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*Author ([email protected]) for correspondence Washington, FridayRoad, Harbor, University of WA 98250,USA. University, AL36849,USA. Auburn, 592 1 of theanimaltreefocusesmainly onfivetaxa:,Porifera, Understanding relationshipsbetween extantlineagesnearthebase very different fromwhatistraditionally understood. structures suchasnerves,musclesandsensorymay be all otherextantmetazoanlineagesandthatearlyevolution of 2014) suggestthatthectenophorelineage,notsponges,issister to transcriptomic andgenomicdata(Ryan et al.,2013;Morozet reshaping ourideasabouttheseanimals.Inparticular, studiesusing on cellularbiology, developmentandphysiologyofspongesis that theyareofthecellulargradeconstruction.Recentresearch distinct bodyplanandevenorganized tissues,leadingtotheidea they lackorgans. Incontrast, spongesareoftenconsideredtolacka ‘ grade’ ofbodyconstruction,whichimplies,incorrectly, that textbooks inNorthAmerica,consideredcombjelliestobeofthe Hyman (Hyman,1940),andthusmostsubsequentinvertebrate surprisingly complex,withdistinctnerveandmusclecells,although called ctenerows.Despitetheirgelatinousnature,ctenophoresare covered byeightrowsofciliaryplatescomposedcompoundcilia, considered amongoneofthesimplest.Gelatinouscombjelliesare phyla,whereasporiferans,orsponges,havelongbeen Ctenophores, orcombjellies,areamongthemostenigmaticof , Choanoflagellata,Neuralevolution,Metazoanorigins KEY WORDS:Ctenophora,Porifera,Combjelly, Phylogeny, are theearliestbranchingextantanimallineage. independent originsregardlessofwhetherctenophoresorsponges mechanisms. Thus,neuralsystemsappeartohaveatleasttwo other animalsandusedifferent setsofcellularandgenetic that neuralsystemsofctenophoresarevastlydifferent fromthoseof consideration oftheevolutionneuronalsystems.Recentdatashow Intertwined withdiscussionofbasalmetazoanphylogenyis determining thepositionsofspongeswithinanimaltree. choanocytes toChoanflagellatadonotprovideusefulcharactersfor aresimpleandthereportedsimilarityofsponge evidence andquestionablepremises.Forexample,theideathat as thebasal-mostextantanimallineagearebasedonlimited raised aboutthisresultandshowthatassumptionsplacingsponges discussion amongresearchers.Here,Icountercommoncriticisms all otheranimals.Evenbeforepublication,thisresultprompted datasuggestthatctenophores,orcombjellies,aresisterto Recent phylogeneticanalysesresultingfromcollectionofwhole Kenneth M.Halanych right! Orcanit? The ctenophorelineageisolderthansponges?Thatcannotbe REVIEW © 2015.PublishedbyTheCompanyofBiologistsLtd|JournalExperimentalBiology(2015)218,592-597doi:10.1242/jeb.111872 Phylogenetic hypotheses Introduction ABSTRACT Department of BiologicalSciences, 101Life SciencesBuilding, Auburn of