The complex origin of Astyanax cavefish


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The complex origin of Astyanax cavefish

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Title:
The complex origin of Astyanax cavefish
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BMC Evolutionary Biology
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Gross, Joshua B.
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Springer Nature
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Regressive Phenotypic Evolution ( local )
Cave Biology ( local )
Genetics ( local )
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Background The loss of phenotypic characters is a common feature of evolution. Cave organisms provide excellent models for investigating the underlying patterns and processes governing the evolutionary loss of phenotypic traits. The blind Mexican cavefish, Astyanax mexicanus, represents a particularly strong model for both developmental and genetic analyses as these fish can be raised in the laboratory and hybridized with conspecific surface form counterparts to produce large F2 pedigrees. As studies have begun to illuminate the genetic bases for trait evolution in these cavefish, it has become increasingly important to understand these phenotypic changes within the context of cavefish origins. Understanding these origins is a challenge. For instance, widespread convergence on similar features renders morphological characters less informative. In addition, current and past gene flow between surface and cave forms have complicated the delineation of particular cave populations. Results Past population-level analyses have sought to: 1) estimate at what time in the geological past cave forms became isolated from surface-dwelling ancestors, 2) define the extent to which cave form populations originated from a common invasion (single origin hypothesis) or several invasions (multiple origin hypothesis), and 3) clarify the role of geological and climatic events in Astyanax cavefish evolution. In recent years, thanks to the combined use of morphological and genetic data, a much clearer picture has emerged regarding the origins of Astyanax cavefish. Conclusions The consensus view, based on several recent studies, is that cave forms originated from at least two distinct ancestral surface-dwelling stocks over the past several million years. In addition, each stock gave rise to multiple invasions of the subterranean biotope. The older stock is believed to have invaded the El Abra caves at least three times while the new stock separately invaded the northern Guatemala and western Micos caves. This renewed picture of Astyanax
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BMC Evolutionary Biology, Vol. 12, no. 105 (2012-06-12).

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REVIEWOpenAccess Thecomplexoriginof Astyanax cavefish JoshuaBGross * Abstract Background: Thelossofphenotypiccharactersisacommonfeatureofevolution.Caveorganismsprovide excellentmodelsforinvestigatingtheunderlyingpatternsandprocessesgoverningtheevolutionarylossof phenotypictraits.TheblindMexicancavefish, Astyanaxmexicanus ,representsaparticularlystrongmodelforboth developmentalandgeneticanalysesasthesefishcanberaisedinthelaboratoryandhybridizedwithconspecific surfaceformcounterpartstoproducelargeF 2 pedigrees.Asstudieshavebeguntoilluminatethegeneticbasesfor traitevolutioninthesecavefish,ithasbecomeincreasinglyimportanttounderstandthesephenotypicchanges withinthecontextofcavefishorigins.Understandingtheseoriginsisachallenge.Forinstance,widespread convergenceonsimilarfeaturesrendersmorphologicalcharacterslessinformative.Inaddition,currentandpast geneflowbetweensurfaceandcaveformshavecomplicatedthedelineationofparticularcavepopulations. Results: