On the long arm of chromosome 3, three loci, Est1, Est2 and Est4, for esterase isozymes are located as complex loci. One of the resistance loci, Ym, to Barley Yellow Mosaic Virus (BaYMV) has been found to be tightly linked with the complex loci with a recombination value of 2.45%, suggesting that selection of the BaYMV resistance can be performed by use of the isozyme marker (Konishi and Kaiser, 1991). Recently, RFLP mapping of another recessive gene for the BaYMV resistance, ym4, revealed that one of RFLP markers (MWGIO) was linked with the ym4 locus and the Est1 locus as 1.2 cM and 3.5 cM, respectively, in order of ym4-MWG10-Est1 (Graner and Bauer, 1993). Furthermore, the other resistance locus to Japanese type 11 of the BaYMV is linked with the esterase complex loci with less than 10% (lida and Konishi, unpublished). These results suggest that some resistance loci are scattered on the long arm of chromosome 3, similar to resistance loci to powdery mildew on the short arm of chromosome 5.

DNA markers such as RFLPs and RAPI)s are much more effective for the location of quantitative trait loci (QTL) on chromosomes. One of the QTLs, the recessive denso dwarfing gene in 'Magnum', is found to be 7.8 cM distal to Xsprl70 on the long arm of chromosome 3 by use of RFLP markers (Laurie et al., 1993).

References

Graner, A. and E. Bauer. 1993. RFLP mapping of the ym4 virus resistance gene in Barley. Theor. Appl. Genet. 86: 689 - 693.

Konishi, T. and R. Kaiser. 1991. Genetic differences in Barley Yellow Mosaic Virus resistance between Mokusekko 3 and Misato Golden. Japan. J. Breed. 41: 499 - 505.

Laurie, D.A., N. Pratchett, C. Romero, E. Simpson and J.W. Snape. 1993. Assignment of the denso dwarfing gene to the long arm of chromosome 3 (31-1) of barley by use of RFLP markers. Plant Breeding I 11: 198 - 203.