Department 1,2, 2 * Friday HarborLaboratories, 620University have ahighlydivergent mtDNA genome~11 mtDNA (Bridgeetal.,1992).Ctenophores,however, also 20 some spongeshaveaslightlylarger mtDNA genome(approaching additional mitochondrialgenes (e.g.ATPase 9and extratRNAs), way ofcomparisontobilaterians, somespongesandcnidarianshave that placozoanswerethebasal-mostlineageofextantanimals. By observation ledDellaportaetal.(Dellaportaal.,2006)tosuggest genomes (Dellaportaetal.,2006;Signorovitch2007). and haveadditionalgenesnotfoundinotheranimalmtDNA missing several –mosttRNAsareabsent (Pettetal.,2011; Placozoans havethelargest mtDNA genome(43,079 of thetreehaveintermediatesizedmitochondrialgenomes. much larger insize.Animal lineages thatbranchedoff nearthebase typicallyhavemanymoremitochondrialgenesand are genes) (Boore,1999;Vallès andBoore,2006).Incontrast,other genes (13proteincodinggenes,tworDNA genesand22tRNA circular mtDNA genomethatisabout15–17 provide clearsupportforanyonehypothesis.Bilaterianshave a Phylogenetic inferencebasedonwholemtDNA genomesdoesnot (mtDNA) genomes,wholenucleargenomesandtranscriptomicdata. been addressedwithmorerecentdataincludingmitochondrialDNA (Philippe etal.,2009;Ryan etal.,2013;Moroz2014). 2001). Thecurrentlyfavoredviewisthatspongesaremonophyletic (Zrzavy etal.,1998;Medina2001;PetersonandEernisse, created adebateastowhetherspongesweremonophyleticornot not supported(Halanych,2004).Also,nuclearribosomaldata concept ofCoelenterata(cnidariansplusctenophores)wastypically placozoans werenotincludedinmanyoftheseanalysesandthe al., 1999;Podaret2001;Medina2001).Importantly, the baseofanimaltree(Fieldetal.,1988;Collins,1998;Kim thatusuallyplacedporiferanandctenophorelineagesclosestto first moleculardatatobearontheissuewerenuclearribosomalgene is themostbasalamongextantanimals(e.g.Nielsen,2012).The longstanding viewbasedonmorphologyisthatthespongelineage which moststudieshaveverylimitedtaxonsampling),whereasthe alternative taxonomicplacementssuggestedbymoleculardata(for Ryan etal.(Ryan etal.,2013).Muchofthisdebatehasbeendueto Halanych (Halanych,2004),Kocotetal.(Kocotal.,2010)and concerning theevolutionofearlyanimalsanddataispresentedin Nosenko etal.,2013).Morecompletediscussionofhypotheses Schierwater etal.,2009;Srivastava2010;Kohn2012; 2007; Dunnetal.,2008;Hejnol2009;Philippe during thelast10 amorphous ),poriferansandctenophoreshasbeenlively debate aboutlineagesthatleadtoplacozoans(smallverysimple , ,)atthebaseofanimaltree,but relatives) orthebilaterianlineage(e.g.,mollusks, that placethecnidarianlineage(anemones,coralsandtheir , CnidariaandBilateria.Therearenowell-supporteddata Questions ofrelationshipsnearthebaseanimaltreehave kb) (Lavrovetal.,2005)and somecnidarianshavelinear

