BIPN 144 Week 6 Notes (part 1)

BIPN 144 Week 6 Notes (part 1) - BIPN144Week6Notes(Part1)

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Unformatted text preview: BIPN144Week6Notes(Part1) Note:ThishasnotbeenreviewedbyDr.Bierandmayhaveerrors.Always trusttheprofessoroverthesenotes. Thiswasanodd,largelyphilosophicallecture,butIwilldomybesttohighlightthe keypoints. Dr.Bierstartedoffthelecturebyreviewingthesevenlessmosaicassay.Hethen highlightedtwomainpointsabouttheeye: 1) Theimaginaldisc(invertebrate)originatesfromepithelialtissue,buttheeye (vertebrate)comesfrombrainmaterial(weird). 2) Astheopticvesiclesthatwillbecomethefutureeyedevelop,theypressup againsttheoverlyingepidermisandtheysecretesignalsbackandforth,this isactivecommunication. AlsoImportant: ReminiscentofwhatgoesonintheflywithEGFRandsevenlesssignaling,cellsengagein activecommunicationtodeterminetheirfates.Thisisincommonbetweenfliesand vertebrates. DrBierthenshowedthedifferentcellsthatmakeuptheeye.Heexplainedthat,oppositeto whatwewouldassume,thephotoreceptorsarestrangelythecellsthatarefurthestaway fromtheopeningoftheeye.Heexplainedhowconesaremoreresponsiblefordetecting color,whereasrodsaremoresensitivetolight.Oureyesprefertofocusonthefovea,which isconerich,andgoodatcolordetermination.However,ifyouarehavingahardtimeseeing atmight,itmayhelptolookslightlyoffcenter,anduseamoreroddenseareaofyoureye. Inmyownexperience,thisworks,butisdifficulttomaintain.Healsomentionedthat differentspeciesdoadifferentamountofprocessingofvisualinformationattheeyelevel. Forexamplesomespecieshavespecializedcellsintheireyesthatdetectedgesorlines. Prettycool.Healsopointedoutthatthecellsthatwilleventuallyformtheeyeseemto figureoutwhichcellswillbecomewhatinasimilarfashiontotheprocessintheflyeye differentphotoreceptorsariseinasomewhatrandomfashiondeterminedbyactive signalingbetweencells. DrBierthendiscussedtwoevolutionarytheoriesaboutthedevelopmentoftheeye.Some evolutionarybiologistsbelievethattheeyehasdevelopedcompletelyseveraltimesover throughouttheevolutionaryhistoryoflife.Thisconceptofthesameproduct(likeaneye) beingproducedmultipletimesduetoselectiveevolutionarypressures,iscalledconvergent evolution.Thesquideye,forexample,looksalotlikeavertebrateeye,butitdevelops completelydifferently,throughwavesofcellmigrationratherthancellcellinteractions. Otherspreferamorerecentschoolofthought,believingthatallcreatureswitheyesshared acommonphotoreceptorpossessingancestor.DrBierprovidedthislovelyimageofwhat thiscommonancestormayhavelookedlike.Lovelyisn'tit? DrBierthenwentontodiscussPax6oreyeless,a"mastergene." Pax6(calledeyelessinflies)isacommongenefoundininvertebratesand vertebrates,andisassociatedwitheyeformation.Pax6givessupporttothe commonancestortheory. OnescientistwassoenamoredwithPax6thathecalleditamastergeneforeyes. HerearesomeideasaboutPax6asamastergene. 1) Pax6invertebratesiseyelessinflies,theyarevirtuallyidentical. 2) Mutantswithreducedpax6functionhavesmallorabsenteyes 3) Pax6isalwaysexpressedinearlyeyeprimordialinnearlyallseeinganimals 4) Misexpressionofpax6canbesufficientforinducingeyeexpression 5) Pax6andeyelessareTF's Hereisanormalwing,andthewingofaflywithmisexpressedpax6,notethat thereisaneyegrowingoutofthewin Datasuchastheseledascientisttodeclarethatpax6isamastergeneforeyes.That wherethereisnopax6therewillbenoeyes,andthatwherethereispax6therewill beeyes. Dr.Bierhowever,feelsthatcallingpax6amastergeneistoostrong.Herearesome problemswiththemastergenetheory. 