Black bars show BAC locations. Deer Park High School; David Prescott, St. John's Ravenscourt School; Jeannie Cuervo, Cleveland High School; Jeremy Barlow, Uniondale High School; PAIRED-LIKE HOMEODOMAIN TRANSCRIPTION FACTOR 1. Where would you expect Pitx1 to be expressed in the above scenario? Construct models to represent the regulation of gene transcription. check mark next to the tissue(s) that will express Pitx1.) These fossils come from a quarry site in Nevada that used to be a freshwater lake about 10 million years ago. repressors, general transcription factors, mediators, promoters, RNA polymerase, phenotype, evolution, Accessibility StatementFor more information contact us atinfo@libretexts.org. As a result, the PITX1 gene is abnormally active during arm and hand development. What happens when you dump an ocean fish into a freshwater lake? Mutational scans reveal differential evolvability of. Legal. View UCBerkeleyOfficials profile on Instagram, View UCZAXKyvvIV4uU4YvP5dmrmAs profile on YouTube, A $25-an-hour minimum wage for medical workers could benefit everyone, Julie Chavez Rodriguez, visionary Berkeley grad, to run Biden campaign, Community engagement improves wildlife restoration outcomes, UC Berkeley computer scientist wins 2023 Guggenheim Fellowship, Berkeley political scientist Scott Straus named to prestigious fellowship, UC Berkeley breaks ground on new Engineering Center, Berkeley graduate programs succeed once again in new U.S. News rankings, Counting fish teeth reveals DNA changes behind rapid evolution. with limb enhancers and is enriched on hindlimb cis-regulatory elements. Accessibility An evolutionary geneticist looks at how small genetic changes can have big evolutionary effects. (Put a check mark next to the Marine Stickleback Jaw Transcription is ON. 5. These changes affect regions of DNA known as regulatory elements, which help turn on or turn off genes (known as enhancers or repressors, respectively). Sorry, your blog cannot share posts by email. Watch the short film, The Making of the Fittest: Evolving Switches, Evolving Bodies. J, Lohan S, Dathe K, Nardone AM, Ferrari P, Landi A, Wittler L, Timmermann B, This site needs JavaScript to work properly. Regulating Evolution. Scientific American, May 2008: 60-, AUTHORS Stickleback fish provide genetic road map for rapid evolution By Robert Sanders, Media relations | June 21, 2021 Three threespine sticklebacks, each about 2 1/2 inches long, that were captured from an Alaskan lake in 2010 as part of a three-decade study of their evolution from sea-faring fish to freshwater fish. The Pitx1 DNA of marine stickleback fish contains a pelvic switch, while the Pitx1 DNA of freshwater 2023 May 22;378(1877):20220054. doi: 10.1098/rstb.2022.0054. stop codon, resulting in a nonfunctional truncated protein. Possible answers could Dev Cell. You isolate DNA from jaw, pelvic, eye, and pituitary 4. Nature. hb```VE!10p`0dQ4q`( 2DG gene as a result of a mutation in the regulatory switch. In which tissues would you expect Pitx1 to be expressed in the above scenario? Low ionic strength water and absence of predatory fishes are associated with reduction of the pelvic skeleton, and lack of Pitx1 expression in the pelvic region is evidently . IST-1.K, IST-1.N, IST-2.D, IST-2.A, IST-2.B, IST-2.C, IST-2.E; SP2, SP3, Teaching Gene Regulation Using Corn and Stickleback Fish, The Making of the Fittest: Evolving Switches, Evolving Bodies, Schooling Behavior of Stickleback Fish from Different Habitats, Using Genetic Crosses to Analyze a Stickleback Trait, Simulating Evolution of a Rock Pocket Mouse Population, Icefish Blood Adaptations: Antifreeze Proteins, Beaks As Tools: Selective Advantage in Changing Environments, The Making of the Fittest: Natural Selection and Adaptation. of stickleback fish with reduced pelvises? government site. Besoke R, Held K, Mundlos S, Kurth I. Deletions in PITX1 cause a spectrum of While the predictability of evolution is not an exact science, the authors believe their understanding of how the stickleback colonizes fresh water provide important insights into the genomics behind vertebrate evolution and provides insight into how evolution might proceed species-wide in the future. The basic elements of transcription regulation in eukaryotes are similar to the very well-studied lac and trp operon systems found in bacterial cells. List three limitations Synthesis Questions 10. the animals life cycle. The molecular mechanisms underlying major phenotypic changes that have evolved repeatedly in nature are generally unknown. : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Zoo_Scavenger_Hunt : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { Anatomy_Worksheets : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Cell_Biology : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Chapter_6 : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Ecology : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Evolution : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Genetics : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, Investigation: Gene Switches in Stickleback Fish, https://bio.