The variability of resistance durability in different potato genotypes harbouring the same resistance QTL but differing by their genetic background was explored. The indirect consequences of the resistance adaptation in terms of local (i.e. genotype-specific) adaptation and cross-virulence was also investigated. Following the virulence of the potato cyst nematode Globodera pallida in a long-term experimental evolution protocol, the results showed that nematode populations were able to adapt to the resistance of four potato genotypes carrying the QTL GpaV from Solanum vernei, and that the plant genetic background has an impact upon the durability of resistance. The pattern of local adaptation observed here indicates that divergent selection has occurred during the experimental evolution performed from the same initial nematode population, and revealed a trade-off between the adaptation to a resistant potato genotype and the adaptation to another resistant genotype differing in its genetic background. In terms of cross-virulence between potato genotypes derived from different resistance sources (S. sparsipilum and S. spegazzinii), this study shows that the adaptation to resistance QTL GpaVvrn does not necessarily allow the adaptation to collinear GpaV loci. The results presented here could be useful for predicting evolution of nematode populations in natural agro-ecosystems and identifying durable strategies for resistance deployment.
The impact of the chemical treatments traditionally used to manage many plant parasites on environmental and human health has resulted in increased restrictions in their use. This is particularly true in the case of plant parasitic nematodes, for which the use of genetic resistance appears to be the most cost-effective alternative control method. Moreover, a large number of resistant cultivars are now available to control different nematode species. The current challenge consists of minimizing potential loss of resistance efficacy due to the repeated use of the same resistant cultivar (or resistance source), as the creation of new cultivars is always the result of long breeding operations.
The development of strategies that ensure the durability of resistance genes addresses research questions related to the adaptation of the parasite to a new environment, i.e. the host plant in the present case. Indeed, for highly specialized parasites like potato cyst nematodes, the genetic composition of host populations represents an essential environmental factor for parasite adaptation (Thrall et al., 2002). In order to increase resistance durability it is, for instance, possible to (i) use cultivars having the same resistance genes but different genetic backgrounds (Palloix et al., 2009; Brun et al., 2010) or (ii) use cultivars derived from different sources of resistance, i.e. carrying different resistance genes or alleles. Although abundant information is available on the direct consequences of selection by resistant host plants on parasite populations and thus on resistance durability (e.g. van der Plank, 1963; Wolfe, 1993; Castagnone-Sereno, 2002; McDonald & Linde, 2002), little is known about its indirect consequences at the intra-species scale (i.e. in terms of genotype-specific adaptation) and at the inter-species scale (i.e. cross-virulence).
Durability of plant resistance is defined as the persistence of resistance efficacy when resistant cultivars are used over long periods, on large areas and in the presence of the target parasite (Johnson, 1981, 1984). In nematology, virulent populations are defined as those able to reproduce significantly on resistant host plants carrying resistance genes that prevent or suppress reproduction of avirulent populations of the same species (Molinari, 2011). Jones & Pawelska (1963) and Turner et al. (1983) performed pioneering research showing that selection by resistant potato genotypes can occur in populations of the cyst nematodes Globodera pallida and G. rostochiensis. It was also later observed against the Mi resistance genes of tomato in the root-knot nematodes Meloidogyne incognita and M. javanica, which differ mainly from cyst nematodes by their parasitism and clonal reproduction mode (Castagnone-Sereno et al., 1996; Verdejo-Lucas et al., 2009).