1) ifyoueliminatepax6itresultsinfarmoredrasticeffectsthanjust eliminatingeyesflieswhoarecompletepax6knockoutshavenoheads. Vertebratepax6ko'saremissingtheanteriorportionsoftheirbrains.Ifpx6 isjustforeyesthenwhyaretheresuchdrasticdefects? 2) Misexpressionofpax6infliesonlymakeseyesincertainlocationsitworks onthewing,butnoteverywhere.Wingsyes,backofflyNo. 3) Othergenesalsobehavelikepax6.AnumberofTF'sinfliesifeliminatedor misexpressedleadtothesamephenotypesaspax6mutants.Soitseemsa groupofeyesworktogethertomakeeyes. Sothereisnowasortof"MasterGene"evolutionalrybelief/theoryspectrum. Ononeendpeoplebelievethatamastercouncilofgenesthatworktogetherto makeeyes.Ontheotherend,peoplebelievethatgeneswerefordetectinglight, andthatlightdetectorsmayhavebeenunderthecontrolofthesegenes.Butthat otherwise,fullyfunctionaleyesdevelopedseparately. ThatiswhatIhaveforthislecture,however,becauseIfeelitcouldbebetter,I amincludingthenotesofanotherTA,JalenBrickhere.Hopefullybetweenthese twosets,theimportantpointswillbeadequatelycovered.Isubmititasmy opinionthatwecoveredverylittleactualmaterialduringthislectureduetothe amountofclassdiscussion. Jalen'sNotes: 5/4/10 Mosaicfacets:comprisedofcellsthathavefunctional(wildtypecopies)ofgene ornonfunctionalcopiesofgene(/)mutant OfthatfacetsthatsucceedinmakingR7cell,whatcellscanneverbemutant R8cellisonlymakingbossligand WithoutR8cell,thereisnosignaltoactivatesevenless ThesefacetscanmakeR7cell,andsomeofthesefacetsweremutant.Thesecells donotrequirethatfunctionforthatgeneproduct,becausethentherewouldn't besevenlessthen Cellthatisnevermutantwhichcanconcludeafterawhilethatitisrequired Whenlookingforbossgene,stilllookingforR7.MutantblindbecausenoR7. Needsignaling R8cellismakingboss,butnotmakingreceptor.R8needstomakesignal Formationofadulteye Couplemessages Donotneedtoknowgruesomedetails 1)brainlikethespinalcordfallsinfromneuralplatetoinitiallymaketubelike structure 2)theeyesorigingatefromthebraininavertebrate o Neuraltubewherepopsinwhereheadis,overallverysimilar o Fromthistube,outpocketswhichbecomefutureeyesopticvesicles Inflies,eyestartsoffasskin,butinvertebrate,eyeisbrainmaterial Asopticvesiclesthatthenbecomefutureeye,goondeveloping,thepressupagainst overlyingepidermisandendupsecretingsignalsbackandforth.Active communication. Fullyformedvertebrateeye o Photoreceptorsaredownnearbottom o Seriesofinterconnectedcellsaboveit:neuronsandglial o Cellsthatsendimpulsetocellaretopmost:gangliacells.Sendalltheaxonstoto bundlethroughfovia.Thatgoestobrain Twothingsneedtoknow** o Eyesorginatefrombrain.Duringprocessofdevelopment,cellsareveryactivein communicationwithoneanother Reminiscientofwhatgoesonthroughfly.ThruEGRFandsevenlessreceptor.Cells engageincommunicationtodetermine.Thisisincommonbetweenfliesand vertebrates Figure4.21S,R,H Haveoninnersurfaceofeye,thepartfurthermostawayfromlight,havecones photoreceptorsthatdetectcolor.Redaresensitivethatdetectpresenceorabsenceof light Photoreceptorshookuptohorizontal,bipolarcells Complexcircuitryofhowhhookup,thenhookupwithgangliacellsthatgouptobrain Dependingonspecies,amountofvisiualprocessingcanbebasicorcomplicated Circuitsofmanyspeciesthatwilldetectthepresenceofedges o Cellthatisnexttoedge,throughcollateralinteractions,willrecognizethatcell nexttoitisdetectinglight,andtheotheroneisnotthereforeedge o Inotherorganisms,edgegoesoninhigherpartsofbrain.Ie.Fruitflycomputed inbrainnoteye Whatareoriginsofcells o ConecellsandRodcellsphotoreceptor o Othercellslikeglial(orMueller)cells o Howdotheyarrive?Strictlineage