Pastpopulation-levelanalyseshavesoughtto:1)estimateatwhattimeinthegeologicalpastcaveforms becameisolatedfromsurface-dwellingancestors,2)definetheextenttowhichcaveformpopulationsoriginated fromacommoninvasion(singleoriginhypothesis)orseveralinvasions(multipleoriginhypothesis),and3)clarify theroleofgeologicalandclimaticeventsin Astyanax cavefishevolution.Inrecentyears,thankstothecombined useofmorphologicalandgeneticdata,amuchclearerpicturehasemergedregardingtheoriginsof Astyanax cavefish. Conclusions: Theconsensusview,basedonseveralrecentstudies,isthatcaveformsoriginatedfromatleasttwo distinctancestralsurface-dwellingstocksoverthepastseveralmillionyears.Inaddition,eachstockgaveriseto multipleinvasionsofthesubterraneanbiotope.TheolderstockisbelievedtohaveinvadedtheElAbracavesat leastthreetimeswhilethenewstockseparatelyinvadedthenorthernGuatemalaandwesternMicoscaves.This renewedpictureof Astyanax cavefishoriginswillhelpinvestigatorsdrawconclusionsregardingtheevolutionof phenotypictraitsthroughparallelismversusconvergence.Additionally,itwillhelpusunderstandhowthepresence ofcave-associatedtraitsinoldversusyoungcavepopulationsmaybeinfluencedbythetimesinceisolationinthe caveenvironment.Thiswill,inturn,helptoinformourbroaderunderstandingoftheforcesthatgovernthe evolutionofphenotypicloss. Keywords: Regressivephenotypicevolution,Cavebiology,Genetics Background Themechanismgoverningtheevolutionofregressive featuresremainsunknown.Regressivefeaturesarehighly conspicuousincave-limitedorganisms,arisefrequently acrosstheanimalkingdom,andareprobablymorecommonthanadaptivetraits[1].Inrecentyears,agreatdeal ofattentionhasreturnedtotheblindMexicancavefish, Astyanaxmexicanus ,asapowerfulmodelforgenomic andevolutionaryresearch.Thisisacredittothetractabilityof Astyanax asalaboratoryanimalandthe availabilityofbothsurfaceandcavemorphotypes.Until recently,ourabilitytodrawconclusionsonfundamental questionspertinenttotheevolutionof Astyanax has remainedlimitedbyanincompleteunderstandingofthe originofthesefascinatingcreatures. Forinstance,theoriginofatraitthroughconvergent versusparallelevolutionrestsonaclearunderstanding oftheinter-relationshipsofdistinctcavepopulations[2]. Hybridizationbetweencaveandsurfacemorphotypesin naturemayleadtoincorrectgroupingsoffishbasedon sharedallelicbackgrounds[2].Further,theoriginofa cave-limitedtrait,presentwidelyacrossdifferentcave forms,willbeinterpreteddifferently(e.g.,throughrapid selectionversusdrift)dependingontheestimatedtime Correspondence: grossja@ucmail.uc.edu DepartmentofBiologicalSciences,UniversityofCincinnati,312CliftonCourt, Cincinnati,OH45221,USA ©2012Gross;licenseeBioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreative CommonsAttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,and reproductioninanymedium,providedtheoriginalworkisproperlycited. Gross BMCEvolutionaryBiology 2012, 12 :105 http://www.biomedcentral.com/1471-2148/12/105

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sinceisolationofcaveformsfromtheirsurface-dwelling ancestors[3]. Longstandingquestionsregardingtheoriginof Astyanax cavefishhavecenteredonseveraltopics.Oneof theearliestquestionsrelatestothenumberoftimes theElAbralimestonecavesofnortheasternMexico werecolonizedbyancestralsurface-dwellingforms [4,5].Understandingthenumberoftimesthesubterraneanenvironmenthasbeeninvadedcanclarifywhether cave-associatedtraitsarisethroughwidespreadconvergencebymultiple,independentpopulations[6,7].More recently,agreatdealofinteresthascenteredonthe extenttowhichgeneflowoccursbetweensurface-and distinctcave-dwellingforms[2,8,9]. Thisworkcoverstwoprincipaltopics:1)abriefreview ofhistoricalstudiesinthefieldof Astyanax biology,and 2)areviewofrecentanalysesthathaveaddedsignificant claritytolongstandingquestions.Bycomparingand contrastingthesetwotopics,thisreviewoffersasummaryoftheprogressionofworkinthefieldof