Genetic investigation of the nonhost resistance of wild lettuce, Lactuca saligna, to lettuce downy mildew, Bremia lactucae

Abstract Downy mildew (Bremia lactucae) in lettuce (Lactuca sativa) is a devastating foliar disease causing high losses in lettuce cultivation. The wild lettuce and nonhost species, Lactuca saligna, is absolute resistant to downy mildew and cross-fertile with L. sativa, albeit with a low success rate and occasional reduced fertility and/or vitality in later inbred generations. This exceptional availability of hybrid plant offsprings creates a unique opportunity to study nonhost resistance by a genetic approach. The L. saligna nonhost resistance genes might be more durable than the classical monogenic race-specific R genes that are mainly used in lettuce breeding. The identification of genes conferring nonhost resistance is a crucial step in its understanding and usage in breeding. In this thesis the quantitative resistances of three backcross introgression lines (BILs), carrying an individual 30 to 50 cM long introgression segment from L. saligna in a L. sativa background, were fine mapped. Disease evaluation of sub-BILs with smaller introgression segments revealed that the resistance of all three BILs was explained by 17 sub-QTLs with a smaller and plant stage dependent effect, some segments reducing, others even promoting downy mildew infection. Further the potential of stacking quantitative resistances of eight BILs per combinations of two was tested under field conditions. Only three out of ten double-combinations resulted in an increased resistance level compared to their parental individual lines, from which one had additive and two had epistatic interactions between the introgressions. As the studies on individual QTL effects of BILs did not reveal potential genetic interactions that could explain the complete resistance of L. saligna, a novel approach was set out to search for indications of epistatic interactions. ‘Selective genotyping’ was applied on the phenotypic disease extremes of large F2 offsprings, in which multi-locus interactions between L. saligna alleles are still prevalent. In a kind of bulked segregant analysis approach four major resistance regions were identified. Preliminary results showed epistatic interactions between the regions on Chromosome 6 and 1 and between Chromosome 6 and 7. During the development of sub-BILs, a digenic hybrid incompatibility was observed: plants carrying a L. saligna segment on Chromosome 6 always required a L. saligna segment on Chromosome 4. Segregation analysis suggested a prezygotic reproductive barrier by non-transmission of one specific hybrid gametophyte (male and female). In cooperation with the research group of Guido van den Ackerveken of University Utrecht a lettuce germplasm screening was conducted for candidate downy mildew effector proteins, which interact with resistance genes in lettuce and trigger a defence response. One of two responsive effector proteins, ‘BLG01’, triggered a hypersensitive cell death response in most tested L. saligna accessions and its response was mapped on Chromosome 9. Despite the complex interactions between resistance QTLs, this thesis research has delivered many insights that are important steps forward towards understanding the incompatible interaction between B. lactucae and L. saligna and its future application in resistance breeding.