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fbio.libretexts.org%2FLearning_Objects%2FWorksheets%2FBook%253A_The_Biology_Corner_(Worksheets)%2FEvolution%2FInvestigation%253A_Gene_Switches_in_Stickleback_Fish, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), REGULATORY SWITCHES OF THE PITX1 GENE IN STICKLEBACK FISH, PART 3: GENE REGULATION IN DIFFERENT TISSUES. transcription, eukaryotic gene regulation, regulatory switches, enhancers, or cis-regulatory regions, activators. 2023 Feb 17;9(7):eade6529. To identify regulatory sequences that drive limb expression in developing mouse embryos, we surveyed an 850 kb region of the Pitx1 locus using overlapping mouse BAC clones that cover the entire Pitx1 coding region as well as flanking regions ().Note that the scan includes 607 kb of sequence upstream of the Pitx1 coding region. How is Pitx1 expressed in different tissues? The Pitx1 gene has multiple regulatory switches that control the expression of the gene in different tissues: the pituitary, jaw, and pelvic tissues. For example, write letter a in the box pointing at the protein-coding region. The pelvic skeleton of threespine stickleback fish contributes to defence against predatory vertebrates, but rare populations exhibit vestigial pelvic phenotypes. jaw ! Description This film explores how mutations in gene regulatory regions have resulted in major changes in the anatomy of freshwater populations of stickleback fish. Activators bind to regulatory switches in a sequence-specific manner. body morphology, development, genetic switch, mutation, pelvic fin, pelvis, pituitary homeobox transcription factor 1 (Pitx1). Please see the Terms of Use for information on how this resource can be used. Be sure to include the appropriate switches and Pitx1 coding region and label your drawing. How Can Gene Editing Eliminate Lyme Disease? The complete absence of Pitx1 protein from all tissues is lethal to the organism. These gene mutations have also been found in people with extra toes (polydactyly), specifically a severe form of the abnormality called mirror-image polydactyly. L3C]jX;L+nq:Tb67!d Label the boxes in Figure 1 with the letters a-e, which correspond to the terms listed below. Depending on students background, it may be helpful to pause the animation at various points to discuss different features. May;126(9):1805-10. doi: 10.1242/dev.126.9.1805. Check all that apply. activators or repressor proteins present in that cell. the exceptions listed in the Key Concepts and Learning Objectives. official website and that any information you provide is encrypted : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Corn_Genetics_Chi_Square_Analysis : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Dihybrid_Crosses_in_Guinea_Pigs : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", DNA_Coloring : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "DNA_Coloring__Transcription_&_Translation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "DNA_Extraction_\u2013_Strawberry" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", DNA_Mutation_Activity : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Genetics_of_Chicken_Combs : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Genetics_of_Wisconsin_Fast_Plants : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "Genetics_Practice_Problems_-_Easy_Mode" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", Genetic_Crosses_That_Involve_2_Traits : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "How_Can_DNA_Replication_Be_Modeled?" include the appropriate switches and Pitx1 coding region and label your drawing. Most such populations have relied upon the same genes to adapt, and usually within a decade. What is a gene variant and how do variants occur? However, as shown in the film, Pitx The activator or repressor protein acts like the hand that flips the switch, but it can only turn the switch on or off if it can bind to the specific DNA sequence. Student answers will vary. Answers are underlined and bold. Gene transcription is a complex process that involves interactions of proteins and regulatory regions of DNA. fixed in a set population of cells, like in the case of cancer cells. Epub 2023 Apr 3. Schooling Behavior of Stickleback Fish from Different Habitats, The Making of the Fittest: Evolving Switches, Evolving