| Homoeologous pairing of specific Agropyron elongatum
(2n=70) chromosome with wheat chromosomes J. DVORAK and D. R. KNOTT Department of Crop Science, University of Saskatchewan, Saskatoon, Sask., Canada The gene pool present in taxons closely related to the genus Triticum represents a valuable source of genetic variability for wheat breeding. The transfer of genes for disease resistance from related genomes and their fixation in the wheat genome is an important task in wheat genetics. The method of translocating alien chromosome segment into the wheat genome by irradiation was pioneered by SEARS (1956) and KNOTT (1961). However, it appears that the appropriate manipulation of the diploidizing genetic mechanism found in wheat (SEARS and OKAMOTO 1958, RILEY 1958) might provide a more efficient method for gene transfer. The pairing between homoeologous chromosomes is prevented in wheat by the activity of a gene located on the longer arm of chromosome 5B. The removal of the pair of 5B chromosomes or the addition of the Aegilops speltoides or Ae. mutica genomes suppresses the activity of the wheat diploidizing mechanism (RILEY 1960) and results in pairing between homoeologous chromosomes. A method based on this principle has been successfully applied to transfer a gene for resistance to yellow rust from Ae. comosa to common wheat (RILEY et al. 1968). Success with this procedure is, however, limited by the degree of pairing between alien chromosomes and wheat chromosomes. The species of the section Elytrigia of the Agropyron genus have proved to be a valuable source of genes for resistance to various diseases of wheat. The successful transfer of genes from Agropyron to wheat by homoeologous recombination depends on pairing between Agropyron and wheat chromosomes. An Agropyron chromosome homoeologous with the 6th wheat group was found to pair with wheat chromosomes with an average frequency of 4.8% when homoeologous pairing was restored by adding the Ae. speltoides genome (JOHNSON and KIMBER 1967). Homoeologous pairing between the chromosomes of Ae. squarrosa, the donor of the wheat D genome, and the chromosomes of diploid Ag. elongatum (2n=14) also suggests fair pairing affinity (DVORAK 1971a). A chromosome pair of Ag. elongatum (2n=70) carrying a gene for leaf rust resistance was substituted into the genome of common wheat by CALDWELL et al. (1956). This wheat derivative was released under the name Agrus. KNOTT produced disomic, monotelocentric and ditelocentric additions of the alien chromosome by backcrossing Agrus to the common wheat cultivar Thatcher. This Agropyron chromosome has been shown to be homoeologous with the 7th homoeologous group of wheat (NANDA 1968, QUINN and DRISCOLL 1967, KNOTT 1968). |
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