Search

Understanding the relationships between host range and pathogenicity for parasites, and between the efficiency and scope of immunity for hosts are essential to implement efficient disease control strategies. In the case of plant parasites, most studies have focused on describing qualitative interactions and a variety of genetic and evolutionary models has been proposed in this context. Although plant quantitative resistance benefits from advantages in terms of durability, we presently lack models that account for quantitative interactions between plants and their parasites and the evolution of these interactions. Nestedness and modularity are important features to unravel the overall structure of host-parasite interaction matrices. Here, we analysed these two features on 32 matrices of quantitative pathogenicity trait data gathered from 15 plant-parasite pathosystems consisting of either annual or perennial plants along with fungi or oomycetes, bacteria, nematodes, insects and viruses. The performance of several nestedness and modularity algorithms was evaluated through a simulation approach, which helped interpretation of the results. We observed significant modularity in only six of the 32 matrices, with two or three modules detected. For three of these matrices, modules could be related to resistance quantitative trait loci present in the host. In contrast, we found high and significant nestedness in 30 of the 32 matrices. Nestedness was linked to other properties of plant-parasite interactions. First, pathogenicity trait values were explained in majority by a parasite strain effect and a plant accession effect, with no or minor parasite-plant interaction term. Second, correlations between the efficiency and scope of the resistance of plant genotypes, and between the host range breadth and pathogenicity level of parasite strains were overall positive. This latter result questions the efficiency of strategies based on the deployment of several genetically-differentiated cultivars of a given crop species in the case of quantitative plant immunity.

Germplasm collections of cultivated plants constitute the source for further genetic progress. These collections gained further interest with approaches for tracking allelic variants associated to phenotypic variations within core collections. In order to explore the structure of genetic variation in pepper ( Capsicum spp.) and to select core-collections maximizing the genetic and the phenotypic diversity, a pepper collection including 1,352 non redundant accessions from 11 Capsicum species from 89 different countries was genotyped using 28 microsatellite (SSR) markers spanning the whole genome. Model-based analysis structured the collection into 6 clusters, with 3 clusters separating the main species complexes, including cultivated species and wild relatives, according to taxonomic classification ( C. frutescens/C. chinense, C. baccatum, C. pubescens), and 3 additional clusters for C. annuum. The relationships between the cultivated C. annuum species and the wild relative ( C. annnuum var. glabriusculum) was precised. The 3 C. annuum clusters were significantly distinct for plant and fruit descriptors corresponding to cultivar types, showing that the genetic structure of this cultivated species was strongly impacted by the long term human selection of landraces in primary as well as secondary diversification centres. We settled nested core-collections of 8, 16, 32, 64 and 128 C. annuum accessions capturing from 37 to 90 % of the genetic diversity for further sequencing efforts and establishment of high throughput genotyping assays. By compiling phenotypic and genotypic data, a larger core-collection of 332 accessions was established, capturing 97 % of the C. annuum genetic and phenotypic diversity for further genetic association studies. [ABSTRACT FROM AUTHOR]