Bodies, Using Genetic Crosses to Analyze a Stickleback Trait, Regulation of Eukaryotic DNA Transcription. Each enhancer was cloned as a 2x concatemer upstream of an hsp70 promoter and eGFP. 3. In the jaw, there is a different activator (activator 1) that binds to the jaw switch to turn on Pitx1 in the jaw tissue. example, write letter a in the box pointing at the coding region of a gene. The complete absence of Pitx1 protein from all tissues is lethal to the organism. Specimens from one of three populations with right-biased or unbiased pelvic reduction sometimes expressed Pitx1. Michael Bell, currently a research associate in the University of California Museum of Paleontology at UC Berkeley, stumbled across one such natural experiment in 1990 in Alaska, and ever since has been studying the physical changes these fish undergo as they evolve and the genetic basis for these changes. It was a trip to Cooks Inlet in Alaska in 1990 that led to the current paper. He has even created his own experiments, seeding three Alaskan lakes with oceanic sticklebacks in 2009, 2011 and 2019 in order to track their evolution from oceanic fish to freshwater lake fish. You isolate the DNA from the heart of the freshwater stickleback that lack pelvic spines. Be sure to include the appropriate switches and Pitx1 coding region and label your drawing. Explain the role that regulatory switches play in determining whether stickleback embryos will develop pelvic spines. Regulating that expression simply means turning on or off or increasing or decreasing the production of a given protein. Pelvic reduction in stickleback fish (family Gasterosteidae) provides a striking example of parallel evolution in a genetically tractable system. humans and a clubfoot-like phenotype in mice. Many freshwater populations of sticklebacks lack the long spines that project from the pelvis of their marine relatives. Updated August 2015, This hands-on activity supports the short film, The Making of the Fittest: Evolving Switches, Evolving Bodies, and, aims to help students understand eukaryotic gene regulation and its role in body development using the Consider what was revealed in the film. mutation will be the same in all tissues because it is an inherited mutation. Note that the transcriptional orientation of the Pitx1 gene is from right to left in this view (arrow). Epub 2023 Apr 5. Other proteins play a role in these regulatory regions. 2005 Dec;234(4):815-23. doi: 10.1002/dvdy.20564. Gene regulation is essential for the cell to perform the functions needed to live. Other proteins play a role in these regulatory regions. Our records indicate you have visited this interactive video on this device before. jaw ______ pelvis ______ Elife. The binding of the activators to 4 white pipe cleaners, 12-18 inches long (or 18-inch white twine), Magic markers (blue, green, red, yellow, purple). Circle the fish below that lacks Pitx1 expression in the pelvis and EXPLAIN your choice. The basic elements of transcription regulation in eukaryotes are similar to the very well-studied lac and trp operon systems found in bacterial cells. The PITX1 protein may play a role in formation of the pituitary gland and tissues derived from the branchial arch, such as the roof of the mouth, the jaw, and parts of the inner ear. protein can be absent in the pelvis alone, and the fish survives. independently regulates the expression of the gene in different parts of the body at different times in RNA polymerase is placed correctly on the DNA. Watch the short film, The Making of the Fittest: Evolving Switches, Evolving Bodies. 9. Before example of a well-studied gene called Pitx1. 2010 May 6;465(7294):13. doi: 10.1038/465013e. For Alvarado DM, McCall K, Aferol H, Silva MJ, Garbow JR, Spees WM, Patel T, DNA fragility in the parallel evolution of pelvic reduction in stickleback fish. Directional asymmetry of pelvic vestiges in threespine stickleback. In this activity, we will talk about how gene expression is regulated through transcription. Alleles of Pitx1 from pelvic-complete (FRIL, LITC) and pelvic-reduced populations (PAXB) were combined, MeSH During development, different sets of genes need to be turned on and off in the right places, at the right times, and in the right sequence for bodies to be built correctly. However, the Pitx1 protein is actually important in building other body parts and is therefore expressed in multiple . Interestingly, protein production can be regulated at the translational and transcriptional steps, as well as after a protein is produced. During development, different sets of genes need to be turned on and off in the right places, at the right times, and in the right sequence for bodies to be built correctly. Pitx1Pitx1 