The potato cyst nematodes Globodera rostochiensis (Woll.) Skarbilovich and G. pallida (Stone) originate from the Andes region in South America and have been introduced into Western Europe since 1850. Both species are successful colonizers. Once primary founders have established vital populations, an area is rapidly colonized by secondary founding events. The mode of spread results in patchy distribution patterns. Analyses of the processes that influence the spatial variations in virulence are of major importance for the control by means of host plant resistance. The ability to unravel the mosaic distribution patterns of the two species and their pathotypes enables breeders and growers to anticipate on the dynamics of virulent populations. The aim of this thesis was to analyse the intra- and interspecific variation of G. rostochiensis and G. pallida in the Netherlands and to obtain insight in the processes that determine the spatial variations in protein, DNA and (a)virulence polymorphisms.interspecific variationThe molecular variation between the sibling species G.rostochiensis and G.pallida is remarkably large. The RAPD technique revealed a total of 250 DNA fragments, of which only nine DNA fragments were common to both species (Chapter 3). Similar results were obtained with the AFLP assay. A total of 1000 AFLP fragments was amplified, of which only 64 fragments could be identified in both species (Chapter 4). These results agreed with previous investigations using 2-DGE and show that morphologically nearly indistinguishable organisms can be quite distinct the molecular level.The extensive genetic differentiation of G.rostochiensis and G . pallida offers perspectives for the development of a diagnostic assay. In addition the PCR technique enables the amplification of species specific fragments from small numbers of cysts extracted from soil samples. For example, G. rostochiensis specific DNA fragments can be amplified from single juveniles (Chapter 1).intraspecific variationThe intraspecific variation of G.rostochiensis as revealed by RAPDs and AFLPs is relatively low (Chapter 3 and 4). The proportion of polymorphic DNA fragments among nine G. rostochiensis populations was 19% and 15.8%, respectively. Three clusters of populations were identified and each cluster could be described by one or more specific DNA fragments.The intraspecific variation of G.pallida, as revealed with RAPDs or AFLPs, is larger in comparison with G.rostochiensis (Chapter 3 and 4). The proportion of polymorphic RAPD fragments among 17 populations was 46%. For the AFLP assay this figure was 23% polymorphic AFLP fragments among 15 populations. The majority of the populations displayed a continuous range of variations. Only a few clearly diverged clusters could be discriminated on the basis of specific DNA fragments.Various groups have applied the RAPD technique to study the genetic divergence among conspecific populations of plant parasitic nematodes. The validity of this technique was evaluated in chapter 4 by comparison of the clustering of 36 G.pallida populations based on RAPD and 2-DGE data. Both data sets demonstrated that the majority of the G.pallida populations were not clearly differentiated from each other. The overall correlation between the distance matrices derived from both data sets was low. Careful examination of the separate dendrograms showed similarity in clustering only for clearly diverged populations or groups of populations.Although the AFLP and RAPD technique are simple, fast and require only minute amounts of biological material, they are not suitable to resolve the subtle differences among potato cyst nematode populations. Quantitative variation in allele frequencies is often not resolved with those techniques, which is, among others, due to the virtual inability to recognize co-dominant alleles and the non-linear amplification of DNA fragments. In chapter 6 and 7 the genetic variation was studied by analysing pools of individuals with 2-DGE. The ratios between the protein quantities produced by the codominant alleles are appropriate measures for the allele frequencies. The correctness of this method has been confirmed by 2-DGE of single individuals (De Boer etal ., 1992)bottleneck effects on the secondary foundersThe intraspecific variation among potato cyst nematode populations in Europe is predominantly determined by the genetic constitution of the primary founders, directly or indirectly introduced from South America, and the effect of random genetic drift on the secondary founders. To obtain insight in these processes, 226 G.pallida populations from the Netherlands were analyzed with 2- DGE. The results strongly suggest that these populations originate from one source, or in case of multiple introductions, from a number of sources with a similar genetic makeup.The genetic differentiation of the 226 G.pallida populations indicate that the colonization of the Netherlands has been accompanied by extensive random genetic drift. Only a limited proportion of the populations appeared to be identical. It is also shown, that the bottleneck