Browsing by Subject "Hybrids"
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Item Phenotypic and genotypic characterization of white maize inbreds, hybrids and synthetics under stress and non-stress environments(Texas A&M University, 2006-10-30) Makumbi, DanMaize is susceptible to biotic and abiotic stresses. The most important abiotic stresses in Africa are drought and low soil fertility. Aflatoxin contamination is a potential problem in areas facing drought and low soil fertility. Three studies were conducted to evaluate maize germplasm for tolerance to stress. In the first study, fifteen maize inbred lines crossed in a diallel were evaluated under drought, low N stress, and well-watered conditions at six locations in three countries to estimate general (GCA) and specific combining ability (SCA), investigate genotype x environment interaction, and estimate genetic diversity and its relationship with grain yield and heterosis. GCA effects were not significant for grain yield across environments. Lines with good GCA effect for grain yield were P501 and CML258 across stresses. Lines CML339, CML341, and SPLC7-F had good GCA effects for anthesis silking interval across stresses. Additive genetic effects were more important for grain yield under drought and well-watered conditions. Heterosis estimates were highest in stress environments. Clustering based on genetic distance calculated using marker data from AFLP, RFLP, and SSRs grouped lines according to origin. Genetic distance was positively correlated with grain yield and specific combining ability. In the second study, synthetic hybrids were evaluated at seven locations in three countries to estimate GCA and SCA effects under low N stress and optimal conditions and investigate genotype x environment interaction. GCA effects were significant for all traits across low N stress and optimal conditions. The highest yielding synthetic hybrids involved synthetics developed from stress tolerant lines. Synthetics 99SADVIA-# and SYNA00F2 had good GCA for grain yield across low N stress conditions. Heterosis was highly correlated with grain yield. Optimal environments explained more variation than stress environments. The third study evaluated the agronomic performance and aflatoxin accumulation of single and three-way cross white maize hybrids at five locations in Texas. Inbreds CML343, Tx601W, and Tx110 showed positive GCA effects for grain yield. Significant GCA effects for reduced aflatoxin concentration were observed in lines CML269, CML270, and CML78 across locations. Differences in performance between single and three-way crosses hybrids were dependent mostly on the inbred lines.Item Roles for polyploidy, circadian rhythms, and stress responses in hybrid vigor(2014-05) Miller, Marisa Elena; Chen, Z. JeffreyHybrid plants and animals, like corn and the domestic dog, grow larger and more vigorously than their parents, a common phenomenon known as hybrid vigor or heterosis. In hybrids between Arabidopsis ecotypes or species (in allotetraploids), altered expression of circadian clock genes leads to increased starch and chlorophyll content and greater biomass. In plants and animals, circadian clock regulation plays a key role in optimizing metabolic pathways, increasing fitness, and controlling responses to biotic and abiotic stresses. In the allotetraploids, the increased level of heterosis is likely caused by interspecific hybridization as well as genome doubling. However, it is unknown how genome dosage and allelic effects influence heterosis, and whether additional clock output traits, such as stress responses, are altered in hybrids. In three related projects, the effects of genomic hybridization (including parent-of-origin effects) and genome dosage on heterosis were elucidated. In my first project, I found that although ploidy influenced many traits, including seed and cell size, biomass and circadian clock gene expression were most strongly influenced by hybridization. Additionally, parent-of-origin effects between reciprocal hybrids were frequently observed for many traits. In my second project, I described a unique role for RNA-directed DNA methylation (mainly CHH methylation) in mediating the parent-of-origin effect on expression of the circadian clock gene CCA1 in reciprocal hybrids. Altered CCA1 expression peaks were associated with heterosis of biomass accumulation in the reciprocal hybrids. Lastly, I used transcriptome sequencing in hybrids at different times of day to examine changes in downstream clock-regulated pathways. In the hybrids, many genes in photosynthetic pathways were upregulated, while many genes involved in biotic and abiotic stresses were repressed during the morning and afternoon, respectively. Additionally, natural variation between parents in stress-responsive gene expression was found to be crucial for producing vigorous hybrids. These conceptual advances increase the mechanistic understanding of heterosis, and may guide selection of parents for making better hybrids.