Doctoral Degrees (Plant Sciences)

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Browsing Doctoral Degrees (Plant Sciences) by Author "Ali, Shimelis Hussein"

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    A comparison of monosomic and disomic substitution lines in the chromosomal location of leaf rust resistance genes in tetraploid wheats
    (University of the Free State, 2003-05) Ali, Shimelis Hussein; Spies, J. J.; Labusehange, M. T.; Pretorius, Z. A.
    English: Two sets of aneuploids were employed and compared to localize adult plant leaf rust resistance genes in tetraploid wheat accessions. One set was the hexaploid Chinese Spring (CS) A- and B-genome monosomics (2n=6x-1=41, AABBDD) and the other the tetraploid Langdon durum D-genome disomie substitutions (2n=4x-2+2=28). The tetraploid accessions (2n=4x=28, AABB) 104 (Triticum turgidum subsp. dicoccum var. arras) and 127 (T. turgidum subsp. durum var. aestivum) were selected as leaf rust-resistant after evaluating 353 Triticum accessions. To study the chromosomal locations of the resistance genes, crosses were made between the complete sets of aneuploids (maternal parents) and the accessions. From both crosses F1 hybrids were used for meiotic chromosome analysis and to select monosomic plants for F2 segregation analysis. In the cross of the CS AB-genome monosomics with resistant lines, F1 monopentaploid plants (2n=5x-1=34, AABBD) were selected. In the other crosses of the resistant accessions with the substitution lines, F1 double monosomic plants were selected with 13 bivalent and two univalent chromosomes during metaphase I. The F2 segregates of selfed monosomic plants were inoculated at the flag leaf stage with pathotype UVPrt2 of Puccinia triticina. The CS monosomic analysis showed that in accession 104 a Lr gene occurs on chromosome 1A. Another gene in the accession was localized on chromosome 6B by Langdon durum substitution analysis. The second gene in this accession could not be localized from CS analysis since the F1 monopentaploid hybrid of that cross was sterile making the F2 segregation analysis incomplete. The gene localized on chromosome 1A in accession 104 by the CS analysis could not be localized by the substitution analysis owing to the presence of a suppressor gene brought from the 0 chromosome of substitution line 101 A. In accession 127 the resistance gene was located on chromosome 4A using the two sets of aneuploids. The study indicated that the tetraploid O-genome substitution lines are more commendable stocks than the hexaploid CS monosomics for chromosomal mapping of leaf rust resistance genes in tetraploid wheats. The trustworthiness of the tetraploid cytogenetic stocks is that the F1 double monosomic hybrids resulting from crossing with the tetraploid did not show sterility or poor germination. These would furnish complete F2 segregation analysis. Besides, the relatively few numbers of chromosomes in the F1 hybrids would ease meiotic chromosome analysis. However, it would be necessary to consider the CS monosomic stocks during gene interaction from D-genome chromosomes of certain substitution lines on genes present on the A- or B-genome chromosomes of the tetraploid wheat under study. The analysis of variance of important agronomic traits in the substitution lines suggested that three substitution aneuploids namely 202B, 707 A and 707B were phenotypically divergent when compared to the other lines and the recurrent parent. These lines are reportedly backcrossed for 12 generations to Langdon. It appears, however, that further backcrossings to the recurrent parent and further targeted selections are necessary to increase traits such as plant height, number of spikelets per spike, kernel weight, and seed yield in line 202B. The same selection schemes are required to improve the number of spikelets per spike, number of kernels per spike, kernel weight, and seed yield in line 707B. Additionally traits such as number of spikelets per spike, kernel weight and seed yield require further improvement in line 707 A. The path analysis revealed true associations of seed yield with kernel weight and heading date. This association was also supported by a simple correlation analysis. The direct path value of the path coefficient analysis exposed kernel weight as a key selection criterion to improve seed yield in the substitution aneuploids. The alternate path values further indicated selection for kernel weight would bring simultaneous selection of improved number of kernel per spike, spikelets per spike and plant height.