effects differ between regions. Significant variation in population structure was observed between the three investigated areas. The genetic variation within G.pallida populations from north Friesland and the IJsselmeerpolders is in general smaller than within populations from the northeast Netherlands. A plausible explanation for this phenomena is the low multiplication rate of potato cyst nematodes due to wider crop rotation schemes in the IJsselmeerpolders and north Friesland. These factors result in a slow expansion of newly founded populations, which enhances the effect of drift.gene-pool similarity conceptThe gene-pool similarity concept rests on the hypothesis that in the absence of selection pressure by host plant resistance, degrees of similarity between populations revealed by molecular techniques are also reflected at virulence loci, including those not yet resolved. To test this concept, the genetic variation revealed by 2-DGE among 102 G.pallida populations was compared with the variation in virulence towards two resistant cultivars. This analysis showed that a decrease in genetic distance among populations is accompanied with a decrease in variation in virulence. In addition it is demonstrated that the gene pool similarity concept is also applicable to loci determining the variation in fitness among populations. The variation in Pf/Pi values among the 102 populations on Désirée is in general smaller between closely related populations than between distantly related populations.breeding for resistanceBreeding for resistance has been dominated by trial-and-error approaches, which has stimulated the view that control by means of host plant resistance is unavoidably a short-term approach due to the 'appearance' of virulent populations. The pervasive myth that breeding for resistance against potato cyst nematodes is a lost arms race is challenged by the results of this thesis. Until recently it was assumed that the genetic variation of G.pallida in the Netherlands was too large to produce potato cultivars with broad-spectrum resistance. In this thesis it is shown that the genetic diversity introduced from the Andes region has been limited and that the variation among the Dutch G.pallida populations is mainly the result of random genetic drift. The elaborate analysis of 226 Dutch G.pallida populations offers perspectives to obtain potato cultivars with broad and durable resistance. The gene pool similarities revealed by 2-DGE can be used as guidance in testing the effectiveness of new sources of resistance.

The potato cyst nematodes Globodera rostochiensis (Woll.) Skarbilovich and G. pallida (Stone) originate from the Andes region in South America and have been introduced into Western Europe since 1850. Both species are successful colonizers. Once primary founders have established vital populations, an area is rapidly colonized by secondary founding events. The mode of spread results in patchy distribution patterns. Analyses of the processes that influence the spatial variations in virulence are of major importance for the control by means of host plant resistance. The ability to unravel the mosaic distribution patterns of the two species and their pathotypes enables breeders and growers to anticipate on the dynamics of virulent populations. The aim of this thesis was to analyse the intra- and interspecific variation of G. rostochiensis and G. pallida in the Netherlands and to obtain insight in the processes that determine the spatial variations in protein, DNA and (a)virulence polymorphisms.interspecific variationThe molecular variation between the sibling species G.rostochiensis and G.pallida is remarkably large. The RAPD technique revealed a total of 250 DNA fragments, of which only nine DNA fragments were common to both species (Chapter 3). Similar results were obtained with the AFLP assay. A total of 1000 AFLP fragments was amplified, of which only 64 fragments could be identified in both species (Chapter 4). These results agreed with previous investigations using 2-DGE and show that morphologically nearly indistinguishable organisms can be quite distinct the molecular level.The extensive genetic differentiation of G.rostochiensis and G . pallida offers perspectives for the development of a diagnostic assay. In addition the PCR technique enables the amplification of species specific fragments from small numbers of cysts extracted from soil samples. For example, G. rostochiensis specific DNA fragments can be amplified from single juveniles (Chapter 1).intraspecific variationThe intraspecific variation of G.rostochiensis as revealed by RAPDs and AFLPs is relatively low (Chapter 3 and 4). The proportion of polymorphic DNA fragments among nine G. rostochiensis populations was 19% and 15.8%, respectively. Three clusters of populations were identified and each cluster could be described by one or more specific DNA fragments.The intraspecific variation of G.pallida, as revealed with RAPDs or AFLPs, is larger in comparison with G.rostochiensis (Chapter 3 and 4). The proportion of polymorphic RAPD fragments among 17 populations