The framework developed by evolutionary biologists to explore the concept of local adaptation is fully relevant to test the genotype-specific adaptation of a parasite to its hosts. Parasites are usually expected to be locally adapted to sympatric host populations (Kaltz & Shykoff, 1998; Gandon & Michalakis, 2002; Hoeksema & Forde, 2008). Two distinct criteria can be used to test for local adaptation (Gandon & Van Zandt, 1998; Kawecki & Ebert, 2004): (i) comparing the fitness of a parasite population in different host populations (the ‘home vs away’ criterion) or (ii) comparing the fitness of a parasite population in its host of origin with that of other parasite populations from different host populations (the ‘local vs foreign’ criterion). When hosts show different levels of resistance, the ‘home vs away’ criterion is poorly informative regarding host adaptation, because it is biased by host quality; therefore, the relevant comparison to highlight divergent selection is the ‘local vs foreign’ criterion (Thrall et al., 2002; Kawecki & Ebert, 2004; Montarry et al., 2008). The issue of local adaptation is usually addressed using naturally evolved populations (for reviews see Kaltz & Shykoff, 1998; Kawecki & Ebert, 2004; Greischar & Koskella, 2007; Hoeksema & Forde, 2008). However, in an agricultural host–parasite system, where the evolution of the host is driven by man, a one-sided experimental evolution experiment in which only the parasite is allowed to evolve seems more relevant.
While the concept of cross-resistance has been commonly used regarding the response of bacteria to the selection imposed by antibiotic molecules (Pantosti et al., 2007), the concept of cross-virulence has been scarcely used regarding the interactions between resistant hosts and parasites (but see Heinrichs & Rapusas, 1985; Jyoti & Michaud, 2005). The term cross-resistance has been used if the gain of resistance to an antibiotic is associated with the gain of resistance to another antibiotic without any prior confrontation between that last antibiotic and the bacteria (e.g. Levy, 2000; Via et al., 2010). From an evolutionary point of view, cross-resistance and cross-virulence are possible examples of exaptation (Gould & Vrba, 1982). Moreover, from an agronomic point of view, a case of cross-virulence between two resistant cultivars (of the same species or not) is an important aspect to take into account for the identification of durable strategies to deploy plant resistances. For root-knot nematodes, virulent populations selected on tomato cultivars carrying Mi-1.2 did not reproduce on tomato genotypes possessing other Mi genes (Mi2–Mi9) or on resistant pepper lines. Similarly, populations virulent on pepper containing the Me3 gene were not able to develop on pepper containing the Me1 gene, nor on resistant tomato cultivars (Castagnone-Sereno et al., 1996; Williamson, 1998). This suggests an absence of cross-virulence to different resistance genes, but that pattern has not been tested yet regarding collinear resistance loci.
This case study involved the nematode G. pallida, one of the main soilborne pests of potato worldwide (Oerke et al., 1994). Globodera pallida is a cyst nematode which achieves one generation per year under field conditions. As in other cyst nematode species, its second stage juveniles (J2) enter the plant roots and establish a specialized feeding structure, the syncytium (Jones & Northcote, 1972), which is a severe nutrient sink for the plant. Adult males leave the root in order to mate females. After mating, the females continue to feed from the syncytium, allowing eggs to develop. Females then die and their corpses form a cyst, which protects the eggs until infective J2 hatch. Creating cultivars with high levels of resistance to nematodes is a major goal of potato breeders, but questions about the durability of these genotypes are raised. Based on an eight-generation experimental evolution, the present study addressed two main questions: (i) does the durability of different resistant potato genotypes differ due to their genetic backgrounds; and (ii) what are the indirect consequences of the resistance adaptation in terms of local adaptation and cross-virulence?
