Browsing by Subject "molecular markers"

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    The Enigma of Genetic Linkage in Molecular Breeding for Maize
    (2017-06) Sleper, Joshua
    Linkage among quantitative trait loci prevents the release of hidden genetic variation, but also preserves desirable gene combinations. This dissertation, which includes three studies, shows the continuing enigma of linkage in maize (Zea mays L.) breeding. The first study aimed to determine if the additional recombinations in doubled haploids induced from F2 instead of F1 plants leads to a larger genetic variance and a superior mean of the best lines. In two maize populations, inducing doubled haploids from F2 plants did not improve the mean, and it increased the genetic variance for moisture, but not for yield and plant height. The second study aimed to determine if multi-allelic markers or haplotypes improve the prediction accuracy of genomewide selection in three-way breeding populations, which could have three alleles per locus. In both simulated and empirical maize populations, accounting for multiple alleles did not improve the prediction accuracy over a biallelic model. The third study aimed to determine if genomewide markers can be used to partition trait effects into independent and correlated portions, and if selection on the independent portion was more effective than selection on the entire trait. Results from four cycles of selection showed that selection only for the independent portion did not lead to higher responses for yield, moisture, and plant height. Overall, genetic linkage both assists and confounds molecular breeding efforts in maize.
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    Identification and mapping of a leaf rust resistance gene in barley line Q21861
    (Genome, 1997) Steffenson, Brian; Borovkova, I.G.; Jin, Y.; Kilian, A.; Blake, T.K.; Kleinhofs, A.
    Barley line Q21861 possesses an incompletely dominant gene (RphQ) for resistance to leaf rust caused by Puccinia hordei. To investigate the allelic and linkage relations between RphQ and other known Rph genes, F2 populations from crosses between Q21861 and donors of Rph1 to Rph14 (except for Rph8) were evaluated for leaf rust reaction at the seedling stage. Results indicate that RphQ is either allelic with or closely linked to the Rph2 locus. A doubled haploid population derived from a cross between Q21861 and SM89010 (a leaf rust susceptible line) was used for molecular mapping of the resistance locus. Bulked segregant analysis was used to identify markers linked to RphQ, using random amplified polymorphic DNAs (RAPDs), restriction fragment length polymorphisms (RFLPs), and sequence tagged sites (STSs). Of 600 decamer primers screened, amplified fragments generated by 9 primers were found to be linked to the RphQ locus; however, only 4 of them were within 10 cM of the target. The RphQ locus was mapped to the centromeric region of chromosome 7, with a linkage distance of 3.5 cM from the RFLP marker CDO749. Rrn2, an RFLP clone from the ribosomal RNA intergenic spacer region, was found to be very closely linked with RphQ, based on bulked segregant analysis. An STS marker, ITS1, derived from Rrn2, was also closely linked (1.6 cM) to RphQ.
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    A resistance gene to Ustilago nuda in barley is located on chromosome 3H
    (Canadian Journal of Plant Pathology, 2010-06-07) Menzies, J.G; Steffenson, Brian; Kleinhofs, A
    Loose smut of barley is a common disease which can be controlled using resistant varieties. Information on the chromosome location of loci controlling loose smut resistance and the development of molecular markers to aid in selection for these genes can be beneficial in the resistant variety development process. The objectives of this work were to determine the resistance or susceptibility of doubled haploid barley lines arising from a cross of the varieties ‘Steptoe’ and ‘Morex’ to Ustilago nuda, the causal agent of loose smut of barley, and map the chromosome location of the loose smut resistance locus in ‘Morex’. The reaction to Ustilago nuda of the doubled-haploid barley plants was determined by inoculating spikelets of each line at anthesis by injection of a teliospore suspension using a needle inoculation method. Mature seeds from the inoculated spikelets were grown to determine the percentage of plants that developed with smutted heads. The lines were classified as susceptible if greater than 10% of the plants were smutted. The loose smut resistance locus from the resistant source ‘Morex’ was mapped using an existing DNA marker map of the ‘Steptoe’/‘Morex’ population. The distribution of the resistant and susceptible progeny from the loose smut testing fit a single gene model. The resistance gene was mapped to chromosome 3 (3H).