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Title:A waterhemp saga: Seed production, genetics, hybridization, and the creation and discovery of quad-stack individuals
Author(s):Bell, Michael S.
Advisor(s):Tranel, Patrick J.
Department / Program:Crop Sciences
Discipline:Crop Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Amaranthus tuberculatus
Herbicide resistance
Seed maturation
Protoporphyrinogen oxidase (PPO)
Acetolactate synthase (ALS)
Gene amplification
5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)
Quantitative real-time polymerase chain reaction (qPCR)
Real-time polymerase chain reaction (PCR)
genetic linkage
glyphosate resistance
four-way resistance
Multiple resistance
Smooth pigweed
Amaranthus hybridus
Abstract:Waterhemp is an old weed posing new problems for corn and soybean producers in the midwestern United States. The weed is indigenous to the Midwest, but has only become a major problem within the last two decades, and it is now one of the most prevalent weeds in Illinois. One of the most troubling aspects of this species is its propensity to evolve resistance to herbicides—a feat which it has now accomplished for herbicides with four different modes of action, with the evolution of resistance to other herbicide modes of action expected in the future. Options for chemical control of this species—particularly for postemergence control in soybean—are rapidly declining. In fact, of the four herbicide chemistries currently available for broadleaf control in soybean, some waterhemp populations have evolved resistance to three. This thesis addresses several facets of this fascinating species, beginning with a literature review in Chapter 1 on the history of weed control, some background on how weeds evolve resistance to herbicides, the biology of waterhemp, and the evolution of herbicide resistance within waterhemp specifically. Chapter 2 addresses a study on an aspect of the reproductive biology of waterhemp—namely the amount of time required for female plants to produce mature seeds after pollination. It was found that some seeds may become viable in as little as 7–9 days after pollination, and that seed dormancy drops if seeds remain on the plant for at least 15–30 days after pollination. These findings could be helpful in future studies requiring the crossing of waterhemp, such as the study reported in Chapter 3, in which the inheritance and genetics of glyphosate resistance in a Missouri waterhemp population are investigated. Glyphosate resistance was determined to be a nuclear-inherited dominant or partially dominant trait, although the number of genes involved could not be determined. Investigations into gene amplification of EPSPS, which has been shown to confer glyphosate resistance in the related species, Palmer amaranth, did reveal elevated copy number in the Missouri population. However, analysis of copy number in F1 and F2 populations showed that copy number does not strictly cosegregate with resistance level, indicating that at least one other factor is necessary for resistance. Several of the F2 populations created for the study in Chapter 3 (involving the crossing of a population resistant to ALS inhibitors, PPO inhibitors and PS II inhibitors with the glyphosate-resistant Missouri population) were investigated in Chapter 4 for the presence of four types of resistance, and individual plants were identified containing all four resistance types, indicating no significant barriers to the combination of four herbicide resistance types within a single plant. Further studies showed tight linkage between ALS and PPO resistance, but no linkage among other types of resistance was detected. In Chapter 5, two waterhemp populations collected from fields in Illinois are examined for multiple herbicide resistance. One population was found to be resistant to glyphosate as well as ALS inhibitors, and the other population was found to be resistant to glyphosate, ALS inhibitors, PPO inhibitors, and PS II inhibitors. Individuals from this population were also identified as being four-way resistant, thus confirming what was observed in greenhouse experiments in the previous chapter. Chapter 6 addresses an attempt at transferring glyphosate resistance from plants of the Missouri waterhemp population into smooth pigweed through hybridization. Progeny were confirmed as hybrids by use of ITS markers, and hybrid plants were found to be resistant to glyphosate. Hybrids were backcrossed (BC) to smooth pigweed, but produced very few seed, preventing the screening of the BC progeny. The BC progeny were again backcrossed to smooth pigweed and found to segregate for seed production, although little seed was produced overall. Although incomplete, this study suggests that such transferal of glyphosate resistance in nature is unlikely. Finally, Chapter 7 discusses concluding remarks, implications and future research.
Issue Date:2011-01-14
Rights Information:Copyright 2010 Michael Scott Bell
Date Available in IDEALS:2011-01-14
Date Deposited:December 2

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