(Dellaportaetal.,2006;Signorovitch

kb inlengthwith37

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The Journal of Experimental psuedocoel and eucoel).Thecellularlevel,the simplestandmost (diploblast andtriploblast)grouped bycoeloms (acoel, tissue-level andorgan-level), talkedaboutnumbersoftissue layers belonging tothreegradesofbody planconstruction(cellular-level, a lingeringimpactonourunderstanding. Sheconsideredanimalsas particular, the wayshecodifiedthecomplexityofanimalsstillhas influence hasreachedacrossseveral generationsofbiologists.In al., 2003;BruscaandBrusca,Pechenik,2009)].Thus, her many invertebratetexts[atleastinNorthAmerica(e.g.Ruppert et previous worksandinterpretationsofanimalbiologystilldominate Libbie Hymanisparticularlyrelevanthereashercompilations of Naturae pervaded howwethinkaboutanimalssinceAristotle’s basedoncomplexity ofbodyplans.Thisapproachhas simple. Foreons,scientistshavebeenthinkingabouthowtoarrange hypothesis isbecausespongesareconsideredtobemorphologically Perhaps theprimaryreasonmostpeopleacceptPorifera-sister analyses published, multiplescientistsquestionedwhatwas‘wrong’ withthe taxa). EvenbeforetheMorozetal.(Morozal.,2014)paperwas (e.g. Articulatahypothesisplacingarthropodsandannelidsassister hypothesis theyhadassumedtobetrueforthecourseoftheircareer experience, someresearchershaddifficulty criticallyevaluatinga hypotheses withequalrigorisoftenprudent.Basedonprevious dogma, steppingbackandassessingbothnoveltraditional sponges inthisposition.Inconsideringhypothesesthatchallenge other extantanimalsiscontrarytoaccepteddogmathatplaces remained sistertoallotheranimalsforthemajorityofanalyses. look forbiasesindataandpotentialsourcesoferror. Ctenophores sampling. Bothstudiesexploredvariousphylogeneticapproachesto genes (242forRyan etal.and586forMorozal.)richertaxon with previousworkonthistopic,thesestudiesemployedmanymore show strongsupportforthectenophorelineageasbasal.Compared content, andexplicitlikelihoodapproachesforhypothesistesting) these papers(basedonlarge aminoaciddatasets,analysesofgene focused onthecombjelly genome ofthectenophore than previousefforts. Ryan etal.’s paperfocusedonreportingthe lineage withinanimalsandhavesomewhatbettertaxonsampling considerable supportforctenophoresasthebasal-mostextant analyses (Ryan etal.,2013;Moroz2014),however, show support forthedeepestnodesinanimalphylogeny. Two recent of theseanalyseshavebeenverytaxonlimitedorlackstrong et al.,2010;Nosenko2013)showvariableresults,butmost data (e.g.Philippeetal.,2009;SchierwaterSrivastava not providedresolutionontheissueofearlyanimalrelationships. cnidarians insomeanalyses.To date,mitochondrialinformationhas variation andevenalackofmonophylyforsponges Osigus etal.(Osigusal.,2013)ofmitochondrialdatashow example, recentanalysesbyBerntetal.(Berntal.,2013)and to besensitivetaxonomicinclusionorphylogeneticmethods.For Kohn etal.,2012).Trees basedonmtDNA aminoaciddatasetsseem animals. concerning thewhetherspongesorcombjelliesaresistertoallother question someofassumptionsthathavebeen,andwillbe,made sister-to-all-other-animals (Ctenophora-sister)hypothesisbutto sister-to-all-other-animals (Porifera-sister)hypothesisorctenophores- present paperisnottoargue specificallyfororagainsttheporiferans- REVIEW Sponges are simple Sponges are A phylogenetichypothesisthatplaces ctenophoresassistertoall Analyses basedonnucleartranscriptomicandwholegenomic through Linnaeus,HaeckleandHymantomoderntimes. − beforetheysawanydataorresults.Theintentionofthe