INFORMATION TO USERSThis was produced from a copy of a document sent to us for microfilming. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the material submitted.The following explanation of techniques is provided to help you understand markings or notations which may appear on this reproduction. 1. The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting through an image and duplicating adjacent pages to assure you of complete continuity. When an image on the film is obliterated with a round black mark it is anindication that the fîlm inspector noticed either blurred copy because of movement during exposure, or duplicate copy. Unless we meant to delete copyrighted materials that should not have been filmed, you will find a good image of the page in the adjacent frame.3. When a map, drawing or chart, etc., is part of the material being photo graphed the photographer has followed a definite method in "sectioning" the material. It is customary to begin filming at the upper left hand comer of a large sheet and to continue from left to right in equal sections with small overlaps. If necessary, sectioning is continued again-beginning below the first row and continuing on until complete.4. For any illustrations that cannot be reproduced satisfactorily by xerography, photographic prints can be purchased at additional cost and tipped into your xerographic copy. Requests can be made to our Dissertations Customer Services Department.5. Some pages in any document may have indistinct print. In all cases we have filmed the best available copy. Signature was redacted for privacy. University Mlcrdfilms InternationalSignature was redacted for privacy.Signature was redacted for privacy.ii Consequently, a mutation caused by insertion of Ds is unstable only in the presence of In the subsequent discussion, these properties will be described in detail under separate subjects.Like the Ac-Ds system, the Spm(En) system contains regulatory and receptor elements. The regulatory element of this system was designated Suppresscr-mutator (Spm) by McClintock (1954) and Enhancer (En) by Peterson (1953a). The receptor element was not named by McClintock and was termed Inhibitor (I) by Peterson (1953a) . The Spm and jto elementswere identified as members of the same system through the activation cri These interactions between two elements and the action of Spm (En) will be discussed in subsequent sections. Chromosome Rearrangements General mechanisms of chromosome rearrangementIn the early studies, McClintock ( Transposition of Transposable Elements in MaizeIn a previous section a general overview about the interaction of regulatory and receptor elements in two-element systems has been presented (Section 2.1). The receptor element is prese...
ACKNOWLEDGMENTS 374 of these two genotypically different daughter cells give rise to adjacent twin sectors in the kernel that are deeply pigmented (Bz Bz-Wx/C bz wx) and bronze (Wx/C bz wx) respectively. The BFB cycle continues to yield and ^ subsectors in ^ ^ sector and vx subsectors in tissue. The consequence of acentric-dicentric formation at D£ is shown graphically in B of Figure 2.5. Other aberrations often arise. One of them is the formation of an internal deficiency in the short arm of chromosome 9. Such deficiencies include the regions adjacent to Translocation between chromosome 9 and another chromosome may arise with one of the points of breakage at the Ds position. Duplications or inversions of segments within chromosome 9 may also be produced, one of the breakage points being at Ds. This phenomenon will be discussed in the section on transposition of Ds (Section 2.3.1.1). 2.2.2.3. Spm(En) system on chromosome breakage The Spm(En) system differs from the Ac-Ds in several ways. One is the induction of chromosome rearrangements. Thus far, no systematic formation of acentricdicentric chromosomes having the characteristics of Ds-induced BFB cycles has been observed in the Spm(En) system, though some Spm(En)-mediated chromosome breakage may occur (Fedoroff, 1983). 2.2.3. The molecular basis of minor chromosome aberrations induced by transposable elements in maize According to McClintock (1978), the BFB cycle can trigger innate systems capable of inducing genome reorganization. The triggering results in various degrees of reorganization, ranging from complex
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