Materials and methods
To create selected lineages of G. pallida, different resistant potato genotypes were used. The genotypes were derived from three different resistance sources: Solanum vernei, S. spegazzinii and S. sparsipilum (Table 1). The susceptible cultivar Désirée was used in order to create the unselected control lineages. The clones AM78.3736 and AM78.3778 share the same resistant parents in their genealogy, including S. vernei 24/20, S. vernei LGU 8 and S. vernei 1-3, and are thus considered here as S. vernei resistance sources. Genotype 60.96.1 corresponds to the commercial potato cultivar Îledher, which is the first resistant cultivar to G. pallida registered in the French catalogue. All three resistance sources depend on a QTL explaining a large part of the genetic variance and responsible for the development of most nematodes into adult males. This QTL was mapped in the three sources in a collinear position on the potato chromosome V, and is designated as the GpaV locus. Additional minor QTLs were also mapped on chromosomes VI, IX, XI and XII (Rouppe van der Voort et al., 1998, 2000; Bryan et al., 2002; Caromel et al., 2003, 2005; Bakker et al., 2011). In this study, the locus Gpa5 described by Rouppe van der Voort et al. (2000) was renamed GpaVvrn, and the collinear QTLs from the S. spegazzinii and S. sparsipilum resistance sources were designated as GpaVspg and GpaVspl, respectively. For all potato clones used here, the presence of the GpaV locus was previously confirmed by cleaved amplified polymorphic sequence (CAPS) analysis (data not shown) as described by Caromel et al. (2011). The potato genotypes 96D.31.68 (from S. sparsipilum) and 96D.32.77 (from S. spegazzinii) have additional minor-effect QTLs (see Table 1). The presence of minor QTLs in the resistant material from S. vernei was not determined.
Table 1. Characteristics of the different resistant potato genotypes used to create the evolutionary lineages of Globodera pallida and to test local adaptation and cross-virulence
(AM78.3778 * Fanette) * Mélissa
AM78.3778 * Van Gogh
(AM78.3778 * Mondial) * 90F.136.3
(Karna * AM78.3736) * (Katja * Amex)
Caspar H3 * spl188S329.18
Caspar H3 * spl188S329.18
Rosa H1 * spg88S334.19
GpaVspg GpaVIspg GpaXIIspg
Rosa H1 * spg88S334.19
Nematode lineages were established using cysts of two French natural G. pallida populations, GpSM (near Saint Malo) and GpN (on the island of Noirmoutier), coming from two highly infested fields (3 J2 and 82 J2 per g of soil for GpSM and GpN, respectively). The lineages were obtained by rearing both populations during eight successive cycles (i.e. eight generations) on the resistant genotypes 60.96.1, 94T.146.52, 96F.376.16 and 360.96.21 and on the susceptible cv. Désirée, to create selected and control lineages (Fig. 1).
Because of the high level of resistance of the potato genotypes used here, obtaining lineages able to develop on them required exposure of large numbers of plants to each population. The first three generations were thus made directly into the fields. In both locations, fields were split into five sections, and each section was seeded with the same (resistant or susceptible) potato genotype for three consecutive years. The next two generations were obtained by sampling, in both fields, 20 L of soil in each section, placing these soil samples in tanks under greenhouse conditions and growing the corresponding potato genotype for two years. After these first five cycles, for each lineage, all the cysts (corresponding to generation 5x) were extracted from the soil using a Kort elutriator. The following generations (6x, 7x and 8x) for each lineage were then obtained by using 10 cysts (on average 1500 J2 individuals), coming from the previous cycle, contained in a tulle bag and placed in a 13 cm pot filled with a soil mixture free of cysts (2:1 sand:natural field soil). Tubers of the corresponding genotypes were then planted and covered with the same soil mixture. Four replicate plants were used for each potato genotype. Potatoes were grown in the greenhouse, under controlled conditions (20°C ± 4), for 120 days. Newly formed cysts from the four replicates were then extracted from the soil, using a Kort elutriator, pooled and stored at 4°C for a minimum of four months (corresponding to the diapause time). The four samples of 10 cysts required for the next generation were then randomly selected.
The nematode lineages used as control were GpSM-Désirée-8x and GpN-Désirée-8x (x corresponds to the number of successive generations on the corresponding potato genotype). These controls allow the effects of repeated selection imposed by the resistant potato genotypes to be separated from those of the laboratory multiplication process.
