Centromere function and evolution in maize (Zea mays)

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Title from title screen of research.pdf file (viewed on August 3, 2007)Vita.Thesis (Ph. D.) University of Missouri-Columbia 2006.A dispensable supernumerary chromosome present in maize, the B chromosome, was the focus of many centromere studies. First, I examine the variation in copy number of centromeric elements and other repeats among different maize lines. Then, I demonstrate that centromeric elements are present away from the centromere on the B chromosome indicating that centromeric elements are not sufficient for centromere function in maize. I demonstrate that the B centromere can be inactivated in dicentric chromosomes to produce stable, functionally monocentric chromosomes. Next, I examine the rate of divergence for centromeric elements in maize and its relatives in relation to other repetitive elements in the genome. I examined the genomic distribution of repetitive elements showing that certain families of retrotransposons are enriched in the heterochromatic regions flanking the centromere. Finally, I describe a novel chromosomal rearrangement, an inversion with a breakpoint in the centromere that splits the tract of centromere repeats creating a chromosome with two distinct sites of centromere elements.Includes bibliographical reference

Retension of knobs in chromosome tips in maize

Abstract only availableKnobs are deeply staining chromosomal sites on maize chromosomes. Molecularly, they are composed of a 180 base pair repeat. Their positions on the chromosomes are variable but usually internal in maize. In relatives, the knobs are usually found on the tips of chromosomes. They have been observed for a long time, yet their function remains a mystery. Knobless maize lines do not appear to have knobs. I used fluorescence in situ hybridization (FISH) to test whether cryptic knob sequences exist at the chromosome tips in maize but have avoided normal detection. Long exposure time detects weak signals near the ends of most chromosomes and some cryptic internal sites. Knobless lines are ideal because they do not have the large knobs which can make such detection difficult, if not impossible. I found the Knobless Tama Flint and Knobless Wilbur Flint lines to have cryptic knobs on most chromosomes. Zea diploperennis exhibited knobs on every chromosome, usually at the tips. Thus, although knobs as usually detected in maize are internal, maize has cryptic knob sequences at the ends of most chromosomes in a similar situation as its relatives suggesting a conserved function at chromosome termini.NSF-REU Program in Biological Sciences & Biochemistr

Analysis of mitochondrial DNA insertions into a nuclear chromosome of the maize B73 line

Abstract only availableMitochondrial DNA (mtDNA) is known to have integrated into the nuclear DNA of plants and animals. The purpose of this project is to investigate the on-going migration of mtDNA into the nuclear DNA of maize plants. Specific objectives are to discover the amount of DNA incorporated, whether it is the whole mitochondrial genome or sections, and to see if it has replicated after migration. The maize inbred line B73 has a particularly large mt DNA insert on chromosome 9. Using the fluorescent in situ hybridization (FISH) method, the arrangement of inserted mitochondrial DNA was examined. The FISH method uses fluorescently labeled mtDNA as probes for hybridization to chromosomes. Regions of the chromosomes that contain mtDNA can then be detected using a compound microscope with fluorescent attachments. Locations that contain more mtDNA are brighter. Three combinations of probes that cover different parts of the mitochondrial genome were employed. In order to analyze the arrangement of the DNA, the chromosomes were prepared from a stage of meiosis called pachynema in which the chromosomes are elongated and have not yet begun to condense. The results have confirmed the presence of all three probes within the large insertion of mtDNA on chromosome 9 of B73. The data suggest that either different parts of the mitochondrial genome are incorporated preferentially or that there is selective replication of portions of the mitochondrial genome after incorporation.MU Monsanto Undergraduate Research Fellowshi