o WhenmakeESorganorstretchreceptors,thereisstrictlineage o Ingeneral,littlelikeafly.Caninjectaprogenitorcellsthatlieonsurfaceof ventricles(opensurface)andputavirusorsomeotherlabelingmethod(ie. Injectwithenzyme).Whatcellsinheritthatgeneticmaterialorlabelingmethod o Isprogenitorcellthatonlygivesrisetohortizontalcells,orconecells,orrod cells?AnswerisNOOOOO Examplewithdyelikemethod o Dyeinjectedwherezoneisproliferative o Singlecellwheregetinjectedandlookatprogenythatcomefromit o Fromoneprogenitorcell,getstringorlineofcells o Thenwillseethatphotoreceptors, o Photoreceptorsclosestandgangliacellsfartherst o Oneprogenitorthatgiverisetoalltypesofcellsintheeye:p,b,m,a,andg o Interactionthroughcellsignaling,becomeonecelltypeoranother o Takefromvertebrateeyedevelopment Eyeoriginatesfrombrainratherthanepidermisasinfly********* Signalingisveryimportantindeterminingpartsofeye.Infly,cell signalingtakesplacetoseewhathappensinfacet.R7v.R8.Done throughcellcommunication.Notdictatedbycelllineage,asincaseof formationofextrasensoryorgans.*********** Figure65 Flydoesnotlikevertebrate KEYPOINTTTT Whathappensinfly.Haveepidermis,havepacketofcellsthatmakesprogenitor. Iamaginaldisk.Hhturingondpp.Facetsforming.Signaltooneanother.Bossand sevenless.Eachonebecomesfacet.Eachfacethasanerve.Nervesjointogethertomake brain Invertebrate,eyecomesfrombrain.Inductiveevents.Oneholisticopticnervethatgoes tobrian Donotlookalike Basedonthat,biologistsorevolutionarybiologistshavespeculatedthateyesare exampleofconvergentevolutionperformverysimilarfunction,butarose independentlyfromdifferentlineages Puzzlethoughstillaroseinconvergentevolutionwasthesquid o Squidisalsoinvertebrateorbelongingtothemolluskorarthropodfamily o Nonetheless,squidmakeseye Howeyeformsinsquid o Evenifoutcomelooksremarkablylikesame,completelydifferent o Madecompletelydifferent o Infly,thereisaneuralregioninside, o Insteadofhavinginductiveeventsthoughinvertebrates,cellsthatareadjacent undergoseriesofcreepingevents.Wherecellmigratesoverexistingorcells belowthem Marsupials(koala,)evolvedformsuncannilylikedeersanddogsinsouthAmerica,but whereN.andS.Americafuse,fawnamixedsoallthetruecaninesandtruedeer displacedincompetiton,thosethathadderivedfrommarsiupals.Marsiupalshad alreadybeencommittedtoonefateandhadnotdoneitmoreefficiently Evolutionisfullofexamplesofconvergentevolution Evolutionarybiologistshavesaidthateyehaveinvolved7independentways Nowthereisnewevidenceandunclearwhatoriginsofeyeis Certainlygoodevidencethatthereisacommonancestorofasinglephotoreceptor o Evidencecomesfrom o 1)eyeless(flies)andpax6 o Eyeless(flies)=pax6(vertebrates) Argumentsforacommonoriginofeyes?orphotoreceptors(betterterm) 1)eyeless(flies)andpax6(vertebrates)thesearetranscriptionfactors 2)mutantsthatreduceeyeless/pax6functionsmallorabsenteyes 3)eyeless/pax6arealwaysexpressedinearlyeyeprimordial(Cellsthatgiveriseto eye)ofnearlyallseeingorlightsensitiveanimals 4)Misexpressionofeyeless/pax6canbesufficientforinducingeyeformation o Wingmisexpressinghumanpax6gene o Seeeyecomingoutofthewing o Eventriestosendoutnervetobrainfromfacets o Eyeless/pax6ismastergeneforeyetheory o Hethoughtbyextremelabelthatthisgenetellscellsthatyouwillbeeye Whydoyoumakeflyeyeandnothumaneye?Flydoesnothavestufftomake humaneye.Ideaisthatgeneisnotdirectlymakingeye.Itislikeelectricalor controlswitchthatflicksonflygeneticprogram o Whenreducefunctionofgeneortakeawaysomeoftheirfunction,theresultis same:reductioninsizeofeye(seenumber2)Genesgetturnedonearly.