Selection effect and resistance durability
For both populations (GpSM and GpN), response to selection was measured by estimating the virulence level of each lineage after five and eight generations and comparing it with the virulence of the unselected control lineage. The virulence of a given lineage was estimated as described by Mugniery & Person (1976). Briefly, pieces of germinated tubers were allowed to emit roots in a Petri dish containing 20 g L−1 water agar, and one of these roots was inoculated with 10 newly hatched larvae coming from cysts placed into root exudates. After inoculation, Petri dishes were kept in total darkness, under controlled conditions (20 ± 1°C), for 15 days. After this period, every inoculated root was dissected in water under a transillumination unit for bright-field/dark-field, and the number of developed females was counted. Virulence level was then calculated for each lineage as the ratio between numbers of females and inoculated larvae. Virulence of each nematode lineage was assessed in this way on its corresponding potato genotype, with 12 to 29 roots inoculated independently. The effect of the number of generations (control population, 5x and 8x) on female numbers was tested for each nematode population (GpSM and GpN) and for each potato genotype (60.96.1, 94T.146.52, 96F.376.16, 360.96.21 and Désirée) through a one-way anova. The resistance durability of each potato genotype was evaluated by comparing the virulence level of each corresponding lineage after eight generations of selection using a second anova model.
Local adaptation was tested by comparing the virulence levels of the selected lineages GpSM-60.96.1-8x and GpSM-94T-8x on the resistant genotypes 60.96.1 and 94T.146.52, using the same experimental procedure as described in the first set of experiments. The test was independently replicated (i.e. the number of inoculated roots) between 12 and 32 times for each nematode population–potato genotype combination.
The effects of adapted nematode lineages (GpSM-60.96.1-8x and GpSM-94T-8x), potato genotypes (60.96.1 and 94T.146.52) and their interaction on the female production were tested through a multiway anova. Local (mal)adaptation results in a significant interaction term between nematode lineage and potato genotype. Contrast tests were used to explore the ‘local vs foreign’ (on each test cultivar) effects.
The occurrence of cross-virulence was determined using the selected and unselected lineages obtained on the resistant genotype 60.96.1, derived from S. vernei, and on the susceptible cultivar Désirée, respectively. Virulence levels of the selected lineages GpSM-60.96.1-8x and GpN-60.96.1-8x were estimated using the same experimental procedure as described in the first set of experiments, on the resistant genotypes 96D32.32 and 96D32.77, derived from S. spegazzinii, and on the resistant genotypes 96D31.51 and 96D31.68, derived from S. sparsipilum. The unselected lineages GpSM-Désiré-8x and GpN-Désiré-8x were used as controls. The test was independently replicated between 9 and 20 times for each nematode population–potato genotype combination.
The cross-virulence of the adapted populations GpSM-60.96.1-8x and GpN-60.96.1-8x on the resistant potato genotypes 96D.32.32, 96D.32.77, 96D.31.51 and 96D.31.68 was tested using a multiway anova. The effects of nematode population (GpSM or GpN), selection (i.e. the comparison between the unselected control populations and the adapted populations), potato genotype (96D.32.32, 96D.32.77, 96D.31.51 and 96D.31.68) and their interactions on the female production were tested. Contrast tests were used to explore the selection effects on each potato cultivar.
All statistical analyses were performed using R software version 2.12.0 (R Development Core Team, 2010). Normality and homogeneity of variances were checked by the Shapiro–Wilk and the Leven tests, respectively, and all parametric tests were confirmed by the non-parametric Kruskal–Wallis test. When significant effects were detected, multiple comparisons of means were performed with the Tukey contrasts test (α = 0.05).