HHS Vulnerability Disclosure, Help Am J Hum Genet. Because the PITX1 protein normally directs lower limb structure, bones, muscles, and tendons in the arms and hands develop more like those in the legs and feet, leading to the features of Liebenberg syndrome. marine and freshwater stickleback fish? We have to consider the following mutation that knocks out the Jaw View the full answer Transcribed image text: Chan D, Mennen U, Klopocki E, Mundlos S. Homeotic arm-to-leg transformation This piece will not be used in the activity, but you o Pituitary switch binding activator (yellow). A fish inherits a mutation that results in a new By Robert Sanders, Media relations| June 21, 2021June 21, 2021, Three threespine sticklebacks, each about 2 1/2 inches long, that were captured from an Alaskan lake in 2010 as part of a three-decade study of their evolution from sea-faring fish to freshwater fish. The Pitx1 protein has many important functions in various tissues during stickleback development. Modeling the Regulatory Switches of the Pitx1 Gene in Stickleback Fish PART 2: Gene Regulation in Different Tissues As you saw in the film, the presence or absence of pelvic spines in the stickleback fish is controlled by whether the Pitx1 gene is expressed in the pelvic tissue. Please enable it to take advantage of the complete set of features! This video explores the evolution of a stickleback fish population over tens of thousands of years, based on fossil data. representations). Gene transcription is a complex process that involves interactions of proteins and regulatory regions of DNA. The animation in the film (8:00-8:24) and Figure 1 show some of the factors involved but not all. Regulatory switches (or enhancers) 6. o General transcription factors (optional, blue) (Put a In addition, students should understand the role of genetic variation in a. species in natural selection and evolution. Bethesda, MD 20894, Web Policies Most marine stickleback fish have a pelvis with a protective spine, but stickleback fish from certain freshwater populations have a missing or reduced pelvis. Klopocki E, Kahler C, Foulds N, Shah H, Joseph B, Vogel H, Luttgen S, Bald R, How can a spined stickleback fish have the exact same PitX1 sequence as a spineless stickleback? Students should have two sets of pipe cleaners. The drawings in the surrounding boxes show the Pitx1 gene region and activator proteins present in the jaw, pelvis, eye, or pituitary tissues. 10. Each switch Epub 2006 Aug 31. Pay close attention to how the switches regulate the expression of the Pitx1 gene in stickleback embryos. a. Protein-coding region Epub 2012 Nov Gene transcription is a complex process that involves the interactions of proteins and regulatory regions of DNA. The known Pel enhancer maps upstream of the stickleback Pitx1 gene . Next Generation Science Standards HS-LS1, HS-LS3, HS-LS3, HS-LS4, HS-LS4 This page titled Investigation: Regulatory Switches of the PITX1 Gene in Stickleback Fish is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Shannan Muskopf (Biology Corner) . Over time, the lake fish came to look quite different from their marine forefathers. 2011 Dec;138(24):5301-9. doi: affecting the genes protein product (shape and function). As you saw in the film, the presence or absence of pelvic spines in the stickleback fish is controlled by whether the Pitx1 gene is expressed in the pelvic tissue. lower-limb malformations including mirror-image polydactyly. 0 They will then create models showing how Pitx1 gene transcription is regulated. In multicellular eukaryotes, gene regulation is also important in building bodies. 11. eye ! Would you like email updates of new search results? According to the worksheet (as a whole), Pitx1 is expressed in: (select all that apply) eyes jaws pituitary gland pelvis tail An Example of Gene Regulation in Stickleback Fish Gene expression (what genes are actively being transcribed) is a tightly regulated process. d. mRNA Students review eukaryotic gene transcription using the example of a gene called Pitx1, which is involved in the development of pelvic spines in stickleback fish. The drawings in the surrounding boxes show the Pitx1 gene region and activator proteins present in the jaw, pelvis, eye, or pituitary tissues. Increased chromatin accessibility promotes the evolution of a transcriptional silencer in, P50 HG002568/HG/NHGRI NIH HHS/United States, P50 HG002568-09/HG/NHGRI NIH HHS/United States, HHMI/Howard Hughes Medical Institute/United States, P50 HG02568/HG/NHGRI NIH HHS/United States, NCI CPTC Antibody Characterization Program. ( Note : Some textbooks refer to regulatory switches as enhancers.) Describe the function of regulatory switches. Freshwater via targeted growth