Since it has become appreciated that soil nematode assemblages are abundant, diverse and contribute to soil nutrient turnover, they have been increasingly used as indicators of soil condition. Use of nematodes as functional indicators relies on the allocation of nematodes to feeding groups and reproductive strategies; in both cases groupings are uncertain. Species within feeding groups vary in their food resources and response to environmental variables, as shown by the difficulties in managing plant-pathogenic nematodes. Therefore species-level discrimination is necessary to permit further advances in understanding the role of nematodes in soil processes and thus in ecosystem resilience. Analysis of published nematode lists shows that among the bacterial-feeding nematodes Cephalobidae are often the most abundant group in soils; Rhabditidae may increase following a resource pulse; in stressed, natural environments Plectidae may be important. To be comparable with other biota, nematode biodiversity assessment requires species-level identification. In many jurisdictions such identification will be difficult due to inadequate systematic knowledge of the nematode fauna.

Nematodes as soil indicators: functional and biodiversity aspects

A major dominant locus conferring resistance against several pathotypes of the root cyst nematode Globodera rostochiensis was mapped on the linkage map of potato using restriction fragment length polymorphism (RFLP) markers. The assessment of resistance versus susceptibility of the plants in the experimental population considered was based on an in vivo (pot) and an in vitro (petri dish) test. By linkage to nine RFLP markers the resistance locus Gro1 was assigned to the potato linkage group IX which is homologous to the tomato linkage group 7. Deviations from the additivity of recombination frequencies between Gro1 and its neighbouring markers in the pot test led to the detection of a few phenotypic misclassifications of small plants with poor root systems that limited the observation of cysts on susceptible roots. Pooled data from both tests provided better estimates of recombination frequencies in the linkage interval defined by the markers flanking the resistance locus.

Localization by restriction fragment length polymorphism mapping in potato of a major dominant gene conferring resistance to the potato cyst nematode Globodera rostochiensis

The nematode resistance locus Gpa2 was mapped on chromosome 12 of potato using information on the genomic positions of 733 known AFLP markers. The minimum number of AFLP primer combinations required to map Gpa2 was three. This demonstrates that a reference collection of potato AFLP markers may be a valuable tool for mapping studies in potato. By use of RFLP probes, Gpa2 was more precisely mapped at the distal end of chromosome 12. Gpa2 confers resistance to a distinct group of populations of the potato cyst nematode Globodera pallida and originates from the same potato accession as locus H1, conferring resistance to pathotype Ro1 of G. rostochiensis. This study shows that these two nematode resistance loci are unlinked and that Gpa2 is linked to the Rx1 locus conferring resistance to potato virus X. The efficiency of AFLPs for genetic mapping of a highly heterozygous crop like potato is discussed and compared with the RFLP technique.

Mapping of the cyst nematode resistance locus Gpa2 in potato using a strategy based on comigrating AFLP markers

Resistance to the root cyst nematode Globodera rostochiensis is an agronomic trait that is at present incorporated into most new potato varieties. Major dominant genes are available that originate from wild and cultivated Solanum species closely related to the cultivated European potato (Solanum tuberosum ssp. tuberosum). One of those genes, H1, from S. Tuberosum ssp. andigena, was mapped to a distal position on potato chromosome V using restriction fragment length polymorphism (RFLP) markers. The H1 locus segregates independently from Gro1, a second dominant gene presumably from S. Spegazzinii that confers resistance to G. Rostochiensis and which has been mapped to chromosome VII. One marker, CP113, was linked without recombination to the H1 locus.

Identification of RFLP markers closely linked to the H1 gene conferring resistance to Globodera rostochiensis in potato

The tomato Hero A gene is the only member of a multigene family that confers a high level (>80%) of resistance to all the economically important pathotypes of potato cyst nematode (PCN) species Globodera rostochiensis and G. pallida. Although the resistance levels of transgenic tomato lines were similar to those of the tomato line LA1792 containing the introgressed Hero multigene family, transgenic potato plants expressing the tomato Hero A gene are not resistant to PCNs. Comparative microscopy studies of in vitro infected roots of PCN-susceptible tomato cv. Money Maker, the resistant breeding line LA1792, and transgenic line L10 with Ro1 pathotype have revealed no statistically significant difference in the number of juveniles invading roots. However, syncytia (specialized feeding cells) induced in LA1792 and L10 roots mostly were found to have degenerated a few days after their induction, and a few surviving syncytia were able to support only the development of males rather than females. Thus, the ratio between males and females was biased towards males on LA1792 and L10 roots. A series of changes occur in resistant plants leading to formation of a layer of necrotic cells separating the syncytium from stellar conductive tissues and this is followed by degradation of the syncytium. Although the Hero A gene is expressed in all tissues, including roots, stems, leaves, and flower buds, its expression is upregulated in roots in response to PCN infection. Moreover, the expression profiles of the Hero A correlates with the timing of death of the syncytium.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-18-0158

Characterization of susceptibility and resistance responses to potato cyst nematode (Globodera spp.) infection of tomato lines in the absence and presence of the broad-spectrum nematode resistance Hero gene.

