Ch 9 - Drosophila Development

The pair-rule gene
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The ___________ allows sperm to pass through it one at a time and probably prevents polyspermy in Drosophila.micropyleBy the time the sperm enters the egg, the egg has already begun to specify the body axes; thus, the sperm enters an egg that is already organizing itself as anembryo.The sperm and egg cell membranes do not fuse. Rather,the sperm enters the egg intact. - The DNA of the male and female pronuclei replicate before the pronuclei have fused, and after the pronuclei fuse, the maternal and paternal chromosomes remain separate until the end of the first mitosisKaryokinesisnuclear division (mitosis)Ctyokinesisdivision of the cytoplasmKaryokinesis occurs without cytokinesis so as to create a _______________, a single cell with many nuclei residing in a common cytoplasmsyncytiumThe rapid rate of division is accomplished by repeated rounds of alternating S (DNA synthesis) and M (mitosis) phases in the absence ofAbsence of a G phaseDuring the ninth division cycle, approximately ____________ reach the surface of the posterior pole of the embryo.five nucleiThe five nuclei become enclosed by cell membranes and generate the _______________ that give rise to the gametes of the adult.pole cellsDuring these stages of nuclear division, the embryo is called a __________________, since no cell membranes exist other than that of the egg itself.syncytial blastoderm - called this until division 13After cycle 13, the _________________ forms by ingression of cell membranes between nucleicellular blastodermHow are cells arranged in the cellular blastoderm?all the cells are arranged in a single-layered jacket around the yolky core of the eggthe formation of the cellular blastoderm involves a delicate interplay between these two cytoskeletal elementsmicrotubules and actin filamentsThis slowdown of nuclear division, cellularization, and concomitant increase in new RNA transcription is often referred to as themid- blastula transitionmaternal-to-zygotic transitionIt is at this stage that the maternally provided mRNAs are degraded and control of development is handed over to the zygote's own genomeIt is at this time that the genes of the nuclei become activeThe mid-blastula transitionSuch genes that are active in the mother to make products for the early development of the offspring are called: ___________ and the mRNAs in the oocyte are often referred to asmaternal effect genes maternal messagesthe activation of the embryo's own genes begins around cycle 11 and is greatly enhanced at cycle 14Zygotic gene transcriptionGastrulation begins shortly after themid-blastula transition.Segmental body plandistinct head end and a distinct tail end, between which are repeating segmental units. Three of these segments form the thorax, while another eight segments form the abdomen.the outer tissue layer (ectoderm) bends to form the ____________. (looks like a cap)cephalic furrowThis furrow eventually pinches off from the surface to become a ventral tube within the embryo.ventral furrowAnterior end of the embryo is pointing _____ and the posterior end is pointing _______up downDorsal side of the embryo shows:pole cells in posterior midgut invaginationVentral side of the embryo shows:ventral furrow anterior midgut invagination celphalic furrowThe processes of embryogenesis may officially begin at fertilization, but many of the molecular events critical for Drosophila embryogenesis actually occur duringoogenesisThese germline cells include 15 metabolically active _____________--that make mRNAs and proteins that are transported into the single cell that will become the oocyte.nurse cells16 germline cells, along with a surrounding epithelial layer of somatic follicle cells, constitute the _________________ in which the oocyte will developegg chamberThe pattern is established by maternal effect genes that form gradients and regions of morphogenetic proteins. These proteins are transcription factors that activate the ______________, which define broad territories of the genesThe gap genes enable the expression of the ____________ genes, each of which divides the embryo into regions about two segments widepair-ruleThe _______________ genes then divide the embryo into segment-sized units along the anterior-posterior axis.segment polarityMaternal gene effects orange: Blue:O: bicoid mRNA B: Nanos mRNAGap genes Orange: Green:O: Hunchback mRNA G: Krupell mRNAPair-rule gene Pink:P: Fushi tarazu mRNASegmented polarity genes Green:G: engrailed mRNAbicoid mRNAanteriorNanos mRNAposteriorgap genesmutations in them cause gaps in the segmentation pattern - Gap genes encode transcription factors - regulate the transcription of pair-rule genespair-rule genesdivide the embryo into periodic units. - results in a striped pattern of seven transverse bands perpendicular to the anterior-posterior axis.segment polarity genesmRNA and protein products divide the embryo into 