Browsing by Author "Vermaak, Charmaine"

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    The genetic stability of a recombinant form of the Lr19 translocation of wheat
    (Stellenbosch : Stellenbosch University, 1999-12) Vermaak, Charmaine; Marais, G. F.; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics.
    ENGLISH ABSTRACT: The Lr19 gene is an excellent source of leaf rust resistance worldwide. It occurs on a translocated segment on chromosome 7DL in Triticum aestivum and was derived from Thinopyrum ponticum. Known genes on the translocation are: centromere - Sd1 (segregation distortion) - Xpsr165 - Xpsr 105 - Xpsr129 - XcsIH81-1 - Lr19 - WSP-D1 (water soluble protein) - Sr25/Y (stem rust resistance/ yellow endosperm). Following the disruption of meiotic pairing behaviour in a Lr19 heterozygote, a recombinant, Lr19 (149), was selected (Marais, 1992c). In the recombination event Lr19 (149) was relocated to chromosome arm 7BL with wheat chromatin on both sides of the translocation. Lr 19 (149) has lost Y1, Sr 25 and Sd1. In translocation heterozygotes, gametes with Lr19 (149) have a strong tendency to self eliminate. The purpose of this study was (1) to determine if self-elimination occurs in heterozygotes of both sexes, (2) if the Lr19 (149) translocation can recombine with homoeologous regions on 7BL in the presence of PhI (pairing inhibitor) gene, (3) to determine whether the selfelimination tendency of the translocation is accompanied by an increased incidence of mutations. Strong self-elimination of Lr19 was detected in F2 and F3 populations. The self-elimination, which is influenced by the genetic background, was found to be more pronounced when the segment was transmitted via pollen. No recombination was detected between Lr19 and two proximally located loci: Xus-OPK91350 and XcsIH81-1, and also not between two distally located loci: Wsp-D1c and X12c. This suggests that the translocation is transmitted as a single, large linkage block during meiosis. The reason for this is probably the PhI gene which regulates homology recognition along the entire length of the chromosome. No mutations were found at the four marker loci. Thus, if Lr19 (149) is used in breeding, its transmission will be impaired on the segregating generations and the selection of superior Lr19 (149) homozygotes will be complicated. Fortunately, this will not be accompanied by an increased tendency for mutation. An attempt to convert the csIH81-1 probe into a STS (sequence-tagged-site) marker was not succesful as no useful polymorphisms could be obtained, even after using different enzymes to cut the amplification product.