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AnIntroductiontoModelOrganismsofDevelopmentGas:2000litersofmethanegasreleased/day!
Whatmakesagoodmodelorganism?Gas:2000litersofmethanegasreleased/day!
Size:6tons250kgfoodeatenevery100kgofelephantdung/dayGestation:23monthsFemalesgivebirthtosingleoffspringeveryfiveyearsSexualmaturityatage12Whatmakesagoodmodelorganism?Size:1mminlength
LiveonadietofbacteriaGestation:500,000offspringin1weekfromsingleorganismSexualmaturityin3daysGenome:
Sequenced!DrosophilamelanogasterCaenorhabditiselegansArthropodsNematodesXenopuslaevisAmphibiansMammalsMusmusculusHomosapiensCaenorhabditiselegansNematodeWormNematodesaccountforanestimatedfourofeveryfiveanimalsintheworld!Smooth-skinned,unsegmentedwormsfirstusedasamodelorganismbySydneyBrennerin1965C.elegansisdiploidandhasfivepairsofautosomalchromosomes(namedI,II,III,IVandV)andapairofsexchromosomes(X).Mostadultsarehermaphrodite(XX)but.05%oflabpopulationsaremale(XO)Lifespanis2to3weeksWormsareusuallykeptonpetriplatesandfedE.coliAbout10,000wormsfitonasingleplate959Somaticcells-allvisiblewithmicroscope-300neurons-81musclecells-131cellsundergoprogrammedcelldeathDevelopmentalfateofeverycellisknowninC.elegans8to17roundsofdivisionarerequiredforcelldifferentiationdependingontissuetypeandcellfunction
C.elegansgenomecontains19,000genesandisfullysequenced70%ofhumangeneshavewormhomologues
C.eleganswillbethefirstandpossiblyonlyanimalthatweknoweverythingaboutatacellularandmolecularlevelGenefunctionstudieshavebecomerelativelysimplewiththerecentdiscoveryofRNAiUsingRNAinterferenceforlocalandsystemicgenesilencing
inC.elegansA.C.eleganshermaphroditeexpressingGFPtransgenesinthepharynxandthenucleiofbody-wallmusclecellsB.C.eleganshermaphroditeexpressingGFPtransgenes+GFPdoublestrandedRNAinthepharynxA.B.SmallRNAsasregulategeneexpressionduringdevelopment
Lookforheterochronicdefectsinmutagenesisscreen--cellsbehaveasifinanearlierorlaterdevelopmentalstageRegulatorycascadesunveiledwhichinvolvesmallRNAs(21-22nts)Lin-4andLet-7encodeshortuntranslated
RNAsandfunctionbybindingtocomplementarysequencesinmRNAsofspecificgenescontrollingdevelopmentLin-4expressionallowscellstoprogressfromlarvalstage1to3Let-7expressionallowscellstoprogressfromlatelarvaltoadultstagesAAAAALin-4DevelopmentalmRNANotranslationofmRNAMoleculargeneticoflife-spaninC.elegansC.eleganscanlive2Xlongerundersomeexperimentalconditions:MutationsinDAF-16,AGE-1,DAF-2,orCLK-1;heatshockproteins;feedingbehavior;freeradicalexposureandoxidativestressPathwaysinvolvedappeartobeconnectedGeneralfeaturesoflongerlifespanincludelessreproduction,lessgrowthandmoreDNA/cellularrepairCouldberelatedtocaloricrestrictionobservationsinmammalsDrosophilamelanogasterCommonFruitFlyMoststudiedanimalmodelLifeCycleof2weeks(fertilizationtosexualmaturity)Fourpairsofchromosomes:theX/Ysexchromosomesandtheautosomes2,3,and414,000Genes,sequencedgenomeand2/3ofhumandiseasegeneshaveflyhomologuesLargerepositoriesofmutantfliesavailableConservationofpatterningbetweenfliesandmammalsInsituhybridizationofwholeembryocanrevealpatternsofgeneexpressionduringdevelopmentRNAorDNAprobesandlabeledantibodiesareused.PolyteneChromosomes
PresentinsalivaryglandsoffliesOriginatefromchromosomalduplicationwithnocelldivisionHavepatternsofdarkandlightbandsuniqueforeachchromosomalsectionvisiblewithalightmicroscopeCanbelabeledwithnucleicacidprobesCanbeusedtodeterminebindingsiteoflabeledproteinsChromosomalrearrangmentsanddeletionscanbevisualizedAntennapediamutant:Antennaaretransformedintometathoracic(secondsecondthoracicsegment)legsWildtypeflyStudyingOrganogenesisinDrosophilaImaginaldiscsaregroupsofundifferentiatedcellsinlarvathatgiverisetoadultorgansandstructuresTransplantationandgenemis-expressionstudiesallowcharacterizationoforganformationatcellandmolecularlevelOrgan-specificgeneshavemammalianhomologuesXenopus
LaevisAfricanclawedtoadAdvantagesofusingXenopusasamodel:Vertebratemodelwithfundamentalfeaturesofland-dwellingvertebrates
OocytesarelargeandundergoexternaldevelopmentFemalescanbestimulatedtoovulatewithhormonesDevelopmentisrapid;fertilizationtofullyformedtadpoleinafewdaysLargesizeallowsstudyofmovementofcellswithinXenopusembryos.Cleavageevery30minutes
Gastrulationat10hours1daytoneurulationGermlayersandstructuralcharacteristicsareeasilyobservedManipulationofembryomayinvolvesurgeryormRNAinjectionStudyofXenopusDevelopmentbyEmbryoInjections
mRNAsandcDNAscanbeinjectedtostudyroleofgenesandproteins
AntisensetoknockoutexpressionOver-andmis-expressionofproteinofinterestAlternateproteinforms:dominantnegative,constitutivelyactive,etc.Reporterforms(GFP,etc.)StudymaternalvszygoticcontributionsSignalingmoleculesandchemicalagentscanbeappliedtodetermineaffectsondevelopmentTheEmbryonicSignalingCenter:Spemann’sOrganizer
ClassicexperimentfirstperformedbySpemannandMangoldin1924Grafteddorsallipofanembryoontoasecondembryo
GastrulationinitiatedatbothsitesSecondwholesetofbodystructuresformedCellfatestudiesinXenopus:NogginNogginexpressionpermitscellstobecomebrainandnervoussystemtissueNoNogginexpressionresultsintissuebecomingskin,boneNogginisaninhibitorofBMPswhichpromotebonegrowthUsenucleicacidmicroinjectiontoknockoutorover-expressnogginMusMusculus
BestmodelformammaliandevelopmentLifecycleapproximately9weeks;21daygestationLittersupto20pupsGenomesequencedManyinbredstrainscharacterized(450available)GeneticmanipulationswelldevelopedHouseMouseEmbryoscanbeperturbedinvariouswaysbutgiverisetonormalmouse
AllowsgeneticmanipulationofembryopossibleEarlyembryocanbesplittoyieldtwo“twins”TwomorulascanbecombinedtoformachimeraCellsfromanembryocanbeinjectedintoanotherblastocysttoformachimeraMouseembryosasasourceofembryonicstemcells
Cultureinnercellmass(givesrisetowholeembryo)EScellswilldivideindefinitelywithoutdifferent
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