Mnemeopsis leidyi backei whereas Morozetal. . Analyseswithin Scala is provingvaluableforunderstandingevolutionaryhistory. elements withinthosegenomes,oracrossmorphologicaldiversity, phylogenetic relationships.However, comparison oforthologous ofagivenlineagehasproceededratherthandeep genomic ormorphologicalcomplexitytellsusmoreabouthowthe Database, http://www.genomesize.com). Thus,considerationof on shorttimescales(forexample,browsetheAnimalGenomeSize We knowgenomesizeandvariationcanshowimpressivechanges is likelynotusefulforphylogeneticreconstructionofdeepnodes. between majoranimallineagesthatdiverged 600millionyearsago ctenophores. Discussionofgenomecomplexity, inandofitself, complement ofgenesandgenesystemswithotheranimalsthando al. (Ryan etal.,2013)pointout,spongesshareamoresimilar of earlyanimals.AsMorozetal.(Morozal.,2014)andRyan et genome complexityandgenecomplementofthecommonancestor arose earlyinmetazoanevolutionhasbeeninterestunderstanding of morphology, butarecentdriverofresearchamonglineagesthat levels. Historically, biologistshaveconsideredcomplexityinterms derived. Hanelt etal.,1996;Pawlowski1996),theywereshowntobe Once molecularevidencebecameavailable(Katayamaetal.,1995; potentially nearthebaseofanimalradiation(Halanych,2004). dicyemids andmyxozoanswereallpreviouslyconsideredlineages as derivedanimals.Forexample,multicellularorthonectids, much moresimpleformsthanspongesandwereadilyacceptthem Importantly, thereareseveral othermetazoansthatarguably have maximum likelihood,Bayesianinference)ofappropriatecharacters. model-driven phylogeneticreconstructionmethodologies(e.g. on synapomorphies(i.e.sharedderivedcharacters)orexplicit the discussion.Inamodernphylogeneticcontext,wewanttofocus ctenophore lineage,considerationofcomplexityisnotrelevantto understand placementofthespongelineagerelativeto grand scaleacrossthediversityofanimallife.Ifwewantto often madewithoutrealization,especiallywhendiscussedonthe always gofromsimpletocomplex!’ Nonetheless,thisassumptionis point manyreaderswillbethinking,‘Ofcourseevolutiondoesnot proceeds fromsimpletocomplex(seeHalanych,1998).Atthis larger tissues.Whatisatissuetheassumptionthatevolution ctenophores intermsofcelltypesandarrangementcellsinto complex thanotheranimalsandareclearlyfarless are puttogetherthanwaspossibleinHyman’s day. Spongesareless we nowhaveamuchdeeperunderstandingofhowanimalbodies animals areatthetissueandorgan levelofconstruction. basic, wasreservedforspongesandplacozoans,whereasother homology discussion andpointsoutsomeinteresting, butunlikely, covers someofthehistory thechoanocyte/ cells are,forthemostpart,lacking. Maldonado(Maldonado,2004) long beenassumedeventhough detailedcomparisonsbetweenthese relationship betweenchoanocytes andcellsofchoanoflagellateshas to becomemorewidelyaccepted. Themoreorlessdirect allowed thesuggestionthatsponges evolvedfromchoanoflagellates discovery ofmulticellularchoanoflagellatecolonies(Tuzet, 1963) hasbeenlongknown(James-Clark,1866), the al., 1984).Whilstthesimilaritybetweenspongechoanocytes and also differentiate intosperm duringsexualreproduction(Gainoet surrounded bymicrovilliandareusedinfeeding.Interestingly, they presence ofchoanocytes.Thesearecellsthatpossessaflagellum One ofthemostidentifiablecharacteristicsspongesis the Choanocytes choanoflagellates from Obviously, complexitycanbediscussedonseveraldifferent Hyman’s treatmentofanimalcomplexityisnotatissuehereand The JournalofExperimentalBiology(2015)doi:10.1242/jeb.111872 593