(see number3)Andmisexpressionofgenecanbesomethingelse.(number4)Based onthatmeansthisgenemakeseye Whatarguesagainstthisverystronghypothesis o Againhypothesisiseyelessandpax6aremastergenes.Theyareancestral, wheresomeorganismdeepinpastcouldseeandthesegeneswereresponsible. Problemswith"mastergenehypothesis 1)Eliminateeyelss/pax6functionmuchbroaderdefects o Completelyheadless o Inmice,pax6inadditiontobeingexpressedineye,alsoexpressedinbrain.And wholeregionofbraindoesn'tform. o Thisisgenethatjustdeterminespositionalidentity.Likehoxgenes.Noseortail. Thiscouldbekindaworkinglikethat,justdefiningazonewheremakeeyes o Lookwhereeyesforminalligatororbat,theydonotalwaysforminexactly sameregiononanterior/posterioraxis,somaynotberightanswereither o Butifgetridofgene,meansnohead,thenitdoesnotmakesensetosaythat eyeless/pax6makeseye 2)Misexpresseyeless/pax6itonlymakeseyeincertainplacesorfromcertaincell types o Ifplaceonbackofhead,itdoesn'thappen o Can'ttransformeverycelltoeye 3)Othergenesalsobehavelikeeyeless/pax6 o Seriesofothertranscriptionfactorsinfliesthatwhenyoueliminatefunction giveyousmallorabsenteyes o Orwhenoverexpressormisexpressthem,givesyoueyessomewhereelse o Generallyallworktogether,sodoesnotcompletelydestroyidea.Butthereis rulingcounselratherthanamonarch o Groupofgenesworkingtogether.Waygenesarehookedtooneanotherisa littlebitdifferentinfliesandvertebrates o Therewasancenstralstatethathadgroupofgenesthatmadeeyes,butnotclear thatanyofthesegenesarededicatedtomakeeyeorevenasgrouptomakeeye o Therearetwoextremerangestomakeeye 1)mastergenehypothesisisright.Councilofgenesthatworktogether asregulatorymachinetospecificeyeness.Doonethingatone developmentalstage,butonethingtheyalwayshaveincommonis makingeyes.Onededicatedfunctionofgene,andcouldhaveevolved otherfunctions,butthatisonededicatedfunction.Cannottransform everycelltype,butcandoit.ancestorhadeyesandcouldsee 2)ancestorcouldonlydetectlight.Couldonlyseeshadows.Couldnot focuslight.Couldnotseeimage.Butbecausecoulddetectlightit evolved.Genesthatrequiredforturningonexpressionofphotoreceptor pigments,thosegenesareincontrolofeyeless,andpax6transcription factors.Soevolutionaryprocessisthenbuiltongeneandhappened severalindependenttimes.Commonancestoronlydetectlightand smallermorphologiesevolvedindependentlyatdifferentlineages o Howcanwetestinthefuture o Forexample,ifmastergenehypothesisiscorrect,eyeishookedupto braininorganism.Inorganism,centersofbrainthatdocomplex calculations.Detectionsofedges.Detectionofmotions.Ifbrainregion thatdetects,isdeterminedbysametranscriptionfactorsorgenesand happensinfliesandvertebrates,thenanimalcouldprocesslight Hardertoprovecasewhereancestorcouldonlydetectlightanddark. Dependsonhowabletoreconstructthatancestor.Ancestorhadonly onephotorecepticcell.Howmuchcoulditseewithonlyone.Morelikely tobesecondhypothesisthen Figure66:Apossiblepictureofthemostrecentcommonancestorofvertebrateand invertebrates Advantagethatallowedittosurvive GenesthatexpressedalongA/Paxisdeterminecelltypesalongbodyinabsenceof genesjustaworm, Wanttodosomethingthatmakeswingsinsomeaprtofthebody,needtobeableto labelthatregion Isthistheadvantage? Abletogetoxygen?Betternervoussystem? Bigmysteries Scienceisjustdonebypeoplewithideas.Intheend,dustsettles, ...
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This note was uploaded on 11/10/2010 for the course BIPN BIPN 144 taught by Professor Bier during the Spring '10 term at UCSD.

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