Identification of RFLP markers for the rhm gene --Yan-San Chyi, Manju Gupta, Matthew J. Frome, F. Dale Park, Bradley Hoo, David Zaitlin, William D. Banks, James Perkins and Philip Filner Correlations between 2 RFLP (restriction fragment length polymorphism) markers on chromosome 6 and the rhm gene for resistance to Bipolaris maydis (Nisik.) Shoemaker, race O were detected by comparing the occurrences of RFLPs in inbred and rhm-converted lines of B73, H95, Mo17, N28 and Va35. The marker SGCR148 detects RFLP's between inbred and rhm-converted line pairs in 5 out of 5 cases. The restriction fragment detected in all 5 rhm lines is the same size and was not observed in 45 other inbred lines lacking rhm. SGCR65 detects RFLPs in 2 out of 5 inbred vs. converted line pairs. Genetic linkage between rhm and the loci detected by SGCR148 and SGCR65 was established by analysis of an F2 population segregating for these loci. The order of the loci is rhm - SGCR148 - SGCR65. However, in contrast to the close linkage expected from the data on converted lines, the calculated map distances from the segregating population are 34.1cM for rhm-SGCR148 and 39.9cM for rhm-SGCR65. These map distances indicate that a chromosomal segment greater than 34cM was retained from the original rhm source line during at least 6 backcrosses in each of 5 separate backcross programs designed to introgress rhm, despite the fact that no barrier to recombination between rhm and the locus detected by SGCR148 exists. We infer the existence of a second gene about 34-40cM from rhm which enhances expression of rhm sufficiently to be selected for consistently by breeders when introgressing rhm.

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