The Journal of Experimental Biology 594 REVIEW textbook: their p.110 (BruscaandBrusca,2003)],aleadinginvertebrate July 2014)tothefollowingstatementinBruscaand[see from layreferences(http://en.wikipedia.org/wiki/Choanocyte, 17 interpretation seemspervasive.Thisiswellillustratedbyeverything first metazoansfromchoanoflagellatesisincorrect.Nonetheless,this choanoflagellate cells,theinterpretationthatspongesevolvedas retention, etc.). forces actingonefficiency (e.g.Reynoldsnumber, sizeofparticle ofthesefeedingapparatuseswereshapedbyselective may helpelucidatehowthefunctionalmorphologyand compare similarcellsoutsideofporiferansandchoanoflagellates occur inotherorganisms aswell(e.g.Phalansterium).Analysesthat possess aflagellumandaresurroundbymicrovilli,collarcells, choanoflagellates] cannotbeassumedwithoutquestion’.Cellsthat et al.concluded,‘thathomology[betweenchoanocytesand Emlet, 1991;Halanych,1996).Afterconsideringtheirresults,Mah and ciliaryfeedingmechanismsisknowninotheranimals(e.g. pressures tooptimizefeedingefficiency. Selectiononultrastructure similarities attheultrastructurelevelmaybeduetoselective the collarinbothcellsandquestionwhetherobserved important functionaldifferences inhowtheflagelluminteractswith morphological andfunctionalsimilaritiesdifferences. Theynote as wellultrastructuralcomponents,inthesecellsanddetailed workers missed.To summarize,Mahetal.lookedatactivefeeding, choanocytes fromthesponge choanoflagellates. Mahetal.(Mahal.,2014)compared ideas abouttherelationshipbetweenspongechoanocytesand such fossilsare notlikelytobefound.Thesetwo concepts(shared Metazoa wouldhelpintheinterpretation ofancestralfeatures,but the commonancestor. Ideally, predatingtheChoanoflagellata/ these taxaarerelativeyoungand arelikelyveryderivedrelativeto Choanoflagellata, Poriferaand Ctenophora,butextantspeciesof years havepassedbetween thelastcommonancestorof record. Thecrown-groupconcept isrelevantbecause~600million relatively recentdiversificationeventsasevidencedbythe ‘crown groups’.Inotherwords,mostspeciesaretheresult of in daughterlineages.(2)Organisms currentlyontheplanetrepresent and theancestormayhavebeenverydifferent fromcurrentforms same setsofcharactersorfeaturestheyinheritedfromtheirancestor, modern lineages.Further, bothdaughterlineages maynothavethe daughter lineagesaswellcharactersthatmaynotbepresent in common ancestorinthepastthatpossessedcharactersfound both directly derivedfromanother. Moderntaxadiverged from a Recent taxashare‘commonancestors’,andrarelyisone 1A).(1) evolutionary biologythatareoftenmisunderstood(Fig. 541 millionyearsago).Thisviewpointexemplifiestwoconcepts of is clear. Incontrast,thesetaxadiverged inthePrecambrian(priorto but theassumptionthatanimalsarederivedfromchoanoflagellates do sayinthenextparagraphthatadirectprecursorislikelyextinct, evolved directlyfrommodernchoanoflagellates.Bruscaand choanoflagellate Choanoflagellates possesscollarcellsessentiallyidenticaltothose Most evidencetodaypointstotheprotist phylumChoanoflagellata Even ifoneassumeshomologybetweenchoanocytesand This quote,incorrectly, reinforcestheideathatmodernsponges found insponges.GenerasuchasProterospongia, and othersare animal-likecolonialchoanoflagellatesandare commonly citedastypifyingapotentialmetazoanprecursor. as thelikelyancestralgroup from whichtheMetazoaarose. Monosiga brevicollis Spongila lacustris and clarifiedwhatother Sphaeroeca and the , phylogeny iscontroversial inpartbecauseofperceived implications The ideaofctenophoresasthe -mostextantlineageinanimal sister argument. of asinglestep,doesnotprovide astrongargument forthe Porifera- structure oronaparsimonyargument basedonalengthdifference choanoflagellates, basedeitheroncurrentknowledgeof their inference. Thus,thesimilarityassumedbetweenchoanocytes and cells tobeanunequivocalsynapomorphyusefulforphylogenetic sponges andchoanoflagellatesaresistertaxawouldallowcollar than withchoanoflagellates.Fig. and spongesshareamorerecentcommonancestorwitheach other choanoflagellate-like cellshavebeenlostinanimals,ctenophores ctenophore-sister hypothesis.Regardlessofthenumber once assumingthesponge-sisterhypothesis,ortwice the evolutionary descent)tochoanoflagellates,theywerelostat least In otherwords,ifchoanocytesarehomologous(i.e.thesame by sponges werethefirstlineagetobranchoff theanimaltree(Fig. collarcells cannotbeanunequivocalsynapomorphytoshow (Halanych, 2004;Ryan etal.,2013;Moroz2014),the ancestor morerecentlythaneitherdidwithchoanoflagellates descended fromchoanoflagellates. paraphyletic. A paraphyleticgrade istheexpectationifsponges as choanoflagellatesaremonophyletic(Carretal.,2008),not extant crowngroup.Thispointissupportedbymolecularevidence, have evolvedfromchoanoflagellates,atermweusetodelineatean ancestors andcrowngroups)helpclarifythatspongescouldnot Fig. Regulation of internal ions internal Regulation of and plussignsindicategains. This lasttreeisnotsupportedbycurrentdata.Minussignsindicatelosses suggests thatthecollarcellcharacterisasynapomorphywithouthomoplasy. requires twolosses.Onlythesponge-sister-to-choanoflagellatetopology least onelossofthischaracterandtheCtenophore-sisterhypothesis homologous tochoanoflagellates.Thesponge-sisterhypothesisrequiresat (B) Alternativeevolutionaryscenariosassumingthatchoanocytesare recent diversificationsandlineagesdivergefromcommonancestors. B A

