DNA-based markers have been developed over the last 10 years, and more than 1000 molecular markers are located on the barley genome. Qi et al. (1996) compared to integrate four independent molecular genetic maps. The integrated map contains 898 markers, covers 1060 cM, and removes many large gaps present in the individual maps. Comparison of the integrated map with the individual maps revealed that the overall linear order markers is in good agreement and that the integrated map is consistent with the component maps. The integrated map of chromosome 3, comprising 133 markers and spanning 131 cM, represents only one 10 cM gap and one cluster of markers in the 40-50 cM region, while the distribution of markers is fairly uniform in other regions.

These markers have contributed to locate morphological and biochemical loci in molecular genetic linkage maps of chromosome 3, as well as QTLs. As morphological loci, denso or sdw1 for semi-dwarf 1 and Pub for pubescent leaf blade were found in chromosme 3L by Barua et al. (1993) and Kleinhofs et al. (1993), respectively. The denso locus was also mapped in molecular linkage maps by Laurie et al. (1993), Laurie et al. (1995) and Thomas et al. (1995). For isozymes, Est1, Est2 and Est4 complex locus for esterase was mapped in chromosome 3L (Barua et al., 1993; Graner and Bauer, 1993; Graner et al., 1994; Kasha and Kleinhofs, 1994; Ordon et al., 1995; Thomas et al., 1995). The EST-5 locus which was located in the distal region of chromosome 3L by Becker et al. (1995) may be identical to the esterase complex locus, since the Est5 locus is in chromosome 1S (5HS) (Yoshimi and Konishi, 1995). Two other isozyme loci, Adh10 for alcohol dehydrogenase 10 and Bgl1 for alkaline ß-galactosidase 1, were found in chromosome 3L by Graner et al. (1994) and Kleinhofs et al. (1993), respectively. For disease resistance, the ym4 resistance gene to barley yellow mosaic virus (BaYMV) and barley mild mosaic virus (BaMMV) was mapped in the distal region of chromosome 3L using RFLP markers (Graner and Bauer, 1993) and also RAPD markers (Weyen et al., 1996). Meanwhile, the Yd2 gene for barley yellow dwarf virus resistance was mapped close to the centromere of chromosome 3L (Collins et al., 1996). The Rhynchosporium resistance (Rh) gene was found in the proximal region of chromosome 3, while the second resistance gene was located in the distal region of chromosome 3L (Thomas et al., 1995). The resistance gene with smaller effect to Rhynchosporium was also determined in the same chromosome (Backes et al., 1995).

For quantitative traits, several agronomically important loci have been found in RFLP maps. The eps3L locus for earliness per se was associated with Xpsb177 in chromosome 3L (Laurie et al., 1995). This earliness gene was detected in the autumn sowing, but not in the glasshouse and the spring-sown field experiments. Using a set of DH lines derived from 'Blenheim' x E224/3, one of two increasing QTLs from 'Blenheim' for yield was detected on chromosome 3, locating very close to the denso locus (Thomas et al., 1995). In contrast, all of the QTL alleles from 'Blenheim' decreased the specific weight indicating kernel plumpness, and two of them were located on chromosome 3. The one QTL with the greatest effect was located close to the denso locus on chromosome 3L, while the other was probably located on chromosome 3S. A team of the North American Barley Genome Mapping Project (NABGMP) found a main QTL for grain yield on chromosome 3, using a population of DH lines derived from 'Steptoe' x 'Morex'. Chromosome 3 was the site of the largest yield QTL, and both positive alleles were contributed by 'Steptoe'. One QTL occupied 17.6 cM interval (CDO113B - His4B) on the long arm of the chromosome, and the second occupied a 6.4 cM interval (Dor4A - ABG396) supported by spans of a 46 cM interval containing the centromere (Hayes et al., 1993). The second QTL seems to a single main QTL for grain yield located in the ABG396 - BCD828 interval with a length of 3.5 cM near the centromere of chromosome 3 by Romagosa et al. (1996). However, any effective QTL for grain yield could not be found on chromosome 3 when different series of DH lines derived from 'Harrington' x 'TR306' and 'Igri' x 'Danilo' were grown in North America (Tinker et al., 1996) and Germany (Backes et al., 1995), respectively.