was 46%. For the AFLP assay this figure was 23% polymorphic AFLP fragments among 15 populations. The majority of the populations displayed a continuous range of variations. Only a few clearly diverged clusters could be discriminated on the basis of specific DNA fragments.Various groups have applied the RAPD technique to study the genetic divergence among conspecific populations of plant parasitic nematodes. The validity of this technique was evaluated in chapter 4 by comparison of the clustering of 36 G.pallida populations based on RAPD and 2-DGE data. Both data sets demonstrated that the majority of the G.pallida populations were not clearly differentiated from each other. The overall correlation between the distance matrices derived from both data sets was low. Careful examination of the separate dendrograms showed similarity in clustering only for clearly diverged populations or groups of populations.Although the AFLP and RAPD technique are simple, fast and require only minute amounts of biological material, they are not suitable to resolve the subtle differences among potato cyst nematode populations. Quantitative variation in allele frequencies is often not resolved with those techniques, which is, among others, due to the virtual inability to recognize co-dominant alleles and the non-linear amplification of DNA fragments. In chapter 6 and 7 the genetic variation was studied by analysing pools of individuals with 2-DGE. The ratios between the protein quantities produced by the codominant alleles are appropriate measures for the allele frequencies. The correctness of this method has been confirmed by 2-DGE of single individuals (De Boer etal ., 1992)bottleneck effects on the secondary foundersThe intraspecific variation among potato cyst nematode populations in Europe is predominantly determined by the genetic constitution of the primary founders, directly or indirectly introduced from South America, and the effect of random genetic drift on the secondary founders. To obtain insight in these processes, 226 G.pallida populations from the Netherlands were analyzed with 2- DGE. The results strongly suggest that these populations originate from one source, or in case of multiple introductions, from a number of sources with a similar genetic makeup.The genetic differentiation of the 226 G.pallida populations indicate that the colonization of the Netherlands has been accompanied by extensive random genetic drift. Only a limited proportion of the populations appeared to be identical. It is also shown, that the bottleneck effects differ between regions. Significant variation in population structure was observed between the three investigated areas. The genetic variation within G.pallida populations from north Friesland and the IJsselmeerpolders is in general smaller than within populations from the northeast Netherlands. A plausible explanation for this phenomena is the low multiplication rate of potato cyst nematodes due to wider crop rotation schemes in the IJsselmeerpolders and north Friesland. These factors result in a slow expansion of newly founded populations, which enhances the effect of drift.gene-pool similarity conceptThe gene-pool similarity concept rests on the hypothesis that in the absence of selection pressure by host plant resistance, degrees of similarity between populations revealed by molecular techniques are also reflected at virulence loci, including those not yet resolved. To test this concept, the genetic variation revealed by 2-DGE among 102 G.pallida populations was compared with the variation in virulence towards two resistant cultivars. This analysis showed that a decrease in genetic distance among populations is accompanied with a decrease in variation in virulence. In addition it is demonstrated that the gene pool similarity concept is also applicable to loci determining the variation in fitness among populations. The variation in Pf/Pi values among the 102 populations on Désirée is in general smaller between closely related populations than between distantly related populations.breeding for resistanceBreeding for resistance has been dominated by trial-and-error approaches, which has stimulated the view that control by means of host plant resistance is unavoidably a short-term approach due to the 'appearance' of virulent populations. The pervasive myth that breeding for resistance against potato cyst nematodes is a lost arms race is challenged by the results of this thesis. Until recently it was assumed that the genetic variation of G.pallida in the Netherlands was too large to produce potato cultivars with broad-spectrum resistance. In this thesis it is shown that the genetic diversity introduced from the Andes region has been limited and that the variation among the Dutch G.pallida populations is mainly the result of random genetic drift. The elaborate analysis of 226 Dutch G.pallida populations offers perspectives to obtain potato cultivars with broad and durable resistance. The gene pool similarities revealed by 2-DGE can be used as guidance in testing the effectiveness of new sources of resistance.

