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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