Selection effect and resistance durability
The four resistant plant genotypes derived from S. vernei showed the same resistance efficacy against the control populations, but contrasted in their resistance durability (Fig. 2). The lineages GpSM-360.96.21, GpN-94T and GpN-96F were lost during the experimental evolution. For the other lineages, the anova model showed a significant generation effect on the female production whatever the resistant potato genotype (F2,49 =286.610, P <0.0001, F2,54 =34.288, P < 0.0001 and F2,58 =11.703, P < 0.0001 for GpSM on the genotypes 60.96.1, 94T.146.52 and 96F.376.16, respectively, and F2,53 =100.30, P < 0.0001 and F2,54 = 18.376, P <0.0001 for GpN on the genotypes 60.96.1 and 360.96.21, respectively) revealing that the nematode populations were able to adapt to those resistant potato genotypes. The absence of a significant generation effect on the susceptible potato genotype Désirée (F1,50 = 0.293, P = 0.5909 for GpSM-Désirée and F1,37 = 3.216, P = 0.0811 for GpN-Désirée) indicates that the differences in female numbers between generations observed on the resistant potato genotypes result from the selection pressure imposed by the resistance rather than from the way the selected lineages were created. For both nematode populations, female numbers on potato genotype 60.96.1 were significantly higher for the 5x lineages than for the controls, and significantly further increased in the 8x lineages (Fig. 2). The same result was observed for lineage GpN-360.96.21 on its host of selection. By contrast, on potato genotypes 94T.146.52 and 96F.376.16, no significant difference in female numbers between GpSM-94T-5x or GpSM-96F-5x and their respective unselected counterparts was observed, but female production was significantly higher for the selected 8x lineages GpSM-94T-8x and GpSM-96F-8x (Fig. 2). After eight generations of selection, the virulent lineage GpSM-96F produced only 8% of developed females, compared to 20% for the virulent lineage GpSM-94T, 28.2% for the virulent lineage GpN-360.96.21, and 62.5% and 53.2% for the virulent lineages GpSM-60.96.1 and GpN-60.96.1, respectively. These last two values were identical (statistics not shown) to those obtained with the unselected control lineages on the susceptible cv. Désirée (Fig. 2).
Because of the limited number of cysts extracted, the nematode population adapted to the potato genotype 96F.376.16 could not be used for the local (i.e. genotype-specific) adaptation experiment, which was therefore tested using the nematode population GpSM adapted to the genotypes 60.96.1 and 94T.146.52. All tested explanatory factors (i.e. nematode lineage, potato genotype and their interaction) had a significant effect on female production (Table 2). As expected, the anova model showed a highly significant host effect; the comparison of means showed that genotype 94T.146.52 was three times more resistant than genotype 60.96.1 (Figs 2 and 3). The highly significant interaction term between nematode lineage and host genotype (Table 2) is evidence for local adaptation in this pathosystem. For each potato genotype, the performance of the sympatric population (i.e. the local or resident pathogen population) was twice as high as the performance of the allopatric population (i.e. the foreign or non-resident pathogen population): GpSM-60.96.1-8x produced more females than GpSM-94T-8x on the potato genotype 60.96.1 and GpSM-94T-8x produced more females than GpSM-60.96.1-8x on the potato genotype 94T.146.52 (Table 2; Fig. 3).
Table 2. Results from the analyses of variance (anova) assessing the effects of nematode lineage (GpSM-60.96.1-8x and GpSM-94T-8x), of the potato genotype (60.96.1 and 94T.146.52), and of the corresponding two-way interaction of these factors on female production
Source of variation
P > F
Contrasts show the ‘local vs foreign’ comparisons for each potato genotype. The statistically significant effects are indicated as *P < 0.05, **P < 0.01 and ***P < 0.001.
Nematode lineage * Potato genotype
Potato genotype 60.96.1
Potato genotype 94T.146.52
Two different patterns were observed regarding cross-virulence to other resistance sources (S. spegazzinii and S. sparsipilum) in lineages selected on the resistant genotype 60.96.1 (derived from S. vernei). Populations GpSM-60.96.1-8x or GpN-30.96.1-8x did not differ significantly from control nematode populations on host genotypes 96D.31.51 and 96D.31.68, derived from S. sparsipilum (Table 3; Fig. 4), for which resistance remained effective. Conversely, the nematode lineages GpSM-60.96.1-8x and GpN-60.96.1-8x produced more females than the unselected control nematode populations on both potato genotypes derived from S. spegazzinii, 96D.32.32 and 96D.32.77 (Table 3; Fig. 4). These patterns were observed for the two independent nematode populations (GpSM and GpN), and the nematode origin effect and the interactions with the other main effects were statistically not significant (Table 3).