Effects of gene bank seed-increases on the genetic integrity of potato germ plasm is a major concern of gene bank managers. Thus the Association of Potato Inter-gene-bank Collaborators (APIC), a consortium of world potato gene bank leaders, initiated this joint research project using RAPD markers to determine genetic relationships between increased generations within accessions. Solanum jamesii (2n = 2x = 24) and S. fendleri (2n = 4x = 48), two wild potato species native to North America, were used as plant material. These species represented two major breeding systems found among Solanum species: out-crossing diploids and inbreeding disomic tetraploids, respectively. Comparisons were made between populations one generation apart and between sister populations generated from a common source. Fourteen such comparisons within S. jamesii accessions had an average similarity of 96.3%, and 21 such comparisons within S. fendleri accessions had an average similarity of 96.0%. No pairs of populations were significantly different, despite the fact that RAPD markers easily separated all of these very similar accessions within their respective species. Only one of six S. jamesii accessions analyzed showed a significant change in gene frequencies among generations. These findings indicate that there has been minimal loss or change of genetic diversity in ex situ germplasm using the gene bank techniques standard at NRSP-6 and other world potato gene banks.

The application of AFLPs, RAPDs and SSRs to examine genetic relationships in the primary northwestern European cultivated potato gene pool was investigated. Sixteen potato cultivars were genotyped using five AFLP primer combinations, 14 RAPD primers, and 17 database-derived SSR primer pairs. All three approaches successfully discriminated between the 16 cultivars using a minimum of one assay. Similarity matrices produced for each marker type on the basis of Nei and Li coefficients showed low correlations when compared with different statistical tests. Dendrograms were produced from these data for each marker system. The usefulness of each system was examined in terms of number of loci revealed (effective multiplex ratio, or EMR) and the amount of polymorphism detected (diversity index, or DI). AFLPs had the highest EMR, and SSRs the highest DI. A single parameter, marker index (MI), which is the product of DI and EMR, was used to evaluate the overall utility of each marker system. The use of these PCR-based marker systems in potato improvement and statutory applications is discussed.Abbreviations: PCR, polymerase chain reaction; AFLP, amplified fragment length polymorphism; RAPD, randomly amplified polymorphic DNA; DNA, deoxyribonucleic acid; EMR, effective multiplex ratio; DI, diversity index; MI, marker index; RFLP, restriction fragment length polymorphism.

The application of AFLPs, RAPDs and SSRs to examine genetic relationships in the primary northwestern European cultivated potato gene pool was investigated. Sixteen potato cultivars were genotyped using five AFLP primer combinations, 14 RAPD primers, and 17 database-derived SSR primer pairs. All three approaches successfully discriminated between the 16 cultivars using a minimum of one assay. Similarity matrices produced for each marker type on the basis of Nei and Li coefficients showed low correlations when compared with different statistical tests. Dendrograms were produced from these data for each marker system. The usefulness of each system was examined in terms of number of loci revealed (effective multiplex ratio, or EMR) and the amount of polymorphism detected (diversity index, or DI). AFLPs had the highest EMR, and SSRs the highest DI. A single parameter, marker index (MI), which is the product of DI and EMR, was used to evaluate the overall utility of each marker system. The use of these PCR-based marker systems in potato improvement and statutory applications is discussed.

At least 25 species of root and tuber crops from 16 genera and 15 families are native to South America. Apart from the 7 species of potato (Solanum spp.), there are nine lesser known species native to the Andes that are grown for their edible underground organs and are traditionally, but not exclusively, cultivated by indigenous people who use them for subsistence or as cash crops. This book is the first of two volumes to deal in depth with the biology and genetic resources of these Andean root and tuber crops, and, following an introduction, is divided into 4 multiauthor sections: (1) ahipa (Pachyrhizus ahipa) by M. Sorensen, W. J. Gruneberg and B. Orting; (2) arracacha (Arracacia xanthorrhiza) by M. Hermann; (3) maca (Lepidium meyenii) by C. F. Quiros and R. Aliaga Cardenas; and yacon (Smallanthus sonchifolius) by A. Grau and J. Rea. Each section follows the standard format for the series and reviews aspects of their taxonomy, botany, origin, ecology, properties, uses, conservation, evaluation of diversity and breeding. In addition, full accounts are given of crop production areas, agronomy, limitations, prospects and research needs. A single appendix lists research workers by country. (Abstract © CAB ABSTRACTS, CAB International)