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Fundamental features of 3D genome organization are established de novo in the early embryo, including clustering of pericentromeric regions, the folding of chromosome arms and the segregation of chromosomes into active (A-) and inactive (B-) compartments. However, the molecular mechanisms that drive de novo organization remain unknown.sup.1,2. Here, by combining chromosome conformation capture (Hi-C), chromatin immunoprecipitation with high-throughput sequencing (ChIP-seq), 3D DNA fluorescence in situ hybridization (3D DNA FISH) and polymer simulations, we show that heterochromatin protein 1a (HP1a) is essential for de novo 3D genome organization during Drosophila early development. The binding of HP1a at pericentromeric heterochromatin is required to establish clustering of pericentromeric regions. Moreover, HP1a binding within chromosome arms is responsible for overall chromosome folding and has an important role in the formation of B-compartment regions. However, depletion of HP1a does not affect the A-compartment, which suggests that a different molecular mechanism segregates active chromosome regions. Our work identifies HP1a as an epigenetic regulator that is involved in establishing the global structure of the genome in the early embryo. The heterochromatin protein HP1 has an essential role in establishing several features of the 3D nuclear organization of the genome during early embryonic development in Drosophila., Author(s): Fides Zenk [sup.1] [sup.2] , Yinxiu Zhan [sup.3] [sup.4] , Pavel Kos [sup.3] , Eva Löser [sup.1] , Nazerke Atinbayeva [sup.1] [sup.2] [sup.5] , Melanie Schächtle [sup.1] , Guido [...]

Combinatorial spatial and temporal patterning of stem cells is a powerful mechanism for the generation of neural diversity in insect and vertebrate nervous systems. In the developing Drosophila medulla, the neural stem cells of the outer proliferation center (OPC) are spatially patterned by the mutually exclusive expression of three homeobox transcription factors: Vsx1 in the center of the OPC crescent (cOPC), Optix in the main arms (mOPC), and Rx in the posterior tips (pOPC). These spatial factors act together with a temporal cascade of transcription factors in OPC neuroblasts to specify the greater than 80 medulla cell types. Here, we identify the enhancers that are sufficient to drive the spatially restricted expression of the Vsxl and Rx genes in the OPC. We show that removal of the cOPC enhancer in the Muddled inversion mutant leads to the loss of Vsx1 expression in the cOPC. Analysis of the evolutionarily conserved sequences within these enhancers suggests that direct repression by Optix may restrict the expression of Vsxl and Rx to the cOPC and pOPC, respectively. Key words: Drosophila, optic lobe, enhancer, Vsx1, Rx. Le deploiement combinatoire, a la fois spatial et temporel, des cellules souches est un mecanisme puissant pour donner naissance a la diversite neurale chez les systemes nerveux d'insectes et de vertebres. Au sein de la medulla en developpement chez Drosophila, les cellules souches neurales de la zone de proliferation externe (OPC pour << outer proliferation centre >>) se developpent grace a l'expression mutuellement exclusive de trois facteurs de transcription de la famille homeoboite : Vsx1 au centre du croissant OPC (cOPC), Optix dans les bras (mOPC) et Rx dans les extremites posterieures (pOPC). Ces facteurs spatiaux travaillent ensemble avec une cascade temporelle de facteurs de transcription dans les neuroblastes de l'OPC pour specifier plus de 80 types de cellules medullaires. Dans ce travail, les auteurs identifient les amplificateurs qui operent l'expression spatialement restreinte des genes Vsx1 et Rx dans l'OPC. Ils montrent que la deletion de l'amplificateur cOPC qui survient chez l'inversion Muddled entraine la perte d'expression de Vsx1 dans l'OPC. Une analyse des sequences conservees au cours de revolution au sein de ces amplificateurs suggere que la repression directe d'Optrix pourrait etre respon-sable de l'expression de Vsxl et de Rx dans le cOPC et le pOPC, respectivement. Mots-cles : Drosophila, lobe optique, amplificateur, Vsx1, Rx., Introduction In recent years, the Drosophila optic lobe, comprised of the lamina, medulla, and lobula complex, has emerged as a powerful system in which to study the regulatory mechanisms that [...]

