Genetic dissection of nonhost resistance of wild lettuce, Lactuca saligna, to downy mildew

Lettuce downy mildew is the most destructive disease in lettuce (Lactuca spp.) cultivation and is caused by Bremia lactucae. The successful cross between its host L. sativa and the nonhost, L. saligna, and offers a rare chance to study the genetics of the nonhost resistance. From a set of 29 Backcross Inbred Lines (BILs) representing in total 96% of the L. saligna genome, 15 introgressions were identified to contribute to this resistance at one to four tested lettuce developmental stages and were provisionally considered as 15 QTLs. QTL pyramiding of four “target QTLs” and the subsequent disease evaluations displayed that the combination of two to three QTLs was enough for almost complete resistance. This shows a redundancy of quantitative genes for nonhost resistance in L. saligna. In histological studies, the pathogen development was obviously arrested earlier in L. saligna than in the BILs with the “target QTL(s)”. The fine mapping of the four “target QTLs” via a Near Isogenic Line (NIL) approach suggested the presence of both single- and multi-QTLs per introgression. In two L. sativa –L. saligna crossing combinations, we observed typical ‘hybrid necrosis’ symptoms, like necrotic lesions on leaves and stems, and retarded growth. This hybrid necrosis is caused by a RIN4-R interaction between RIN4 allele(s) from L. saligna and a probable R gene in L. sativa.