References:

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Barua, U.M., K.J. Chalmers, W.T.B. Thomas, C.A. Hackett, V. Lea, P. Jack, B.P. Forster, R. Waugh, and W. Powell. 1993. Molecular mapping of genes determining height, time to heading, and growth habit in barley (Hordeum vulgare). Genome 36:1080-1087.

Becker, J., P. Vos, M. Kuiper, F. Salamini, and M. Heun, 1995. Combined mapping of AFLP and RFLP markers in barley. Mol. Gen. Genet. 249:65-73.

Collins, N.C., N.G. Paltridge, C.M. Ford, and R.H. Symons. 1996. The Yd2 gene for barley yellow dwarf virus resistance maps close to the centromere on the long arm of barley chromosome 3. Theor. Appl. Genet. 92:858-864.

Franckowiak, J.D. 1995. Notes on linkage drag in Bowman backcross derived lines of spring barley. BGN 24:63-70.

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

Graner, A., E. Bauer, A. Kellermann, S. Kirchner, J.K. Muraya, A. Jahoor, and G. Wenzel. 1994. Progress of RFLP-map construction in winter barley. BGN 23:53-59.

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Laurie, D.A., N. Pratchett, N., C. Romero, E. Simpson, and J.W. Snape. 1993. Assignment of the denso dwarfing gene to the long arm of chromosome 3 (3H) of barley by use of RFLP markers. Plant Breed. 111:198-203.

Laurie, D.A., N. Pratchett, J.H. Bezant, and J.W. Snape. 1995. RFLP mapping of five major genes and eight quantitative trait loci controlling flowering time in a winter x spring barley (Hordeum vulgare L.) cross. Genome 38:198-203.

Ordon, F., E. Bauer, K.J. Dehmer, A. Graner, and W. Friedt. 1995. Identification of a RAPD-marker linked to the BaMMV/BaYMV resistance gene ym4. BGN 24:123-126.

Qi, X., P. Stam, and P. Lindhout. 1996. Comparison and integration of four barley genetic maps. Genome 39:379-394.

Romagosa, I., S. Ullrich, F. Han, and P.M. Hayes. 1996. Use of the additive main effects and multiplicative interaction model in QTL mapping for adaptation in barley. Theor. Appl. Genet. 93:30-37.

Thomas, W.T.B., W. Powell, R. Waugh, K.J. Chalmers, U.M. Barua, P. Jack, V. Lea, B.P. Forster, J.S. Swanston, R.P. Ellins, P.R. Hanson, and R.C.M. Lance. 1995. Detection of quantitative trait loci for agronomic, yield, grain and disease characters in spring barley (Hordeum vulgare L.). Theor. Appl. Genet. 91:1037-1047.

Tinker, N.A., D.E. Mather, B.G. Rossnagel, K.J. Kasha, A. Kleinhofs, P.M. Hayes, D.E. Falk, T. Ferguson, L.P. Shugar, W.G. Legge, R.B. Irvine, T.M. Choo, K.G. Briggs, S.E. Ullrich, J.D. Franckowiak, T.K. Blake, R.J. Graf, S.M. Dofing, M.A. Saghai Maroof, G.J. Scoles, D. Hoffman, L.S. Dahleen, A. Kilian, F. Chen, R.M. Bivashev, D.A. Kudrna, and B.J. Steffenson. 1996. Regions of the genome that affect agronomic performance in two-row barley. Crop Sci. 36:1053-1062.

Weyen, J., E. Bauer, A. Graner, W. Friedt, and F. Ordon. 1996. RAPD-mapping of the distal portion of chromosome 3 of barley, including the BaMMV/BaYMV resistance gene ym4. Plant Breed. 115:285-287.

Yoshimi, R. and T. Konishi. 1995. Linkage analysis of several isozyme loci in barley. BGN 24:35-37.