Surface chemical compounds are key components of survival and reproduction in many species. Cuticular hydrocarbons (CHCs) are chemical compounds produced by all insects that are used for both desiccation resistance and chemical communication, including communication related to mating. In the species pair of Drosophila melanogaster and D. simulans, female CHCs stimulate conspecific males to mate and repel hetero-specific males. While CHCs are a critical contributor to both reproductive success within a species and isolation between species, few genes underlying species variation in CHC profiles are known. Here, we use genetic mapping of the 3rd chromosome to test a suite of candidate genes for interspecies variation in CHCs. Candidate gene CG5946 was found to be involved in species differences in the production of 7,11-heptacosadiene and 7-tricosene between D. melanogaster and D. simulans. This is therefore a new candidate locus contributing to species-specific variation in the CHC profile. In the process of mapping genes for CHCs, we also identified 29 candidate genes for the reduced survival or inviability of interspecies hybrids. Key words: pheromone, cuticular hydrocarbons, species differences, Drosophila melanogaster, Drosophila simulans. Les composes chimiques de surface sont des composantes cles de la survie et de la reproduction chez de nombreuses especes. Les hydrocarbures cuticulaires (HC) sont des composes chimiques produits par tous les insectes et ils sont utiles a la fois pour la resistance a la dessiccation et pour la communication chimique, incluant la communication liee a l'accouplement. Chez la paire d'especes Drosophila melanogaster et D. simulans,les HC chez la femelle stimulent l'accouplement chez les males de la meme espece et repoussent les males hetero-specifiques. Tandis que les HC contribuent de maniere importante au succes en matiere de reproduction au sein d'une espece et a l'isolement entre especes, peu de genes contribuant a la variation interspecifique pour les profils des HC sont connus. Dans ce travail, les auteurs ont eu recours a la cartographie genetique chez le chromosome 3 pour tester un ensemble de genes candidats pour la variation interspecifique dans les profils des HC. Le gene candidat CG5946 a ete identifie comme etant implique dans les differences interspecifiques dans la production de 7,11-heptacosadiene et de 7-tricosene entre le D. melanogaster et le D. simulans.Celocus constitue donc un nouveau candidat contribuant a la variation interspecifique pour les profils en HC. Au cours de cette cartographie de genes pour les HC, les auteurs ont egalement identifie 29 genes candidats pour une reduction de la survie ou la non-viabilite des hybrides interspecifiques. Mots-cles : pheromone, hydrocarbures cuticulaires, differences d'especes, Drosophila melanogaster, Drosophila simulans., Introduction How species diverge and remain distinct is of key importance in the study of evolutionary processes. Of particular interest are the genetic mechanisms underlying the isolating barriers that arise [...]

It has long been acknowledged that changes in the regulation of gene expression may account for major organismal differences. However, we still do not fully understand how changes in gene expression evolve and how do such changes influence organisms' differences. We are even less aware of the impact such changes might have in restricting gene flow between species. Here, we focus on studies of gene expression and speciation in the Drosophila model. We review studies that have identified gene interactions in post-mating reproductive isolation and speciation, particularly those that modulate male gene expression. We also address studies that have experimentally manipulated changes in gene expression to test their effect in post-mating reproductive isolation. We highlight the need for a more in-depth analysis of the role of selection causing disrupted gene expression of such candidate genes in sterile/inviable hybrids. Moreover, we discuss the relevance to incorporate more routinely assays that simultaneously evaluate the potential effects of environmental factors and genetic background in modulating plastic responses in male genes and their potential role in speciation. Key words: gene expression, plasticity, hybrid incompatibility, speciation. II est reconnu depuis longtemps que des changements dans le niveau d'expression genique peuvent entrainer des differences importantes chez les organismes. Cependant, il n'est pas encore pleinement compris comment les changements d'expression genique evoluent et comment de tels changements influencent les differences entre organismes. Encore moins de choses sont connues sur l'impact que de tels changements peuvent avoir sur la restriction des flux geniques entre especes. Dans ce travail, les auteurs se penchent sur des etudes portant sur l'expression genique et la speciation chez l'espece modele Drosophila. Les auteurs passent en revue des etudes qui ont identifie des interactions geniques avec l'isolement reproductif post-accouplement et la speciation, particulierement celles qui modulent l'expression genique chez les males. Les auteurs s'interessent egalement a des etudes qui ont modifie experimentalement le niveau d'expression genique pour en mesurer l'effet sur l'isolement reproductif post-accouplement. Les auteurs font ressortir la necessite d'etudes plus approfondies du role de la selection causant une perte d'expression de genes candidats chez des hybrides steriles ou non-viables. De plus, les auteurs discutent de la pertinence d'incorporer de maniere plus routiniere des essais qui mesurent simultanement les effets potentiels des facteurs environnementaux et du fond genetique dans la modulation de la reponse plastique des genes males et de leur role potentiel dans la speciation. [Traduit par la Redaction] Mots-cles : expression genique, plasticite, incompatibilite hybride, speciation., Introduction In 1975, King and Wilson used data from protein sequence comparisons and allele frequency of polypeptide products to show high protein similarity between humans and chimpanzee despite substantial morphological [...]