Homologous chromosomes display associations in many organisms. Drosophila melanogaster (here after, Drosophila) serves as an excellent model to study pairing interactions since chromosomes are paired in all somatic cells throughout development. For many genes, the degree of homolog association influences gene expression. These effects, collectively referred to as transvection, can promote gene activation or silencing. Requirements for transvection are poorly understood. Chapter One reviews what is known about transvection in Drosophila and chromosome interactions in mammals. Recent cell culture studies implicated a requirement for Topoisomerase 2 (Top2) in chromosome pairing. Top2 encodes an ATP dependent homodimeric enzyme that generates double stranded breaks to change DNA topology. This enzyme is a common target of anticancer drugs due to its role in DNA metabolism. To understand the in vivo role of Drosophila Top2, an EMS screen was completed. Chapter Two describes the identification and characterization of fifteen new EMS generated Top2 mutations. Fifteen null and hypomorphic alleles were obtained, including one that displays temperature sensitivity. Molecular analyses of these alleles uncovered single or multiple base pair substitutions within the coding region of each mutant gene. Even though flies carrying individual missense alleles in trans to a deficiency are inviable, heteroallelic combinations of several missense alleles produced viable flies, including two lines carrying mutations that display resistance to anti-cancer drugs. These data indicate that Top2 activity can be restored by dimerization of defective subunits. Our new Top2 alleles establish a novel allelic series and provide a platform for understanding drug resistance. In Chapter Three, the role of Top2 in chromosome associations was tested to determine whether mutations in Top2 disrupted transvection. Viable heteroallelic combinations of Top2 mutations were used to test transvection at three classically studied loci. For each gene, homologous interactions were analyzed by screening for alterations in pairing-dependent changes in phenotype involving transvecting alleles. Only one of the three genes tested displayed phenotypic changes in Top2 complementing adults that were consistent with an alteration in pairing dependent changes in expression. Transcript levels were assessed at the three genes studied that display transvection. Our studies indicate that changes in the phenotype, due to altered Top2, are likely gene specific transcriptional changes. Further investigation of gene associations in Top2 mutants employed fluorescence in situ hybridization (FISH). These studies showed that all loci examined were paired near wild type levels, suggesting that Top2 does not globally disrupt homolog associations in vivo. The differences observed in Top2 function in vivo and in vitro may be explained by two possibilities. First, the probes studied differ from those used in vitro, indicating that different genetic loci may have different sensitivities to unpairing. Second, Top2 plays a role in the segregation of sister chromatids during anaphase and loss of Top2 causes improper resolution of chromosomes resulting in aneuploidy. In cell culture, cells were allowed to go through one division and then were subsequently fixed, permitting analyses on all cells. It is possible that nuclei exhibiting aneuploidy have undergone cell death in vivo, explaining why we do not see increased amounts of unpairing. In conclusion, Top2 contributions to nuclear functions are complex. Loss of Top2 may result in subtle changes in pairing that may affect transcription and transvection.

<p><!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:HyphenationZone>21</w:HyphenationZone> <w:PunctuationKerning /> <w:ValidateAgainstSchemas /> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables /> <w:SnapToGridInCell /> <w:WrapTextWithPunct /> <w:UseAsianBreakRules /> <w:DontGrowAutofit /> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif]--> <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; line-height:150%; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} p.Standardmedluft, li.Standardmedluft, div.Standardmedluft {mso-style-name:"Standard med luft"; margin-top:14.0pt; margin-right:0cm; margin-bottom:0cm; margin-left:0cm; margin-bottom:.0001pt; text-align:justify; line-height:150%; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 70.85pt 2.0cm 70.85pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 10]><mce:style><! /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} --><!--[endif]--></p><p>Transvection is a widespread phenomenon affecting chromosomal and gene function. There are many examples of epigenetic machineries controlling gene regulation. Nuclear Lamin proteins could have this function. This project shows <em>zeste</em> dependent transvection<strong> </strong>in loss-of-function<strong> </strong><em>lamin</em> mutants in <em>Drosophila melanogaster</em>.<strong> </strong>The <em>zeste</em> locus<strong> </strong>encodes a regulatory gene product affecting the expression of other loci, e.g. <em>white</em>. No transvection effect in loss-of-function <em>lamin </em>mutants has so far been shown. The effect of homozygosity versus heterozygosity of <em>lamin</em> on <em>zeste</em>-dependent transvection at paired <em>white</em> loci was analysed by crossing fruit flies to get homozygous<em> </em><em>z¹</em>; <em>lam</em>^D395 individuals. Whether or not the <em>zeste (z¹</em>) transvection effect on <em>white</em> was affected by <em>lam</em> ^D395 loss-of-function mutation was determined by comparing the eye colour phenotypes of double mutant <em>z¹</em>; <em>lam</em>^D395 females to that of <em>z¹/Y</em>; <em>lam</em>^D395 males, which were used as an internal negative control since they are hemizygous for <em>zeste</em> that is located on the X chromosome. Females homozygous for <em>z¹</em> and <em>lam</em>^D395 displayed the <em>z¹</em>-characteristic yellow eye colour. The conclusion is that <em>zeste</em>-dependent transvection effect at <em>white</em> also occurs in <em>lamin</em> mutants. Future research on transvection is needed in order to understand the exact mechanisms of gene regulation. Even gene therapies for some human diseases can take advantage of <em>trans</em>-acting sequences to correct gene expression.</p><p> </p>

