du groupe Heterodera avenae par la morphometrie et les rDNA-RFLP - L'analyse canonique discriminante sur quatre caracteres morphometriques des juveniles et l'analyse des enzymes de restriction de l'ADN ribosomal ont ete utilisees pour identifier Heterodera arenaria, H. aucklandica, H. avenae, H. filipjevi, H. hordecalis, H. iri, H. latipons, H. litoralis, H. schachtii et une nouvelle espece originaire de prairies. Les resultats de l'analyse UPGMA ont montre que les populations d'H. avenae formaient trois groupes et celles d'H. filipjevi deux groupes a un niveau de similarite de 80%. Le polymorphisme intraspecifique a ete revele par des etudes de rDNA-RFLP et deux types de regions de l'ITS peuvent etre mis en evidence dans les populations d'H. avenae. Les modeles de bandes de restriction obtenus avec BsuRI, PstI et TaqI ont identifie clairement les populations d'H. filipjevi des autres especes du groupe H. avenae. D'autres enzymes et leurs combinaisons ont identifie les autres especes. Aucun enzyme n'a differencie les populations europeennes d'H. avenae de H. arenaria. Les caracteres morphometriques, les cartes de clivage des regions des ITS par l'endonuclease de restriction et un dendogramme des relations phylogenetiques supposees sont donnes.

We studied the effects of intra- and interspecific competition on the abundance of endoparasitic nematodes and assessed the consequences for biomass production of the natural dune grass Ammophila arenaria. Pratylenchus penetrans was limited by intraspecific competition and it suppressed the abundance of Heterodera arenaria, whilst the interaction between H. arenaria and Meloidogyne maritima was neutral. Pratylenchus penetrans and H. arenaria reduced plant biomass, whereas M. maritima did not. Plant biomass was not differently affected by adding one or two nematode species. When added to older plants, numbers of H. arenaria and M. maritima were higher but numbers of P. penetrans were lower, resulting in less reduction of plant biomass. We discuss our results on this natural system with respect to patterns of interspecific nematode competition observed in agricultural systems. [KEYWORDS: COASTAL SAND DUNE ; HETERODERA ARENARIA ; INTERACTION; INTERSPECIFIC COMPETITION ; INTRASPECIFIC COMPETITION ; MELOIDOGYNE MARITIMA; PRATYLENCHUS PENETRANS ; SERIAL INOCULATION]

To date, no study has explicitly addressed effects of variation in species diversity of root-feeding herbivores on host plant biomass. Root-feeding nematodes typically occur in multi-species communities. In a three-year field experiment, we investigated how variation in species diversity of root-feeding nematodes affected nematode dynamics and response of the dune grass Ammophila arenaria to root-feeder activity. This plant species needs regular burial by fresh beach sand to remain vigorous, suggesting that A. arenaria benefits from a temporary escape from root-feeding soil organisms and that root-feeders are involved in plant degeneration in stabilized dunes. We created series of ceased and continued sand burial and added the endoparasitic nematodes Meloidogyne maritima, Heterodera arenaria and Pratylenchus penetrans alone or in combination to A. arenaria. We included treatments with and without the whole soil community, measured plant biomass and quantified numbers of nematodes. Addition of H. arenaria and P. penetrans decreased numbers of M. maritima juveniles and delayed the first appearance in time of both juveniles and females, while numbers of males only decreased when plants had been buried. Burial with sand and addition of the other two endoparasites affected numbers of H. arenaria juveniles, while numbers of P. penetrans were low and not affected. Shoot biomass of A. arenaria was lower when M. maritima had been added alone than when the three species had been added together. Addition of root zone soil decreased biomass of all plant parts. Burial with sand decreased aboveground shoot biomass, whereas it increased belowground shoot and root biomass. Our results point at idiosyncratic effects of nematode diversity on A. arenaria biomass. Heterodera arenaria and P. penetrans protected their host by reducing numbers and delaying activity of M. maritima to a later stage in the growth season, when root-feeding activity was less harmful for plant biomass development.

