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Identification and Characterization of Random Amplified Polymorphic DNA Markers Linked to a Major Gene (Cr2) for Resistance to Cronartium ribicola in Pinus monticola

DNA markers tightly linked to resistance (R) genes provide a very powerful tool for both marker-assisted selection in plant breeding and positional cloning of R genes. In the present study, a linkage of random amplified polymorphic DNA (RAPD) markers to the single dominant gene (Cr2) for resistance to white pine blister rust fungus (Cronartium ribicola) was investigated in western white pine (Pinus monticola). A mapping population of 128 individual megagametophytes was generated from seeds of a heterozygous resistant tree (Cr2/cr2), and the corresponding seedlings of each megagametophyte were subjected to the test of phenotype segregation by inoculation with C. ribicola. Bulked segregant analysis and haploid segregation analysis identified eight robust RAPD markers linked to Cr2. This constitutes the first Cr2 genetic linkage map spanning 84.7 cM with four markers only 3.2 cM from Cr2. One sequence (U256-1385) of these linked markers was significantly similar to the Ty3/gypsy-like long terminal direct repeats retrotransposons. Another marker, U570-843, had no significant similarity to any entry in either GenBank or the loblolly genomics data bank. As presumed that the average physical distance per centimorgan is about 10 Mb in P. monticola, it is probably unrealistic to use these DNA markers for positional cloning of the Cr2 gene.