Both epistatic and additive effects of QTLs are involved in polygenic induced resistance to disease: a case study, the interaction pepper - Phytophthora capsici Leonian.

To study the resistance of pepper to Phytophthora capsici, we analyzed 94 doubled-haploid (DH) lines derived from the intraspecific F1 hybrid obtained from a cross between Perennial, an Indian pungent resistant line, and Yolo Wonder, an American bell-pepper susceptible line, with 119 DNA markers. Four different criteria were used to evaluate the resistance, corresponding to different steps or mechanisms of the host-pathogen interaction: root-rot index, receptivity, inducibility and stability. Three distinct ANOVA models between DNA marker genotypes and the four disease criteria identified 13 genomic regions, distributed across several linkage groups or unlinked markers, affecting the resistance of pepper to P. capsici. Some QTLs were criterion specific, whereas others affect several criteria, so that the four resistance criteria were controlled by different combinations of QTLs. The QTLs were very different in their quantitative effect (R(2) values), including major QTLs which explained 41-55% of the phenotypic variance, intermediate QTLs with additive or/and epistatic action (17-28% of the variance explained) and minor QTLs. Favourable alleles of some minor QTLs were carried in the susceptible parent. The total phenotypic variation accounted for by QTLs reached up to 90% for receptivity, with an important part due to epistasis effects between QTLs (with or without additive effects). The relative impact of resistance QTLs in disease response is discussed.