Astyanax biologyandprovidesaframeworkforunderstanding competinghypotheses.Thisreviewaimstoprovidea heuristictoolthroughwhich Astyanax researcherscan betterevaluatetheevolutionarymechanismsleadingto regressivephenotypicloss.Geologicalandclimaticfactorsaffecting Astyanax originsAstyanax caveformsevolvedinacomplexgeographical regionofpresent-dayMexicothathasrepresenteda transitionalzonebetweentheNeotropicandtheNearcticregionssincethelateCenozoicera(Figure1)[1012].TheSierradeElAbraisalargecarbonatecomplex, situatedtotheeastoftheSierraMadreOriental, extendingroughly150kmfromthenorthwesttothe southeast(Figure2)[13,14].TheregionoftheElAbra cavesharboringblindcavefishrepresentsacalcareous reefcomplexthataroseduringthemiddleCretaceous era(Figure1A)[13,15,16].Aguayo-Camargo(1998)analyzedtheElAbraLimestoneatitstypelocality(near CiudadVallesinSanLuísPotosí,Mexico)utilizingboth petrologicalandfossilcharacters.Thisanalysisindicated thattheageoftheexposed “ lower ” ElAbraLimestone isearlyCenomanian(~93.5to99.6millionsofyears ago(MYa);Figure1B)[13].Theexposed “ upper ” limestoneisdatedtothelateTuronian(~89.3 – 93.5MYa; Figure1B)[13].Together,theseexposedregionsrepresenttwomajorsedimentaryenvironments,respectively: anolder ‘ rudist-reef ’ environment(termedtheTaninul member),andayounger ‘ back-reef ’ environment (termedtheElAbramember).Subdivisionswithineach regioncanbeidentifiedbasedondifferencesinfaunal diversity,compositionandoilemplacement[13,17].The unexposedsubsurfaceregionisolder,datingtothe Albian(~99.6 – 112MYa)[13,18-20].TheElAbra Limestonebecameexposedinthelate-Cretaceousand earlyTertiaryeras(~65MYa;Figure1B),following movementsduringtheLaramideOrogeny[5,16].Gradualerosionofthelimestonebedrockledtoformation ofasignificantcavernousnetworkofsubterranean cavernsbeginningaroundthemid-Tertiaryperiod[5]. Thecavernsthemselvesdevelopedthroughcommunicationwiththeterrestrialsurfaceviatworoutes.Through theprocessofstreamcapture,waterinvadedfracturesin overlyingimpermeablerock,leadingtoasteeprockface atthecaveentryway[16].Themajorityofcaveformations intheElAbraregionharboringcavefisharethesesotanós, suggestingthatperhapscavecolonizationoriginallyoccurredthroughinadvertenttrappingoffishintoacaveentrancepit(e.g.,duringspringflooding;see[5]).The secondmodeofformationinvolvescollapseofacavernous roofthrougherosionthatunitesthecavewiththeoverlyingterrestrialenvironment.Thesecaveformations,cuevas,aredistinguishedthroughoutnorthernMexicobya distinctgeologicalstructure[5]. Animportantfeatureofthecavesinthisregioniselevation(i.e.,vertical)differencesthatlikelyserveasphysicalbarriers(atleastatthecaveentrances)between cave/surfacepopulations[5].Indeed,severalofthecave poolsinthisregionare “ perched ” atsignificantelevationsabovesealevel.Some Astyanax cavepopulations aremoreisolatedthanothers[5].Forinstance,the southernmostChicacaveentranceislocated49meters abovesealevel,whereastheVásquezcaveentrance resides422metersabovesealevel[5].Accordingto Mitchelletal.(1977),theaveragealtitudeofcave entrancesacrosstheregionis~220metersabovesea level[5].Whiletheseauthorsdidnotregardanyofthe cavepopulationsasbeing “ absolutely ” isolated,vertical separationatthecaveentranceandwithinthecave,i.e. “ perchedpools ” ,providesanobviousspatialisolatorbetweenpopulations[5]. Inadditiontothisverticalisolation,thepresenceofan internaldrainagedivideforRíoManteandRíoChoy likelyservesasadivisionforpopulationsfromthe northernandsouthernElAbracaves,respectively(Figure2)[5].ApathwayfromthesouthernmostElAbra cavestothenorthernPachóncaveisbelievedtohave existedinthepast[5].Indeed,Bradicetal.,(2012)argue thatthePachóncavewaslikelyestablishedasanindependentpopulationfromtherestofthesouthernEl Abraclusterofcavepopulations.Thisisbasedonthe highlycomplexgeologyofundergroundpassagesbetweencaves(iftheyareevennavigable)aswellasthe significantFSTvaluesbetweenthePachóncavefishand fishfromtheotherElAbracaves(seebelow)[8].The authorsnotethatthesemeasuresofgeneticdistancereflectboththecurrentisolationaswellasaprobableindependentorigin[8].Gross BMCEvolutionaryBiology 2012, 12 :105 Page2of12 http://www.biomedcentral.com/1471-2148/12/105