14-segment-wide units - establishing the periodicity of the embryo.homeotic selector geneswhose transcription determines the developmental fate of each segmentNüsslein-Volhard et al. showed these 3 things about the bicoid gene(1) Bicoid protein was found in a gradient, highest in the anterior (head-forming) region (2) embryos lacking Bicoid could not form a head (3) when bicoid mRNA was added to Bicoid-deficient embryos in different places, the place where bicoid mRNA was injected became the headWhen injected into the anterior of bicoid-deficient embryos (whose mothers lacked bicoid genes), the bicoid mRNA did what?"rescued" the embryos and they developed normal anterior-posterior polarity.If bicoid mRNA was injected into the center of an embryo, then that middle region became the what?the head, with the regions on either side of it becoming thorax structures.If a large amount of bicoid mRNA was injected into the posterior end of a wild-type embryo (with its own endogenous bicoid message in its anterior pole) what happened?two heads, one at either endHow would a bicoid-deficient mutant develop without manipulation?bicoid-deficient phenotype = two tails (tail-abdomen-tail)Normal development of wild-type drosophila embryo (with bicoid gene at head end)wild-type phenotype= normal arrangement (head-tail)Adding bicoid mRNA to anterior end of bicoid-deficient mutant embryonormal development (head-tail)Adding bicoid mRNA to middle of bicoid-deficient mutant embryo"head in middle" with tails on both ends (tail-head-tail)Adding bicoid mRNA to posterior end of wild type embryotwo heads! (head-tail-head)Bicoid and Nanos protein products (are / are not) bound to the cytoskeleton, they diffuse toward the middle regions of the early embryo, creating the two opposing gradients that establish the anterior-posterior polarity of the embryo.they are not boundhunchback (hb) and caudal (cad)are critical for patterning the anterior and posterior regions of the body plan, respectively - maternally provided mRNAstranslation of the hb mRNAs is repressed by the diffusion gradients ofNanostranslation of the cad mRNAs is repressed by the diffusion gradients ofBicoidIn the anterior region, Bicoid protein prevents translation of thecaudal message - By recruiting this translational inhibitor, Bicoid prevents translation of caudal in the anterior of the embryoIn the posterior region, Nanos protein prevents translation of thehunchback message.The result of these interactions is the creation of ___number__ maternal protein gradients in the early embryofour -An anterior-to-posterior gradient of Bicoid protein - An anterior-to-posterior gradient of Hunchback protein - A posterior-to-anterior gradient of Nanos protein - A posterior-to-anterior gradient of Caudal protein2 proteins which result in: An anterior-to-posterior gradientBicoid, hunchback2 proteins which result in: A posterior-to-anterior gradientNanos, caudalThe terminal gene group the third set of maternal genes whose proteins generate the unsegmented extremities of the anterior-posterior axis:Acron , telsonAcronanterior terminal = brainTelsonposterior terminal = tailMutations in these terminal genes result in the loss ofboth the acron and most anterior head segments and the telson and most posterior abdominal segmentsThe terminal genesThese genes give rise to the extreme anterior-posterior regions that are not segmented.The anterior-posterior axis of the Drosophila embryo is specified by three sets of genes:1. Genes that define the anterior organizing center. (ex. bicoid) 2. Genes that define the posterior organizing center. (ex. Nanos) 3. Genes that define the terminal boundary regions. (acron + telson)Three types of segmentation gene mutations1. Gap: Kruppel 2. Pair-rule : fushi tarazu 3. Segment polarity: engrailedGap mutantslack large regions of the body (several contiguous segments) - gaps in the segmentationPair-rule mutantslack portions of every other segment - defines the pattern and determines the number of segments - Mutations result in half the normal number of segmentsSegment polarity mutantsshow defects (deletions, duplications, polarity reversals) in every segmentParasegments in the Drosophila embryo are shifted ______one compartment forward in relation to the segments.4 main gap genesKrüppel (Kr) knirps (kni) hunchback (hb) giant (gt)Three toggle switches in the gap gene network1. strong mutual inhibition between Hunchback and Knirps, 2. strong mutual inhibition between Giant and Krüppel 3. concentration- dependent interaction between Hunchback and Krüppel.3 main reasons having separate enhancers are important1. a mutation in a particular enhancer can delete its particular stripe and no other 2. If a reporter gene is fused to one of the enhancers, the reporter gene is expressed only in that particular stripe 3. placement of the stripes can be altered by deleting the gap genes that regulate them.Stripe placement is a result of (2)(1) the modular cis-regulatory enhancer elements of the