Genetic and physical characterization of chromosome 4DL in wheat

The long arm of chromosome 4D in wheat (Triticum aestivum L.) has been show n in previous studies to harbor genes of agronomic importance. A major domi nant gene conferring Aluminum (Al) tolerance (Alt2 in 'Chinese Spring' and Alt(BH) in 'BH 1146'), and the Kna1 locus controlling the K+/Na+ discrimina tion in saline environments have been mapped to this chromosome arm. Howeve r, accurate information on the genetic and physical location of markers rel ated to any of these genes is not available and would be useful for map-bas ed cloning and marker-assisted plant breeding. In the present study, using a population of 91 recombinant inbred lines segregating for Al tolerance, w e provide a more extensive genetic linkage map of the chromosome arm 4DL ba sed on RFLP, SSR, and AFLP markers, delimiting the Alt(BH) 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 Alt(BH) gene was physi cally mapped to the distal region of the chromosome, between deletion break points 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 p olymorphism rates in chromosome arm 4DL for the different types of markers used were extremely low, as confirmed by the physical mapping of AFLPs. Fin ally, analysis of 1 Mb of contiguous sequence of Arabidopsis chromosome 5 f lanking the gene homologous to the BCD1230 clone (a cosegregating marker in our population coding for a ribulose-5-phosphate-3-epimerase gene), reveal ed 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.