This update will consist primarily of a compilation of cytogenetic and RFLP studies involving chromosome 7 that were presented at the Sixth International Barley Genetics Symposium.

Cytogenetic studies

Islam and Shepherd (1991) are continuing their studies on the induction of recombination between barley and wheat chromosomes. This is accomplished by using triple monosomic stocks consisting of 19 wheat bivalents plus a monosomic for a marked barley chromosome arm, a monosomic for the related wheat chromosome, and a single dose of wheat chromosome 5B carrying the Ph1 mutation. With regard to barley chromosome 7, double monosomic stocks with 7Hbeta and wheat 7B have been isolated and work is in progress to isolate double monosomics with 7H and 7B. These will be used to induce recombinants over the entire length of chromosome 7.

Pickering (1991) reported that Hordeum bulbosum chromosomes can substitute for H. vulgare chromosomes in backcross progenies involving the interspecific hybrid. Pollen from partially fertile triploid hybrids (VBB) was used to pollinate H. vulgare. In diploid progenies from such backcrosses, frequently one H. bulbosum chromosome was present that substituted for H. vulgare chromosomes 1, 6, or 7. One such backcross plant contained in addition to a 6S telocentric, a H. bulbosum substitution and an interchange involving H. vulgare chromosome 7.

RFLP studies

The North American Genome Mapping Project presented a preliminary barley RFLP map at the Symposium (Blake et al., 1991). That version showed a total of 139 markers predominantly RFLPs but also isozyme and morphological markers. Twenty-two of those markers were assigned to chromosome 7. Chromosome 2 and 7 were considered to have the best maps to date. Chromosome 7 has a map of 245 centimorgans with 5 gaps exceeding 10 cM, 3 gaps exceeding 20 cM, and 2 gaps exceeding 30 cM. A more recent update on the above map (Kleinhofs, personal communications) showed a total of 267 markers with 179 confirmed.

Another version of a barley RFLP map was recently published by Heun et al. (1991). Of the 155 total RFLPs mapped, 16.1% were located on the map of chromosome 7 which covered 17.9% of the total map. The 25 markers located on chromosome 7 were obtained from the following sources: 11 from an oat cDNA library, 9 from a wheat genomic library, 2 from a barley cDNA library, 2 from an Aegilops squarrosa genomic library plus a rDNA marker from wheat.

Another barley RFLP map is being assembled in Germany (Jahoor et al., 1991). In that undertaking 245 probes were assigned to a specific chromosome and 70 located to a position on that chromosome. The numbers for chromosome 7 are 34 probes assigned and 10 located.

Cannel et al. (1991) reported on the assignment of various characterized cDNA clones to individual chromosome arms. They are to serve as a baseline for assigning additional RFLP markers. In the case of chromosome 7, a probable amylase/protease inhibitor was assigned to 7L.

Blake (1991) has recently reported on the identification of QTL loci by means of RFLP studies. A mapping population of 150 doubled haploids derived from a Steptoe × Morex hybrid was examined. It was found that grain protein content was controlled predominantly by genes with minor effects with 10% of the variability controlled by loci on chromosome 7. Kernel weight was controlled by a combination of major and minor effects with 10% of the variability controlled by loci with minor effects located on chromosome 7.

Chromosome 7 was found also to carry a major gene for seed dormancy accounting for 50% of the variability.

References:

Blake, T. 1991. QTLs underlying grain characters, how many, how important. Oat Biotech. VI. Sponsored by Quaker Oats Co. (cited with permission).

Blake, T., et al. (15 coauthors). 1991. An RFLP map of barley, North American Genome Mapping Project. Barley Genet. VI: 245-248.

Cannell, M., A. Karp, P. Shewry, and P. Isaacs. 1991. Restriction fragment length polymorphism analysis in barley and assignment of characterized probes to chromosome arms. Barley Genet. VI: 249-253.

Heun, M., A. E. Kennedy, J. A. Anderson, N. L. V. Lapitan, M. E. Sorrells, and S. D. Tanksley. 1991. Construction of an RFLP map for barley (Hordeum vulgare L.). Genome 34:437-447.

Islam, A. K. M. R., and K. W. Shepherd. 1991. Recombination between wheat and barley chromosomes. Barley Genet. VI:68-70.

Jahoor, A., J. Schondelmaier, A. Graner, R. G. Herrmann, and G. Fischbeck. 1991. Linkage analysis of RFLP markers in the barley genome. Barley Genet. VI: 263-265.

Pickering, R. 1991. Alien substitution of Hordeum bulbosum L. chromosomes into H. Vulgare L. Barley Genet. VI:88-90.