Following joint tests in the Netherlands, Germany and the United Kingdom a new scheme is proposed for naming pathotypes of potato cyst-nematodes. Pathotypes of Globodera ( = Heterodera) rostochiensis and G. pallida are designated separately using a simple nomenclature Ro1 to Ron for G. rostochiensis and Pa1 to Pan for G. pallida. Currently seven clones are used to differentiate five pathotypes of G. rostochiensis and three of G. pallida. Several of the pathotypes recognised in the British, Dutch and German National Schemes are the same. Results of pathotyping tests may show considerable variation; adequate replication and standardisation of method are essential. With an internationally used scheme it is highly desirable that new pathotypes or differential clones are added only after testing at Institutes in several countries.

An International Scheme for Identifying and Classifying Pathotypes of Potato Cyst-Nematodes Globodera Rostochiensis and G. Pallida

Previous suggestions that root exudate of nematode-resistant Solanum vernei hybrids hatches fewer juveniles than that from susceptible potato plants, thus contributing to resistance, are not supported. Although fewer eggs hatched in exudate from some ex vernei clones this was insufficient to contribute significantly to decreased nematode reproduction on these clones.

Hatching of Potato Cyst-Nematodes (Globodera Rostochiensis, G. Pallida) in Root Exudates of Solanum Vernei Hybrids

Changes in the taxonomy of cyst-nematodes in the past decade and the contributions and usefulness of some new techniques are reviewed The problems of distinguishing sibling species and the way in which they may have evolved are discussed The occurrence of pathotypes within Globodera and Heterodera is reviewed and a revised classification of the Heteroderoidea presented Nacobbodera Golden & Jensen is synonymised with Meloinema Choi & Geraert

Recent Developments and Some Problems in the Taxonomy of Cyst-Nematodes, With a Classification of the Heteroderoidea

Development of potato cyst-nematodes in roots of resistant potato plants was studied using a technique which confined invasion to a limited portion of the root system. Comparisons were made between development in plants with the major resistance gene H1 derived from Solanum tuberosum ssp. andigena, plants with polygenic resistance derived from S. vernei, and a susceptible cultivar. One population of Globodera rostochiensis pathotype Ro1 and one of G. pallida pathotype Pa3 were used, with development in roots observed until 37 d after inoculation. Numbers of juveniles invading root systems were not clearly related to resistance but in all combinations of potato and nematode where resistance was found there was a decrease in rate of nematode development which varied in degree but occurred with both types of resistance. This appears to be the main effect of resistance on nematode multiplication. These findings are discussed in relation to earlier reported work on partial resistance.

Development of Potato Cyst-Nematodes in Roots of Resistant Solanum Tuberosum Ssp. Andigena and S. Vernei Hybrids

Host ranges of 35 populations of Globodera spp. from the Americas and pathotypes from Europe were tested on 19 Solanum subgenus Leptostemonum spp. representing different taxonomic sub-groupings and geographical distributions. Cluster and principal co-ordinate analyses showed intraspecific variation in all Globodera spp. except G. tabacum. European populations of pathotypes Ro1 to Ro5 and Pa2 had smaller host ranges than populations of Globodera from the Americas and hypotheses are advanced to explain the phenomenon. North and Central American Globodera spp. could not be differentiated; similarities between populations of different species were often equal to or greater than those within the same species. G. airginiae and the "Mexican cyst-nematode" were most similar followed by G. solanacearum and G. tabacum. G. pallida and G. rostochiensis were more distinct. The pattern of resistance did not correspond directly to taxonomic sub-groupings of Leptostemonum as sections, e. g. Acanthophora and Lasiocarpa, contained resistant and susceptible species. A closer correspondence was found with geographical distribution of solanums; indigenous Old World species with little or no exposure to Globodera were uniformly susceptible, whereas resistance was found in indigenous American solanums likely to have been in contact with Globodera. The hypothesis is advanced that susceptibility is the basic condition in Solanum and that resistance has been selected in response to nematode attack, circumvention of resistance subsequently arising in some Globodera populations.

A.R. Stone

Papers

An International Scheme for Identifying and Classifying Pathotypes of Potato Cyst-Nematodes Globodera Rostochiensis and G. Pallida

Hatching of Potato Cyst-Nematodes (Globodera Rostochiensis, G. Pallida) in Root Exudates of Solanum Vernei Hybrids

Recent Developments and Some Problems in the Taxonomy of Cyst-Nematodes, With a Classification of the Heteroderoidea

Development of Potato Cyst-Nematodes in Roots of Resistant Solanum Tuberosum Ssp. Andigena and S. Vernei Hybrids

Host Ranges of Globodera Species Within Solanum Subgenus Leptostemonum