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Anadditionalgeologicalvariablethathaslikelyinfluencedthesettlementof Astyanax populationsinCentral AmericaistheTrans-MexicanVolcanicBelt(TMVB), whichharboredintensevolcanicactivity3 – 12MYa, throughthepresentday[11].Itisbelievedthatthe TMVBhasservedtolimitgeneflow,howeverrecent studiessuggest Astyanax populationshavecrossedthis geologicalbarrieratleastthreetimes,andthatallthree migrantpopulationshaveinvadedthesubterraneanenvironment[9,21]. Thegeologicalfeaturesservingtoisolatecavepopulationshaveprobablyinfluencedthemorphologicaland physiologicalevolutionoftheinhabitants.Forexample, giventhatPachóncavefishareperchedpopulations,they encounterfarlessseasonalflooding,andexperiencea lowernutrientbase[22].Undertheseconditions,metabolicchangesthatfavorsurvivalindepauperatecaves arepredictedtoevolve[3,22].Indeed,ananalysisof standardandroutineoxygenconsumptioncarriedout betweensurface-dwellingformsandthreecavepopulationsindicatedsignificantvariabilityinmetabolicrate [22].TheisolatedPachóncavefishpopulationdemonstratedtheloweststandardoxygenconsumptionrate (0.230±0.036mgO2g-1h-1;[22])comparedtosurface fish(0.314±0.081mgO2g--1h-1).Chicacavefishdemonstratedanintermediatestandardoxygenconsumption rate(0.277±0.063mgO2g-1h-1;[22])whichmaybe explained,inpart,bypresenceofalargebatroostinthis cavethathasprovidedamoreconstantfoodsupply thatmitigates(orpossiblyeliminatesaltogether)the effectsofpastbottleneckingeventsinthispopulation [5,10,22,23]. Geographicdistancebetweencavelocalitieshasalso likelyplayedaroleinpastcolonization.Bradicetal. (2012)concludedthatthewesternMicosandnorthern Guatemalapopulations(Figure2)likelyrepresentseparateinvasionsintothesubterraneanenvironmentsince thesecavesare~90kmapartfromoneanother,separatedbyaridgeandtwoopenvalleys.Additionally, subterraneankarstsystemsareverydynamic;newcaves arecreated,separated,ormergedwitholderones throughtime[9].Thusdispersalof Astyanax cavefish, Figure1 Numerousgeologicaleventshaveinfluencedthesettlementof Astyanax cavefishinnortheasternMexico. TheElAbra Limestoneformationmaterializedfromthevastdepositionofmarinesedimentduringthemid-CretaceousPeriod( A ;lightblue).TheElAbra Limestonegraduallybecameexposedatthesurface~65MYa( B ;brown).Overthecourseofseveralmillionsofyears,avastnetworkoflimestone cavesevolvedintheareaofpresent-daynortheasternMexico.Thesecavesweresubsequentlyinvadedbyancestralsurface-dwelling Astyanax fish.Surface-dwellingformsmigratednorthwardfromSouthAmericaintwowaves.TheolderwavearrivedinMesoamericaeither~8MYa,viaan incipientlandbridge( D ;dashedredline)thatconnectedSouthandNorthAmerica,orshortlyafter( D ;solidredline)theclosureofthe Panamanian-Columbianseabarrier~3.3MYa( C ;purple).ThisolderwaveseededthecavesoftheElAbraregion(Figure2).Amorerecentwave ofnewepigeanstockcolonizedtheregion~2.1MYa( E ;green)andseededthenorthernGuatemalacavesandthewesternMicoscaves (Figure2). Gross BMCEvolutionaryBiology 2012, 12 :105 Page3of12 http://www.biomedcentral.com/1471-2148/12/105