pair-rule genes (2) the trans-regulatory gap gene and maternal gene proteins that bind to these enhancer sites.The segment polarity genes encode proteins that are constituents of the __________ and ___________signaling pathwaysWnt and Hedgehogwingless to be transcribed solely in the column of cells directly _________ to the cells where engrailed is transcribedanteriorHomeobox genes specifysegment identity in flies as they do in humans→structures to be formed by each segment in the embryoPolytene chromosomesreplicate w/o separatingFruit fly fertilization1. Sperm enters an egg that is already activated 2. Sperm can enter the egg through the micropyle 3. By the time the sperm enters the egg, the egg has already begun to specify the body axes 4. Sperm enters the egg fully intact 5. Sperm and egg cell membranes do not fuse 6. Gigantic sperm→sexual selectionGurken paracrine binds to Torpedo on uncommitted follicle cells resulting ininduced differentiationBicoid is THE anterior morphogenforms the headCaudal proteins (dark stain) enter the nuclei and helps specifyposterior fatesCaudal & hunchback protein gradienthelp establish anterior-posterior axisHunchback help specifyanterior fatesGap genes- Activated and repressed by maternal effect genes - Expressed in 1-2 broad domains along the anterior-posteriorThe gap genes have low expression across the embryo BUT get consolidated into regions with high abundance as nuclear divisions continue. example:• EX: Hunchback gradient→AT division 12, Hunchback is found in high levels across the anterior part of the embryo. • By division 15, Hunchback expression makes a gradient towards the middle of the embryo • At this point Hunchback was undetectable in the posterior portionSpecific promoter regions of the even-skipped (eve) gene controlspecific transcription bands in the embryoThe segment polarity genes - engrailed vs wingless- One row of each cell express wingless (wg) and engrailed (en) - en is expressed if Eve or Fitz genes are expressed. - The cells that produce en also express hedgehog. - wg is expressed in cells that has no Eve or Ftz expression.Interactions b/w engrailed and wingless- Reciprocal interactions btw neighboring cells - Cells secreting hedgehog activate the transcription of wg - Cells secreting wg activate the transcription of en & hedgehogHomeotic gene contains homeobox sequence that codes for60 amino acid homeodomain proteinsHomeodomain is a ______________ motifhelix-turn-helix2 regions of chr III contain most of the genes called homeotic complex or Hom-C - 1st region: Antennapedia complex - 2nd region: Bithoarx complex- 1st region: antennapedia complex: contains labial, Antennapedia, sex combs reduced, deformed proboscipedia - 2nd region: Bithoarx complex: contains Ultrabithorax, Abdominal A and Abdominal Bgurken mRNA that is critical foranterior-posterior axis formation also determine dorsal-ventral patterning. - gurken mRNA localized btw the oocyte nucleus & dorsal follicle cells of the ovary and forms a crescent btw the oocyte nucleus and the oocyte cell membrane.Gurken proteindorsal-ventral patterningGurken stimulate the follicle cells closest to the oocyte nucleus→through torpedo signal those cells to differentiate into dorsal follicle cells.- The proteins translated from the mRNA produce anterior- posterior gradient along the dorsal surface of the oocyte. - Gurken can only diffuse over a short-distanceDorsal Protein Distinguish _______ and _____the Dorsum (Back) and the Ventrum (belly) in the fly embryoHow does the Dorsal Protein ONLY enter the nucleus of ventral cells?- Dorsal forms a dimer with Cactus - Dorsal + Cactus = inactive and remain in the cytoplasm of all cells - Cactus gets phosphorylated by a protein kinase called Pelle. - p-Cactus heads to the proteosome - Leaves Dorsal behind - When Dorsal + Cactus binding is broken down, - Dorsal translocate to the nucleus - This only happens in the ventral cells→unless there is a mutation - Ventral cells become the mesodermDrosophila Gastrulation- 16 ventral most cells with highest Dorsal protein abundance buckle inward→form ventral furrow - This furrow becomes a tube that invaginates into the embryo - Flattens out and differentiate into the mesodermMesoderm gives rise to- All body muscles - All fat cells - GonadsToggle switches in gap gene network1) Strong mutual inhibition btw Hunchback& Knirps 2) Strong mutual inhibition btw Giant&Kruppel 3) Concentration interaction btw Hunchback & Kruppel a) High doses of Hunchback = inhibit Kruppel b) Moderate doses of Hunchback = promote Kruppel formationSpecific promoter regions of the even-skipped (eve) gene control ______________specific transcription bands in the embryoThe segment polarity genes - engrailed vs wingless- One row of each cell express wingless (wg) and engrailed (en) - en is expressed if Eve or Fitz genes are expressed. - The cells that produce en also express hedgehog. - wg is expressed in cells that has no Eve or Ftz expression.engrailed vs wingless