Genetic Mapping of Resistance to Phytophthora cinnamomi for American Chestnut Restoration in the Southeastern United States

Phytophthora cinnamomi (Pc) is a major pathogen of the American chestnut, Castanea dentata, that has been killing trees in the southern range of this species for over 200 years, long before chestnut blight was first reported in North America. To identify and genetically characterize resistance determining regions of the Chinese chestnut genome, we are collaborating on screening hybrid chestnut seedlings from the The American Chestnut Foundation's breeding program for resistance to Pc. Using the backcross families from crosses of American chestnut trees with two resistant Chinese chestnut trees, ‘Mahogany’ and ‘Nanking’, we conducted linkage mapping and QTL detection for Pc resistance introgressed from the Chinese chestnut. Utilizing a genome-wide SNP array, an initial low-density genetic map was constructed for the cross of AdairKY1 × GL158 ('Nanking' background). Resistance to Pc was mapped to linkage group E (LG_E). Using a set of LG_E SSR markers from the Chinese chestnut reference map, we also genotyped additional hybrid populations and generated local LG_E maps for half-sib crosses NK1+NK2 ('Nanking' background) and KY115 × AD98 ('Mahogany' background). Overlapping QTL for Pc resistance were detected in both Pc resistance sources. Subsequently, we extended and phenotyped hybrid populations for resistance to Pc. Altogether, 1408 phenotyped individuals (5 crosses) are now available and are being mapped using “Genotype By Sequencing” (GBS) platforms. We have currently completed a GBS map and QTL analysis for the HB2 and 'Nanking' families. Here we present the results of this collaborative research effort and discuss the implications for breeding Pc resistant American chestnut trees.