control. Description. "YIa0,>Dy@""?m"`Fez{8 $!nNgg0014qN>0 @ 9. In multicellular eukaryotes, gene regulation is also important in building bodies. How is Pitx1 expressed in different tissues? Be sure to label the proteins and DNA The types and amounts of proteins produced by a given cell in the body are very important and carefully regulated. Use your textbook to 27. regulatory switch (eye switch) that regulates Pitx Be sure to Unlike the mutations that cause Liebenberg syndrome (described above), these genetic changes reduce the amount of functional PITX1 protein, which disrupts normal development of the lower limbs. ), Pitx1 is not expressed in any tissue HHS Vulnerability Disclosure, Help and mandible development require the Ptx1 gene. (Photo courtesy of Michael Bell) Transcription factors specifically bind to the promoter to activate transcription. In some cases, these mutations confer a selective advantage for that cell and that mutation will be Sample student drawing: Written by Eriko Clements, PhD, HHMI, and Ann Brokaw, Rocky River High School hb```f``: Abl,/4;>2=Ws The molecular mechanisms underlying major phenotypic changes that have evolved repeatedly in nature are generally unknown. Having multiple switches enables Pitx1 to be used many times in different contexts and expands the versatility of that gene. Marine Stickleback Pelvis Transcription is ON switch controls the transcription of genes in different tissues and at different times in development. Hum Mol Genet. In this activity, you will learn about one particular gene, Pitx1, and how its expression is regulated in different tissues. 90 0 obj <> endobj 119 0 obj <>/Filter/FlateDecode/ID[<6D0891D5A0624A33AB5A40A3DBD84E56>]/Index[90 51]/Info 89 0 R/Length 134/Prev 959151/Root 91 0 R/Size 141/Type/XRef/W[1 3 1]>>stream Would you like email updates of new search results? J Hered. Dev Dyn. (acceptable Activators present in a particular tissue bind to a specific sequence on the DNA and turn Pitx1 on in the appropriate tissues. (The researchers had ruled out the possibility that the Pitx1 gene jumped to chromosome 4 in the ninespine stickleback.) regions in the figure. Development. Accessibility StatementFor more information contact us atinfo@libretexts.org. and thus greatly expand the functional versatility of individual genes. Students will interpret molecular diagrams of eukaryotic gene, transcription to familiarize themselves with the molecular components and mechanisms responsible for We used whole-mount in situ hybridization to study Pitx1 expression in 19 populations of Gasterosteus aculeatus from lakes around Cook Inlet, Alaska, USA. Investigation: Gene Switches in Stickleback Fish is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. Previous studies suggest that cis-acting regulatory changes at the Pitx1locus control pelvic reduction in a population of threespine sticklebacks (Gasterosteus aculeatus). A quarry in Nevada contains fossil stickleback fish that once lived in an ancient freshwater lake at this site about 10 million years ago. The upshot of these 31 years of research is a study, published Friday, June 18, in the journal Science Advances, that details the genomic changes that drive sticklebacks rapid evolution. Use the information from the film and your knowledge of eukaryotic gene transcription to answer questions 5-9. (Put a LeClair EE, Mui SR, Huang A, Topczewska JM, Topczewski J. Dev Dyn. 12. Whether a gene is expressed is dependent on the regulatory switches that the gene possesses and the The center image is that of a stickleback embryo. Parallel evolution of Pitx1 underlies pelvic reduction in Scottish threespine stickleback (Gasterosteus aculeatus). The Pitx1 DNA of marine stickleback fish contains a pelvic switch, while the Pitx1 DNA of freshwater stickleback fish does not. Regulating that expression simply means turning on or off or increasing or decreasing the production of a given protein. 15;20(20):3943-52. doi: 10.1093/hmg/ddr313. Interpret visual representations of eukaryotic gene transcription. Mesocosm experiment reveals scale dependence of movement tendencies in sticklebacks. Field Tested by Amy Peterson, Gananda High School; Cindy Gay, Steamboat Springs High School; Dave Kenyon, Paw Paw High School; David Knuffke, outgrowth through regulation of Tbx4 expression and shapes hindlimb morphologies Transcribing DNA to messenger RNA and translating that RNA to protein is often referred to as gene expression. In this activity, you will learn about one particular gene, Pitx1, and how its expression is regulated in different tissues. Bookshelf Depending Fishing for the secrets of vertebrate evolution in threespine sticklebacks.
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