Files
Abstract
Pecan (Carya illinoinensis (Wangenh.) K. Koch) is a tree nut species native to North America. Desirable nut shape and scab resistance are important targets in pecan breeding. The research conducted as part of this dissertation involves the genetic and molecular characterization of nut shape and scab resistance in pecans. A semi-automated image-based morphometric analysis was conducted to collect 10 different nut shape phenotypes of 34 full-sib families generated from various crosses between 31 different parents. Heritabilities of these traits were estimated using mid-parent offspring regression, which ranged from 0.41 to 0.83. Additionally, a variance component method was employed to estimate heritability for shape phenotypes in 19 additional pecan genotypes over two years, revealing estimates ranging from 0.26 to 0.78. These findings indicate that nut shape traits in pecans exhibit moderate to high heritability, suggesting that selecting these traits would be effective in a pecan breeding program. To systematically characterize the molecular mechanisms of scab resistance, a comparative transcriptomic study was performed on 'Desirable' pecan in response to inoculation with pathogenic and apathogenic scab isolates. Contrasting gene expression patterns were observed in samples showing a resistant and susceptible reaction at three different time points: 24, 48, and 96 hours post-inoculation. Gene ontology enrichment and weighted gene co-expression analysis of differentially expressed genes suggest that the early response of pathogen-related signal transduction and the development of cellular barriers against the invading fungus are likely defense mechanisms employed by pecan cultivars against pathogenic scab isolates. A genetic mapping study was conducted using an F1 mapping population derived from a cross between two pecan cultivars: 'Pawnee' as the female and 'Elliott' as the male parent. Mapping population were genotyped using genotyping by sequencing (GBS), and SNP-based high-density parental genetic maps were constructed. Scab resistance was evaluated through controlled inoculations in the greenhouse using two different scab isolates and a significant QTL for scab resistance was identified on chromosome 5 in ‘Elliott’ that explains 19.1% to 22.7% of phenotypic variation. A number of candidate genes related to disease resistance were identified within 2- 2-logarithm of odds (LOD) interval of the scab-resistant QTL.