<!--[if gte mso 9]><xml> <w:WordDocument> <w:View>Normal</w:View> <w:Zoom>0</w:Zoom> <w:HyphenationZone>21</w:HyphenationZone> <w:PunctuationKerning /> <w:ValidateAgainstSchemas /> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables /> <w:SnapToGridInCell /> <w:WrapTextWithPunct /> <w:UseAsianBreakRules /> <w:DontGrowAutofit /> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><!--[if gte mso 9]><xml> <w:LatentStyles DefLockedState="false" LatentStyleCount="156"> </w:LatentStyles> </xml><![endif]--> <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; line-height:150%; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} p.Standardmedluft, li.Standardmedluft, div.Standardmedluft {mso-style-name:"Standard med luft"; margin-top:14.0pt; margin-right:0cm; margin-bottom:0cm; margin-left:0cm; margin-bottom:.0001pt; text-align:justify; line-height:150%; mso-pagination:widow-orphan; font-size:12.0pt; mso-bidi-font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 70.85pt 2.0cm 70.85pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 10]><mce:style><! /* Style Definitions */ table.MsoNormalTable{mso-style-name:"Table Normal";mso-tstyle-rowband-size:0;mso-tstyle-colband-size:0;mso-style-noshow:yes;mso-style-parent:"";mso-padding-alt:0cm 5.4pt 0cm 5.4pt;mso-para-margin:0cm;mso-para-margin-bottom:.0001pt;mso-pagination:widow-orphan;font-size:10.0pt;font-family:"Times New Roman";mso-ansi-language:#0400;mso-fareast-language:#0400;mso-bidi-language:#0400;} --><!--[endif]--> Transvection is a widespread phenomenon affecting chromosomal and gene function. There are many examples of epigenetic machineries controlling gene regulation. Nuclear Lamin proteins could have this function. This project shows zeste dependent transvection in loss-of-function lamin mutants in Drosophila melanogaster. The zeste locus encodes a regulatory gene product affecting the expression of other loci, e.g. white. No transvection effect in loss-of-function lamin mutants has so far been shown. The effect of homozygosity versus heterozygosity of lamin on zeste-dependent transvection at paired white loci was analysed by crossing fruit flies to get homozygous z1; lamD395 individuals. Whether or not the zeste (z1) transvection effect on white was affected by lam D395 loss-of-function mutation was determined by comparing the eye colour phenotypes of double mutant z1; lamD395 females to that of z1/Y; lamD395 males, which were used as an internal negative control since they are hemizygous for zeste that is located on the X chromosome. Females homozygous for z1 and lamD395 displayed the z1-characteristic yellow eye colour. The conclusion is that zeste-dependent transvection effect at white also occurs in lamin mutants. Future research on transvection is needed in order to understand the exact mechanisms of gene regulation. Even gene therapies for some human diseases can take advantage of trans-acting sequences to correct gene expression.

Nous montrons que les morphismes du groupe de tresses à n brins dans le mapping class group d'une surface de bord éventuellement non vide et de genre inférieur ou égal à n/2 sont soit cycliques (i.e. dont l'image est un groupe cyclique), soit des transvections de monodromie géométriques (i.e. à multiplication près par un élément du centralisateur de l'image, un générateur standard du groupe de tresses est envoyé sur un twist de Dehn, et deux générateurs standards consécutifs sont envoyés sur deux twists de Dehn le long de deux courbes s'intersectant en un point). En corollaire, nous déterminons les endomorphismes, les endomorphismes injectifs, les automorphismes et le groupe d'automorphisme des groupes suivants : le groupe de tresses à n brins lorsque n est supérieur ou égal à 6, le mapping class group de toute surface de genre supérieur ou égal à 2. Pour chacun des énoncés impliquant le mapping class group, nous étudions deux cas : lorsque le bord est fixé point par point ou seulement composante par composante. Nous décrivons également l'ensemble des morphismes entre différents groupes de tresses dont le nombre de brins diffèrent d'au plus un, et l'ensemble des morphismes entre mapping class groups de surfaces (de bord éventuellement non vide) dont les genres (supérieurs ou égal à 2) différent d'au plus un.

Cette thèse est consacrée à l'étude des revêtements galoisiens et à la recherche du revêtement universel et du groupe fondamental pour les algèbres de dimension finie, connexes et basiques sur un corps algébriquement clos. Pour ce faire, nous partons d'une construction déjà existante: le groupe fondamental associé à toute présentation d'une telle algèbre A par son carquois ordinaire Q et des relations admissibles. Nous commençons par comparer les différentes présentations de A. Les automorphismes de l'algèbre kQ des chemins de Q permettent de relier deux présentations de A et parmi ceux-là, nous distinguons les dilatations et les transvections: elles engendrent le groupe des automorphismes de kQ, en outre, les groupes fondamentaux de deux présentations de A reliées par une dilatation ou une transvection sont liés entre eux par un passage au quotient. Ceci permet d'exhiber un groupe fondamental pour A lorsque le corps de base est de caractéristique nulle et lorsque Q n'a pas de double raccourci. Ces raisonnements se transposent à l'étude des revêtements galoisiens de A puisqu'à chaque présentation de A est associé un revêtement galoisien de A et de groupe le groupe fondamental de la présentation. Ainsi, sous les hypothèses précédentes fournissant le groupe fondamental de A, un revêtement universel de A existe. Ce dernier résultat est également démontré pour un corps de caractéristique quelconque, lorsque A est monomiale et lorsque Q n'a ni flèches multiples ni cycle orienté tout en admettant d'éventuels double raccourcis.