GrainGenes

Several groups throughout the world are involved in genetic mapping of barley chromosomes. The traditional markers, morphological and protein/isozyme markers, which represent specific genes, are becoming less popular as molecular markers receive more and more attention. Modern maps of barley chromosomes are based on RFLP and RAPD markers and there is currently much interest in using simple sequence repeats (or microsatellites) and amplified fragment length polymorphisms (AFLPs) to cover the genome more densely. Unfortunately, there appears to be low levels of polymorphisms for molecular markers such as RFLPs and RAPDs on chromosome 4 compared to the other six barley chromosomes. In most of the currently published maps chromosome 4 has the least number of RFLP and RAPD markers. Indeed in some mapping projects, because no polymorphic markers can be detected, chromosome 4 is not represented.

Studies on genetic diversity of wild barley (Hordeum spontaneum) populations have shown that adaptation to environment, particularly water availability and temperature, is controlled to a large extent by genes on chromosome 4 (Chalmers et al., 1992). Chromosome 4 has also been implicated in salt tolerance (Forster et al., 1990) and potential water use efficiency (Handley et al., 1994). One possible explanation for the lack of variation on chromosome 4 is that genes on this chromosome have been heavily selected for in adapting barley to agricultural conditions, an untested hypothesis. One gene known to have major effects on adaptation is the sh gene for vernalisation requirement, this gene is located on the long arm of chromosome 4 (Hackett et al., 1992).

The gene map of chromosome 4 is given in Fig. 1. The map is basically unchanged from that presented in BGN 21. There are 28 loci covering a genetic distance of 136 cM. The centromere is unmapped and the cer-j locus has been assigned the 0 position.

Mapped Genes

f9 - chlorina seedling; rb - ribbon grass; I (int-c) - infertile intermedium; v/5/ - six row; K - hooded; ert-i - erectoides; z (zb2) - zebra striped leaf; lg4 - light green seedling; f10 - chlorina seedling; N182 - reaction to Erysiphe graminis; ari-c (lk5) - breviaristatum; lb2 - long weak basal internode; cer-zh - eceriferum; Ml-g (Reg-2) - reaction to Erysiphe graminis; cer-zg - eceriferum; br2 - brachytic dwarf; cer-j (gl3) - eceriferum; min - semi-minute dwarf; Bl - white aleurone xenia; sid - single elongated internode dwarf; cer-cqu (gs1) - eceriferum; ml-o (reg6) - reaction to Erysiphe graminis; Height - mature plant height; Hs -hairy sheath; Vrn1 (sh) - response to vernalisation; yh - yellow head; Bmy1 - -amylase; Wsp3 water-soluble protein.

References

Chalmers, K.J., Waugh, R., Watters, J., Forster, B.P., Nevo, E., Abbott, R.J. and Powell, W. (1992). Grain isozyme and ribosomal DNA variability in Hordeum spontaneum populations from Israel. Theor. Appl. Genet. 84, 313-322.

Forster, B.P., Phillips, M.S., Miller, T.E., Baird, E. and Powell, W. (1990). Chromosomal location of genes controlling tolerance in salt (NaCl) and vigour in Hordeum vulgare and H. chilense. Heredity 65, 99-107.

Hackett, C.A., Ellis, R.P., Forster, B.P., McNicol, J.W. and Macaulay, M. (1992). Statistical analysis of a linkage experiment in barley involving quantitative trait loci for height and ear-emergence time and two genetic markers on chromosome 4. Theor. Appl. Genet. 85, 120-126.

Handley, L.L., Nevo, E., Raven, J.A., Martinez-Carrasco, R., Scrimgeour, C.M., Pakniyat, H. and Forster, B.P. (1994). Chromosome 4 controls potential water use efficiency (¹³C) in barley. J. Exp. Bot. 45, 1661-1663.

Figure 1. The gene map of chromosome 4.