Moreover, becausespongesandctenophoressharedacommon Choanoflagellates

1. Earlyanimalevolution.

Sponges (+) The JournalofExperimentalBiology(2015)doi:10.1242/jeb.111872 Ctenophores Present (-)

Cnidaria &

Choanoflagellates

(A) Crowngroupsaretypicallytheresultof Choanoflagellates Ctenophores

1B showsthatonlyatreeassuming

(+) Ctenophores

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Cnidaria & Bilateria for allanimals Common ancestor Cnidaria & Bilateria Choanoflagellates Crown groups

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Ctenophores

1B). Cnidaria & Bilateria

The Journal of Experimental Biology not defendableinthecontextofmodernevolutionaryscience. they couldnothaveevolvedtwice.Thispremiseofcomplexity is physiological ormorphologicalrequirementslikebasallamina)that down totheideathatnervesaresocomplex(withotherspecific osmoticcontrol.Moreover, theargument essentiallyboils neural evolutionseemstobebasedonaflawedassumptionabout from thatofotheranimalsasonceassumed. physiological statewithinaspongebodymaynotbeasdifferent (Nickel,2006;Bond,2013).Suchresultsimplythat they arecapableofsomecoordinatedmovement,albeitonaslow have moreadvancedphysiologythanpreviouslyrecognizedinthat al., 2010)].Additionally, evidenceismountingthatspongesmay Ctenophora-sister hypothesis.Therehasbeensome improvementin 2013; Morozetal.,2014) has resultedinsupportforthe data, moretaxaandappropriate outgrouptesting(Ryan etal., Eernisse, 2001;Philippeetal., 2009),andtheinclusionofmore the Porifera-sisterhypothesis(e.g. Medinaetal.,2001;Petersonand contrast, manyprioranalysesbased ononeorafewgenesfavored addition ofmoredataand taxathat‘corrects’ thetree.In needed here.First,inmostcases,LBA isoftendiscoveredbythe and thisispotentiallyavalidargument. However, afewcaveatsare systematics hasstillnotdevisedagoodtestfordetectionof LBA, attraction (LBA)(Felsenstein,1978).Thefieldofmolecular of nucleotidesubstitution,are‘obviously’ subjecttolongbranch et al.,2014)istodeclarethatctenophores,whichhaveelevated rates topologies ofRyan etal.andMoroz (Ryan etal.,2013;Moroz Perhaps themostcommonlyusedexcusetodismiss tree to confirmthepresenceoftightjunctions[seetheirfig. tissue. Transmission electronmicroscopywork inthatreportappears passage intheseanimalswassimilartoobservationsfromvertebrate asymmetrical electrochemicalpotential.Moreover, resistancetoion sponges possessanepitheliumthatisabletogenerate (Adams etal.,2010)demonstratethatfreshwaterhaplosclerid capable ofregulatingionicflowtosomedegree.Adamsetal. relative totheoutsideenvironmentandpossessanepithelium bodies, somespongesdoappeartobeableexertioniccontrol is incorrect.Despitechannelinglarge amountsofwaterthroughtheir too valuabletobelost. neuronal features,andonceosmoticcontrolwasobtaineditwouldbe osmotic controlwouldhavetobegainedpriortheevolutionof . Sucharguments assumethatfunctionalmorphologyfor are basalandthattheycouldnothaveevolvedfromanancestorwith that thelackofabasallaminainspongesatteststofactthey Srivastava etal.,2010).Asaresult,Ihavebeensubjecttoarguments be lackingabasallamina(Mackie,1984;Cereijidoetal.,2004; control theirintercellularenvironment.]Spongesareoftenassumedto instead askthequestionofwhetherspongeshavepropensityto am notconcernedaboutidiosyncrasiesofsemanticdefinitions,but ‘basal epithelia’,‘basementmembrane’ or‘basalmembrane’.Here,I has alongandconvolutedhistoryisoftenmixedtogetherwith ionic concentrationsofextracellularmatrix.[Theterm‘basallamina’ substances intoandoutofananimal’s bodyandassuchhelpscontrol , thatistypicallythoughtofascontrollingthemovement epithelial cellsisanextracellularmatrixormembrane,thebasal the extracellularmatrixmustbecontrolled.Inmostanimals,below across thecellmembraneandthusionsbothinsideneuronin organism withneuronsmustbeabletomaintainanactionpotential for neuralsystemevolution.Becausenervesarepolarized,an REVIEW Long branchattraction Thus, theargument relatingthepresenceofabasallaminato With regardstothisargument, thecommonperceptionofsponges