In coastal foredunes, the grass Ammophila arenaria develops a soil community that contributes to die-back and replacement by later successional plant species. Root-feeding nematodes and pathogenic soil microorganisms are involved in this negative feedback. Regular burial by wind-blown beach sand results in vigorous growth of A. arenaria, probably because of enabling a temporary escape from negative soil feedback. Here, we examine the role of root-feeding nematodes as compared to the whole soil community in causing negative feedback to A. arenaria. We performed a 3-year sand burial experiment in the field and every year we determined the feedback of different soil communities to plant growth in growth chamber bioassays. In the field, we established A. arenaria in tubes with beach sand, added three endoparasitic root-feeding nematode species (Meloidogyne maritima, Heterodera arenaria and Pratylenchus penetrans) or root zone soil to the plants, and created series of ceased and continued sand burial. During three subsequent years, plant biomass was measured and numbers of nematodes were counted. Every year, bioassays were performed with the field soils and biomass of seed-grown A. arenaria plants was measured to determine the strength of feedback of the established soil communities to the plant. In the field, addition of root zone soil had a negative effect on biomass of buried plants. In the bioassays, addition of root zone soil also reduced the biomass of newly planted seedlings, however, only in the case when the field plants had not been buried with beach sand. Addition of the three endoparasitic root-feeding nematodes did not influence plant biomass in the field and in the bioassays. Our results strongly suggest that the negative feedback to A. arenaria is not due to the combination of the three endoparasitic nematodes, but to other components in the soil community, or their interactions with the nematodes.

Several studies have suggested that plants are able to control the development of specialist herbivorous invertebrates, but not that of generalists. Plants are alleged to have evolved tolerance against specialists in order to suppress the development of more damaging generalists through competition. Here, we tested whether specialist plant parasitic nematodes in the root zone of the natural dune grass Ammophila arenaria are able to suppress the development of a generalist plant parasitic nematode and therewith protect the plant. We added a generalist ectoparasite (Tylenchorhynchus ventralis) and specialist endoparasites (Heterodera arenaria, Pratylenchus penetrans, Meloidogyne maritima) in different densities to A. arenaria. We also tested whether sequential inoculation of the specialists had an additional competitive effect on T. ventralis. Our results show that the specialist endoparasitic nematodes indeed suppressed the development of the generalist T. ventralis, but only when the specialists were added to the plant in relatively high densities that exceeded the field density of the specialist endoparasitic nematodes. Therefore, we conclude that competition by specialist nematodes is not a likely mechanism for the regulation of the generalist plant parasite T. ventralis. Sequential inoculation of endoparasites did not influence the development of T. ventralis more than inoculation at the same time. Despite their inhibiting effect on the development of T. ventralis, the endoparasites did not counteract the negative effect of T. ventralis on plant biomass. On the contrary, they themselves had a negative effect on shoot biomass of A. arenaria, although no effect was found on root biomass. We discuss our results in relation to other mechanisms that may regulate the population density of T. ventralis. [KEYWORDS: Competition; Generalist; Plant parasitic nematode; Population regulation; Specialist; Temporal nematode dynamics]

Cyst and root-knot nematodes show high levels of gross morphological similarity. This presents difficulties for the study of their ecology in natural ecosystems. In this study, cyst and root-knot nematode species, as well as some ectoparasitic nematode species, were identified using the second internal transcribed spacer (ITS2) sequence variation detected by polymerase chain reaction-single-strand conformational polymorphism (PCR-SSCP). The ITS2 region was sufficiently variable within the taxa investigated to allow species to be separated on the basis of minor sequence variation. The PCR primers used in this study were effective for 12 species with three genera within the Heteroderinae (Globodera pallida, G. rostochiensis, Heterodera arenaria/avenae, H. ciceri, H. daverti, H. hordecalis, PI. mani, PI. schachtii, H. trifolii, Meloidogyne ardenensis, M. duytsi and M. maritima). However, pathotypes of Globodera pallida and G. rostochiensis could not be distinguished. The method was tested at two coastal dune locations in The Netherlands tone in the lime-poor dunes of the north and one in calcareous dunes of the south) to determine the population structure of cyst nematodes. At each site, cyst nematodes were associated with three plant species: two plant species on the foredune (Elymus farctus and Ammophila arenaria) and one plant species occurring further inland (Calamagrostis epigejos). Two species of cyst nematodes, H. arenaria and H. hordecalis, were found. H. arenaria associated with vigorous A. arenaria and H. hordecalis in association with degenerating A. arenaria and C. epigejos. The field survey demonstrated that in coastal dunes abiotic factors may be the important affecting the distribution of cyst nematodes. [KEYWORDS: Globodera; Heterodera; identification; Meloidogyne; nematode; PCR-SSCP Plant-parasitic nematodes; polymerase chain-reaction; ribosomal dna; rdna; alignment; arenaria]