Table 3. Results from the analyses of variance (anova) assessing the effects of nematode population (GpSM and GpN), selection (selected and unselected lineages), potato genotype (96D.32.32, 96D.32.77, 96D.31.51 and 96D.31.68), and the corresponding interactions of these factors on female production
Source of variation
P > F
anc: not calculable.
Contrasts show the ‘selected vs unselected’ comparisons for each potato genotype. The statistically significant effects are indicated as *P < 0.05, **P < 0.01 and ***P < 0.001.
Nematode population * Selection
Nematode population * Potato genotype
Selection * Potato genotype
Nematode population * Selection * Potato genotype
Potato genotype 96D32.32
Potato genotype 96D32.77
Potato genotype 96D31.51
Potato genotype 96D31.68
As resistant cultivars are often the result of a long process of breeding, the evaluation of resistance durability is essential. This is particularly true in the present case study, G. pallida–potato, because only a single resistant potato cultivar (cv. Îledher) is actually registered in the ware category of the French catalogue. The general objectives of this paper were therefore first, to screen the durability of different highly resistant potato genotypes carrying the major QTL GpaV from S. vernei, and secondly, to test the efficacy of potato genotypes derived from other resistance sources (S. spegazzinii and S. sparsipilum) to control the virulent lineages selected on S. vernei, i.e. to explore the indirect consequences of adaptation to resistance in terms of local adaptation and cross-virulence.
The results demonstrated clearly that from ‘avirulent’ field populations of the cyst nematode G. pallida, which probably included a small number of virulent individuals, the selection by resistant plants, mimicked by repeated inoculations, produced virulent lineages. Contrary to plant viruses, where mutation is one of the main evolutionary forces involved in the breakdown of resistance (Montarry et al., 2011), the fact that some nematode lineages (GpSM-360.96.21, GpN-94T and GpN-96F) were lost during the experimental evolution experiment constitutes an additional argument for the initial presence of virulence alleles in the nematode populations. Accordingly, for G. rostochiensis, the H1 resistance gene which has never been overcome in Great Britain was overcome in the Netherlands (Zaheer et al., 1993), suggesting that the composition of nematode populations is crucial for resistance durability.
Contrary to earlier experiments by Turner et al. (1983) and Phillips & Blok (2008), which were performed on Solanum hybrids with low to moderate resistance levels, all potato genotypes used here showed a similar and very high resistance level, rated between 7 and 9 on the 0 to 9 scale used in the EU Council Directive 2007/33/EC (Anonymous, 2007). Under the specific experimental conditions here, eight generations were sufficient to completely overcome the resistance of genotype 60.96.1, but not that of the three other resistant clones, although all four potato genotypes possess the QTL GpaVvrn. Consequently, the genetic backgrounds of the potato genotypes 94T.146.52, 360.96.21 and 96F.176.16 probably include some additional resistance factors which make their resistance more difficult to overcome. Rouppe van der Voort et al. (2000) found a minor QTL on chromosome IX, and it is also known that some minor QTLs could be present in the susceptible parents. This result is only the third report supporting the idea that the genetic background in which a major QTL is introgressed plays an important role for the durability of the resistance, as recently shown for a virus and a fungus. Palloix et al. (2009) showed that the plant genetic background in which the major-effect resistance gene pvr23, which acts against Potato virus Y (PVY), was introduced plays a critical role on the durability of pvr23. The frequency of resistance breakdown by PVY was high when pvr23 was introgressed into a susceptible genetic background, whereas no resistance breakdown occurred when pvr23 was combined to partial-effect resistance QTLs. A similar conclusion was reached in the pathosystem Leptosphaeria maculans–Brassica napus: the combination between a qualitative and a quantitative resistance enhances the durability of the major resistance gene (Brun et al., 2010).