2 (Adamset that presumablyactasreceptorsfor have adiversesetiGlutamatereceptorsnotfoundinotheranimals ) andlacktypicalneuroreceptors.Moreover, they all otheranimals(or, ifpresent,they arenotactingas scute, patterning (e.g.genesencodingneurogenins, Ctenophores lackmanyofgenescontrollingneuronalfateand cellular mechanisms.However, thiscongruenceisnotobserved. systems seemtobehighlyevolutionaryconservedinfunctionand mechanisms andstructuresespeciallygiventhatbilaterianneural all animalswithaneuralsystem,thenweexpectcongruencein understand metazoanevolution.Ifneuronsarehomologousacross systems’, asasinglecharacter, isoflittlevalueinhelpingto and makingassessmentsofgainsorlosses‘nerves’ or‘neural the neuralsystemasasinglecharacterisgrosslyover-simplified, 1965). Assuch,neuralhomologycannotbeassumed.Considering been definedbasedonfunctionalgrounds(BullockandHorridge, convince usweareexaminingthe‘same’ feature. typically identifysomesimilaritiesorintermediatestagesthat homologous featuresareverydisparateintwodescendants,wecan features beingexamined,whichcanincludedevelopment.Evenif of phylogeneticrelationshipswithknowledgethesimilarity common ancestor. Suchanassessment typicallycombinesknowledge to beableshowthatthesamestructure(s)descendedfroma evolution ofanimallifeontheplanet.To assesshomology, weneed has thepotentialtoalterourunderstandingconcerningearly question ofhomologyamongneuralsystemsisanimportantonethat whether neuralsystemshaveevolvedmorethanonceinanimals.The main discussionssurroundingtheCtenophore-sisterhypothesisis are reluctanttoconsiderctenophoresasbasal.However, oneofthe Above, Ihavedetailedsomeofthediscussionaboutwhyindividuals writing, thereisnoevidenceofLBA inthecaseofctenophores. situations, attractedbranchesoftenappearmuchshorter. Asofthis look atatreeanddeclarethebranchesarelongbecause,inLBA saturation thathavecausedspuriousrelationships.Onecannotjust be abletodemonstratethatthereareproblemswithsubstitutional valid argument thatLBA hasinfluencedtopologicalresultsshould statements canbebackedupwithresultsand/orobservations.Any as withanyscientificendeavor, thereisanexpectationthat sampling (e.g.Morozetal.,2014)helpsalleviatethisissue.Second, branches thanthoseofothertaxaintheiranalyses,butadditional ctenophores usedinRyan etal.(Ryan etal.2013)havelonger tree reconstructionisstillsomewhatsubjective.Struckshowedthat identify longbranches,determiningwhetherbranchesarebiasing method forassessinglongbranches.Whereasthecan testing forLBA.Forexample,Struck(Struck,2014)proposeda question theassumption thatthesestructuresare homologous.Some functional neuronsinctenophores, comparedwithotheranimals, and thereforewillnotbediscussed furtherhere. neural machineryweredetailed inMorozetal.(Morozal.,2014) multicellular animals.Manyof thesimilaritiesanddifferences in type lectins,etc.)requiredfor multicellularityalsoarosepriorto et al.,2003)showedthatmanygeneticmechanisms(cadherins, C- evolution ofthefeatureinquestion.Forexample,Kingetal.(King can expectsomecellularandgeneticcomponentstopredate the evolutionary studiesofdevelopmenthaverepeatedlyshownthat we between ctenophoresandotherneuron-bearinganimals.However, . Thatsaid,therearesomeelementsthatsimilar Origins or neural systems Origins orneural andneuralsystemsarecomplicatedstructureshave The numerousandprofounddifferences incellularmachinery for REST, The JournalofExperimentalBiology(2015)doi:10.1242/jeb.111872 HOX, Otx), lackcanonicalneurotransmittersfoundin L -glutamate, whichactsasa NeuroD, Achaete- 595

