Quantitative resistance has gained interest in plant breeding for pathogen control in low-input cropping systems. Although quantitative resistance frequently has only a partial effect and is difficult to select, it is considered more durable than major resistance (R) genes. With the exponential development of molecular markers over the past 20 years, resistance QTL have been more accurately detected and better integrated into breeding strategies for resistant varieties with increased potential for durability. This review summarizes current knowledge on the genetic inheritance, molecular basis, and durability of quantitative resistance. Based on this knowledge, we discuss how strategies that combine major R genes and QTL in crops can maintain the effectiveness of plant resistance to pathogens. Combining resistance QTL with complementary modes of action appears to be an interesting strategy for breeding effective and potentially durable resistance. Combining quantitative resistance with major R genes has proven to be a valuable approach for extending the effectiveness of major genes. In the plant genomics era, improved tools and methods are becoming available to better integrate quantitative resistance into breeding strategies. Nevertheless, optimal combinations of resistance loci will still have to be identified to preserve resistance effectiveness over time for durable crop protection.

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 GpaV vrn 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.

Understanding the factors that contribute to population genetic divergence across a species' range is a long-standing goal in evolutionary biology and ecological genetics. We examined the relative importance of historical and ecological features in shaping the present-day spatial patterns of genetic structure in two related plant species, Beta vulgaris subsp. maritima and Beta macrocarpa. Using nuclear and mitochondrial markers, we surveyed 93 populations from Brittany (France) to Morocco – the southern limit of their species' range distribution. Whereas B. macrocarpa showed a genotypic structure and a high level of genetic differentiation indicative of selfing, the population genetic structure of B. vulgaris subsp. maritima was consistent with an outcrossing mating system. We further showed (1) a strong geographic clustering in coastal B. vulgaris subsp. maritima populations that highlighted the influence of marine currents in shaping different lineages and (2) a peculiar genetic structure of inland B. vulgaris subsp. maritima populations that could indicate the admixture of distinct evolutionary lineages and recent expansions associated with anthropogenic disturbances. Spatial patterns of nuclear diversity and differentiation also supported a stepwise recolonization of Europe from Atlantic-Mediterranean refugia after the last glacial period, with leading-edge expansions. However, cytoplasmic diversity was not impacted by postglacial recolonization: stochastic long-distance seed dispersal mediated by major oceanic currents may mitigate the common patterns of reduced cytoplasmic diversity observed for edge populations. Overall, the patterns we documented here challenge the general view of reduced genetic diversity at the edge of a species' range distribution and provide clues for understanding how life-history and major geographic features interact to shape the distribution of genetic diversity.