The existence of contrasted durability in genotypes derived from S. vernei, and the fact that, unfortunately, the resistance of cv. Îledher (60.96.1) was the least durable, raise the question of the ability of a potato genotype derived from the same source of resistance to control the virulent lineages selected by cv. Îledher. This question was addressed in a second experiment: the efficacy was investigated of both 60.96.1 and 94T.146.52 genotypes to control the virulent lineages GpSM-60.96.1-8x and GpSM-94T-8x selected on each of them. The pattern of local (i.e. genotype-specific) adaptation observed here indicates that divergent selection has taken place during the eight successive cycles of the experimental evolution performed from an initial nematode population on two different resistant potato genotypes. Because both sympatric populations were fitter than both allopatric populations, a trade-off seems to exist between adaptation to a resistant potato genotype and adaptation to another resistant genotype. Consequently, the pattern of local adaptation highlighted here could be used to predict evolution of populations in agro-ecosystems and thus to identify durable strategies of resistance deployment. A spatiotemporal strategy of diversification combining potato cultivars derived from the same resistant source and carrying the same major QTL GpaVvrn, but differing by their genetic background, could thus be designed. However, because gene flow is the main evolutionary force that acts against local adaptation (Gandon & Van Zandt, 1998; Kaltz & Shykoff, 1998; Hoeksema & Forde, 2008; Montarry et al., 2008) those predictions could be biased by dispersal abilities of nematodes. Indeed, as cyst nematodes have only a limited potential for active dispersal (Crofton, 1971), passive dispersion through human activities such as soil tillage, movement of agricultural material between fields, soil losses due to crop harvesting (Ruysschaert et al., 2007), and possibly also through soil and seed transport on a large scale by wildlife, such as wild boar and deer (e.g. Boulanger et al., 2011) generates high gene flows (Plantard & Porte, 2004; Picard & Plantard, 2006).
Looking for resistant genotypes derived from different resistance sources (in particular S. spegazzinii and S. sparsipilum) could also be a potential way to control populations virulent to S. vernei. Unfortunately, the data show that cross-virulence occurs between S. vernei and S. spegazzinii, whatever the combination of genes used: for both genotypes, the virulent lineages adapted to the genotype 60.96.1 had the capacity to overcome the resistances without any prior confrontation. This phenomenon does not exist for S. sparsipilum. Consequently, adaptation to QTL GpaVvrn does not necessarily allow adaptation to all collinear GpaV loci, such as QTL GpaVspl. From a phylogenetic point of view, the results make sense because Spooner et al. (2005) showed, using AFLP data, that S. vernei and S. spegazzinii are genetically close relatives, both much more distant from S. sparsipilum. Those results have important implications for potato breeders and for the management of potato resistances: the development of new cultivars based only on the S. spegazzinii source will probably not be the most promising way to control nematode populations adapted to S. vernei resistances. In deployment strategies, it would be better to develop new tetraploid cultivars from S. sparsipilum and (i) use them in rotation with clones carrying S. vernei resistance source, or (ii) associate them with a genotype carrying S. vernei resistance.
The present study revealed resistance in different genetic backgrounds with contrasting durability. Moreover, there are now nematode populations with (after the selection by resistant plants) and without (before the selection by resistant plants) the resistance-breaking capacity. Future work will allow researchers to search and find life history traits, of the plant and of the parasite, which are correlated with the durability of resistance, and which could be used to predict the durability of future resistant potato cultivars.
Financial support from the French Ministry of Agriculture through the CTPS grant C07-01-pomme-de-terre is gratefully acknowledged. Genotypes 60.96.1 (cv. Îledher), 360.96.21 and 96F.376.16 were kindly provided by the potato breeding companies Grocep, Comité-Nord and Bretagne-Plants, respectively. The authors gratefully acknowledge Dr D. Mugniéry who initiated the experimental evolution, Dr B. Caromel who provided CAPS markers to follow QTL GpaV, Dr E. Grenier and Dr D. Andrivon for comments on earlier drafts of this manuscript and Dr B. Moury for useful discussions about the concept of cross-virulence.