The Journal of Experimental Biology ulc,T .adHrig,G.A. Horridge, and T. H. Bullock, L.W. Buss, and R. DeSalle, B., Schierwater, C.W., Cunningham, D., Bridge, 596 REVIEW is gratefullyacknowledged. Support fromtheNationalAeronauticsandSpaceAdministration(NNX13AJ31G) The authorsdeclarenocompetingorfinancialinterests. Laboratory. no. 124totheAUMarineBiologyProgramand33TheMoletteBiological Brannock andtwoanonymousreviewersweremosthelpful.Thisiscontribution and T. H.Struckcontributedtothismanuscript.CommentsfromPamelaM. Discussions withLeonidL.Moroz,KevinM.Kocot,NathanV. Whelan,LiYuanning generosity inallowingmetoparticipatethissymposiumismostappreciated. examine theearlyevolutionofanimalnervoussystems.PeterAnderson’s Nervous SystemsII’ meeting(supportedbytheNationalScienceFoundation)to The genesisofthismanuscriptwasasapresentationforthe‘EvolutionFirst their greatestdiversity, extremelychallenging. make accessingoffshore deepwater, wherectenophoreslikelyhave dwindling oceanographicresources(e.g.shipandsubmersibletime) constituentthroughouttheworld’s .However, are moretractablethanthoseoftaxonsampling.Ctenophoresa genes touseandhowanalyzethem.Insomeways,theseissues genomes, buttherewillstillbeconsiderabledebatesoverwhich are enteringtheageofassessingevolutionaryhistorywithcomplete and reconstructinglarge datasets)willhelpaddressthisissue.We but, importantly, developmentofimprovedanalyticalmethods(LBA Obviously, moregeneticdata anddatafrommoretaxaareneeded there willnodoubtbefuturereportsonearlyanimalphylogeny. in theanimaltree.Importantly, thiscontroversyisnotsettledand at leasttwoviablehypothesesforrelationshipsofthedeepestnodes and Morozetal.(Morozal.,2014),researchersshouldconsider phylogeny. Giventherecent papers ofRyan etal.(Ryan etal.,2013) critical ofthelong-heldideathatspongesarebasalinanimal sister hypothesisand(2)tochallengeresearchersbeequally with criticismsthathavebeenespousedagainsttheCtenophora- consequence forunderstandinganimalneuralevolution. homologous rendersrelationshipsamongbasaltaxaoflittle the neuralsystemsofctenophoresandotheranimalsarenot neurons orevenanimals.)Interestingly, acceptingthepremisethat oravoltagepotentialacrossthecellisnotspecificto function andcommunicationaredifferent. (Notethatcellular cnidarians andbilaterians,themostbasicelementsofneuronal an unlikelyscenario.Neuronalmachineryisconservedacross machinery haschangedthroughthecourseofevolution,butthisis might argue thattheneuronishomologousanditsunderlying rsa .C n rsa G.J. Brusca, and R.C. Brusca, J.L. Boore, S.P. Leys, and G. Goss, E.D., Adams, en,M,Bedr,C,Baad . abc,J,Dnt,A,Fizc,G., Fritzsch, A., C. 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