Genome evolution in polyploid wheat

Bread wheat is an hexaploid organism consisting of three different genomes. Despite intensive studies over the last 80 years, little is known about the nature of genomic changes that occurred at the polyploid level and facilitated harmonious coexistence of the three genomes in the same nucleus. To study some of these changes we identified, mapped and sequenced 13 different low-copy, non-coding chromosome-specific sequences and 3 genome-specific sequences. These sequences, despite being non-coding, are highly conserved. While they occur in all the diploid species of the group, in the polyploid species they hybridize only to one chromosome pair in the case of chromosome-specific and to several pairs of the same genome in the case of genome-specific. This indicates elimination of these specific sequences from some of the genomes at the polyploid level. Indeed, probing a wide range of newly-synthesized polyploids with these sequences showed that their elimination was a very rapid process occurring soon after polyploidization. This polyploidy-induced sequence elimination (PISE) of low-copy, non-coding sequences from two of the three genomes of hexaploid wheat brings about "molecular diploidization" of the polyploids. It results in a further differentiation of the homoeologous (partially homologous of the different genomes) chromosomes providing the physical basis for the diploid-like meiotic behavior of hexaploid wheat. Using a series of deletion lines, it was found that these sequences are clustered in several chromosomal regions along each chromosome arm. These highly specific regions are assumed to be involved in homology recognition and initiation of synapsis at meiotic prophase. The structure of the homology determining regions (HDR), the molecular mechanism of elimination and involvement of these sequences in the initiation of pairing is currently being studied (in collaboration with Avi Levy and Gideon Grafi from our department).

Mapping and tagging economically-important genes of wild wheat

Wild tetraploid wheat, the progenitor of most cultivated wheats, contains many useful genes. We produced 28 different chromosome-arm substitution lines of wild wheat in an Israeli commercial variety of bread wheat. These lines were screened for quantitative trait loci (QTLs) as well as for genes for disease resistance and for improved quality. Several QTLs promoting grain yield were identified. These genes are currently being mapped and tagged by DNA markers.

Development of a technology for the production of hybrid wheat

Lines of hybrid wheat surpass conventional true-breeding lines in yield, quality and tolerance to biotic and abiotic stresses. To date, there are no suitable methods for a mass production of hybrid wheat seeds. We are at the final stages of developing and testing a novel method for a commercial production of hybrid wheat seeds.(Patent application submitted).

Recent Publications

Feldman, M., Liu, B., Segal, G., Abbo, S., Levy, A.A., and Vega, J.M. (1997) Rapid elimination of low copy DNA sequences in polyploid wheat: A possible mechanism for differentiation of homoeologous chromosomes. Genetics, 147, 1381-1387.

Segal, G., Liu, B., Vega, J.M., Abbo, S., Rodova, M., and Feldman, M. (1997) Identification of a chromosome specific probe that maps within the Ph1 deletions in common and durum wheat. Theor. Appl. Genet., 94, 968-970.

Liu, B., Segal, G., Vega, J.M., Feldman, M., and Abbo, S. (1997) Isolation and characterization of chromosome-specific DNA sequences from a chromosome-arm genomic library of common wheat. The Plant Journal, 11, 959-965.

Vega, J.M., and Feldman, M. (1998) Effect of the pairing homoeologous gene Ph1 on centromere misdivision in common wheat. Genetics, 148, 1285-1294.

Liu, B., Vega, J.M., Segal, G., Abbo, S., Rodova, M., and Feldman, M. (1998) Rapid genomic changes in newly synthesized amphiploids of Triticum and Aegilops. I. Changes in low-copy non-coding DNA sequences. Genome, 41, 272-277.

Liu, B., Vega J.M., and Feldman, M. (1998) Rapid genomic changes in newly synthesized amphiploids of Triticum and Aegilops. II. Changes in low-copy coding DNA sequences. Genome, 41, 535-542.

Vega J.M., and Feldman, M. (1998) Effect of the pairing gene Ph1 and premeiotic colchicine treatment on intra- and inter- chromosome pairing of isochromosomes in common wheat. Genetics, 150, 1199-1208.

Millet, E., Rong, J.K., and Feldman, M. (1998) Production of wild emmer recombinant substitution lines in a modern bread wheat cultivar and their use in wheat mapping. Proc. 9th Int. Wheat Genet. Symp., Vol. 1, pp. 127-130.

Galili, S., Avivi, Y., and Feldman, M. (1998) Differential expression of three RbcS subfamilies in wheat. Plant Science, 139, 185-193.

Rong, J.K., Millet, E., Manisterski, J., and Feldman, M. (1999) A new powdery mildew resistance gene: introgession from wild emmer into common wheat and RFLP-based mapping. Euphytica (in press).

Feldman, M. (2000) The origin of Cultivated Wheat. In: The Wheat Book, A. Benjean (Ed.), Limagrain Co., Chappes, France (in press).

PATENT:

Feldman, M., and Millet, E. (1997) Method for production of hybrid wheat. (Application no. 120835).

Legends for figures:

Figure 1. DraI-digested genomic DNA of the two parental lines, Triticum durum cv. Langdon (left) and Aegilops speltoides line TS01 (second from the left), of F₁ plants and of newly-synthesized polyploids, hybridized to the genome-specific sequence PSR593. Note that the sequence of Aegilops speltoides is absent from the F₁ and the polyploids.

Figure 2. Diagram showing the physical location of the ten low-copy, non-coding chromosome-specific sequences on chromosome arm 5BL of hexaploid bread wheat. Note the clustering of these sequences in two regions.