Genetic and physical characterization of chromosome 4DL in wheat

The long arm of chromosome 4D in wheat (Triticum aestivum L.) has been shown in previous studies to harbor
genes of agronomic importance. A major dominant gene conferring Aluminum (Al) tolerance (Alt2 in ‘Chinese Spring’
and AltBH in ‘BH 1146’), and the Kna1 locus controlling the K+/Na+ discrimination in saline environments have been
mapped to this chromosome arm. However, accurate information on the genetic and physical location of markers related
to any of these genes is not available and would be useful for map-based cloning and marker-assisted plant breeding. In
the present study, using a population of 91 recombinant inbred lines segregating for Al tolerance, we provide a more extensive
genetic linkage map of the chromosome arm 4DL based on RFLP, SSR, and AFLP markers, delimiting the AltBH
gene to a 5.9-cM interval between markers Xgdm125 and Xpsr914. In addition, utilizing a set of wheat deletion lines for
chromosome arm 4DL, the AltBH gene was physically mapped to the distal region of the chromosome, between deletion
breakpoints 0.70 and 0.86, where the kilobase/centimorgan ratio is assumed to be low, making the map-based cloning of
the gene a more realistic goal. The polymorphism rates in chromosome arm 4DL for the different types of markers used
were extremely low, as confirmed by the physical mapping of AFLPs. Finally, analysis of 1 Mb of contiguous sequence
of Arabidopsis chromosome 5 flanking the gene homologous to the BCD1230 clone (a cosegregating marker in our population
coding for a ribulose-5-phosphate-3-epimerase gene), revealed a previously identified region of stress-related and
disease-resistance genes. This could explain the collinearity observed in comparative mapping studies among different
species and the low level of polymorphism detected in the chromosome arm 4DL in hexaploid wheat