Barley and wheat are related species that belong to the Triticeae tribe of the Gramineae family (Baum, 1978). Even though barley is a diploid, and wheat a hexaploid, the genomes of these two species are similar. Both have the same basic chromosome number (x=7) and approximately the same DNA content per genome (5.5 pg) (Bennett and Smith, 1976). Furthermore, each chromosome in barley is genetically related (homoeologous) to a chromosome in each genome of wheat as previously shown by genetic compensation studies and comparison of morphological, isozyme, and a few RFLP loci (Hart, 1987; Kam-Morgan et al., 1989; Sharp et al., 1989). To better determine the extent of homology between the genomes of barley and wheat, RFLP analysis using a common set of cDNA and genomic clones was conducted. T. tauschii was chosen as the wheat representative because it is diploid and a RFLP map is available for it (Gill et al., 1991).
To determine the proportion of low copy sequences that are shared between barley and T. tauschii, a total of 98 barley Pst 1 genomic and cDNA clones were hybridized to blots containing T. tauschii DNA, while 26 T. tauschii genomic clones were hybridized to barley DNA. Results showed that a very high proportion of the low copy clones tested was common between barley and T. tauschii. Approximately 93% of the barley clones and 88% of the T. tauschii clones detected homologous sequences in the other species.
Changes in copy numbers of barley genomic clones were observed between the genomes of barley and T. tauschii. A higher proportion 67% of the clones tested were single copy in barley, compared to 55% in T.tauschii. The remaining 33% and 45% of the clones represented multicopy and repeated sequences in barley and T. tauschii, respectively.
Twenty-two new RFLP markers were mapped in T. tauschii and compared to the map positions in the barlye Steptoe/Morex RFLP map (North American Barley Genome Mapping Project, unpublished). Only 11 of these markers were found to be present in the same homoeologous chromosomes in the two species. The remaining clones mapped to chromosomes which are non-homoeologous between barley and T. tauschii. Since some of the clones hybridized to more than one band in one or both of the species, it is possible that different bands were scored and mapped in the two species. We are currently testing whether any of the non-polymorphic bands are found in homoeologou chromosomes in the two species, using wheat-barley addition lines and wheat ditelosomic lines.
Figure 2 shows the comparison of gene order between three sets of homoeologous chromosome groups between barley and T. tauschii. In general, gene order is conserved between these two species. Additional markers are being compared to determine the nature of a few observed differences.
Baum, B. R. 1978. Can. J. Bot. 56:374-385.
Bennett, M. D., and J. B. Smith. 1976. Philos. Trans. Royal Soc. Lond. B. 274:227-274
Gill, K. S., E. L. Lubbers, B. S. Gill, W. J. Raupp, and T. S. Cox. 1991. Genome 34: 362-374.
Hart, G. E. 1987. In: Wheat and Wheat Improvement. Heyne, E., R. Morris, and D. R. Knott (eds.). American Society of Agronomy, Madison, WI, pp. 194-214.
Kam-Morgan, L. N., B. S. Gill, and S. Muthukrishnan. 1989. Genome 32:724-732.
Sharp, P. J., S. Chao, S. Desai, A. Kilian, and M. D. Gale. 1989. In: Development and Applications of Molecular Markers to Problems in Plant Genetics. Helentjaris, T., and B. Burr (eds.), Cold Spring Harbor Laboratory, New York, pp. 29-34.