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asreportedwiththecavecrustacean Gammarusminus [1,24],mayhaveoccurredafterthegradualformation anderosionofdifferentcavesystems.Indeed,thedynamicnatureofthekarstmayexplainsomepatternsof geneflowrecentlyreportedamongcaveandsurface fishpopulations[8].Anytheoryseekingtoexplain widespreadconvergenceonsimilartraitsamong Astyanax cavepopulationsmustaccountfordiversegeologicalandecologicalinfluences,suchasperchedpools andlowerfoodavailability,whichvarybypopulation. ArrivalofancestralepigeanformstoMesoamerica Astyanax isaNewWorld,freshwaterfishgenusthatoriginatedinSouthAmerica.Theabsenceofalandbridgehas longbeenregardedastheprincipallimitationtonorthern invasionofthisfishintoCentralAmerica[25].Ithasalso beenassumedthatnorthwardmigrationofancestral Astyanax surface-dwellingformsfollowedformationof thePanamanian-ColombianlandbridgeinthelateTertiary,roughly3.3MYa(Figure1C,Dsolidredline) [5,10,25-28].Arecentstudy,however,arguesthatthe movementof Astyanax formsnorthwardintoCentral Americamayhaveactuallyoccurredmuchearlier(~7.8 – 8.1MYa),migratingviaanincipientlandbridgethat existedwellbeforeclosureofthePanamanian-Columbian seabarrier(Figure2C,Ddashedredline)[11]. IftheancestralformsarrivedintoMesoamericaat somepointbetween~3.1MYaand8.1MYa,whendid thesefishactuallyinvadethecaves?Earlyestimatesof cavecolonizationwerecoarse.Forinstance,Aviseand Figure2 Summaryofpopulation-levelanalysesincaveandsurfacepopulationsof Astyanaxmexicanus . Overthecourseofseveral millionsofyears,ancestral Astyanax (surface)fishrepeatedlyinvadedthelimestonekarstregionofnortheasternMexico(inset).Theseinvasions haveledto29namedpopulationsintheElAbraregion,manyofwhicharesignificantlydistantfromoneanother.Severalreportshaveaddedto ourgrowingunderstandingoftheinterrelationshipsamongcaveforms.Theresultsofthesereports,basedonmitochondrialandnucleargene sequenceanalysis,aresummarizedhere(coloredboxes;seelegend).Notethatseveralcaveshavenotyetbeensampled(e.g.,thenorthernJineo andEscondidocaves).Classicalreportsfirstsuggestedthatrecentcaveformsmayhaveoriginatedfromasinglecolonization[38],however contemporaryreportsargueforamuchearliercolonizationintonortheasternMexicoandsubsequentcolonizationofsubterraneancavesbyan “ old ” (reddashedline)and “ new ” stock(greendashedline)ofsurface-dwellingancestors[7-11,21]. Gross BMCEvolutionaryBiology 2012, 12 :105 Page4of12 http://www.biomedcentral.com/1471-2148/12/105

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Selander(1972)suggestedtroglobitescolonizedcaves priortotheendofthePleistoceneera – between~2 millionand~10,000yearsago.Thistimingwasextrapolatedfromworkperformedintemperatezonecavesin whichcaveanimalswerearguedtohavebecomeisolated duringawarmingtrendofPleistoceneinter-glacial events[3,29,30].Thus,ifusingtheHoloceneasaminimumtime-since-isolation,subterraneanformsintemperatecavesmayhavebeenisolatedasrecentlyas ~10,000years[3]. But,whataboutaquatictropicalcaves?Evidencesuggeststhatthetropicsexperiencedmultipleglacial/interglacialphasesthroughoutthePleistocene[31,32].Further,incontrasttonon-aquaticorganisms,aquaticspeciesintropicalcavesareassumedtohavebeenisolated foramuchlongerperiodoftime[33].Forinstance, HobbsandBarr(1960)arguedthecave-adaptedcrayfish Orconectes haslikelybeenisolatedfromitsancestralsurfacecounterpartfor~2MYa[34]. Laterstudiesin Astyanax estimatedthatthetimingof ancestralinvasionsintothecavesrangedfromthelate Pliocene[15]tothelatePleistocene[5,25,27,35,36].A seriesofrecentstudies,however,havesuggestedthatthe latterestimateisfartoorecent.Anearlierestimated timeofinvasionintothecaveenvironmentcomesfrom recentworksuggestingcertaincaveswerecolonized quiteearlybymarineanimals( Speocirolanabolivari, Troglocubanusperezfarfanteae and Spelaeomysisquinterensis ;[10]).Themostrecentview,basedonconsensus drawnfromseveralreports,isthattwoancestralwaves ofsurface-dwellingstockinvadedtheregionofthecaves. AnolderstockofsurfacefishinvadedthecavesoftheEl Abraregion(seebelow;Figure1D)[8],andayounger stocksubsequentlyinvadedthewesternMicoscavesand thenorthernGuatemalacaves(seebelow;Figure1E)[8]. Ofcourse,theprecisetimingofinvasionsintothecaves bysurface-dwellingformsisunknown.However,tilapia fishhavebeennotedincertainElAbracaves(e.g.,Yerbaniz;[8]).Sincethesefishwereonlyintroducedto Mexicointhe1980s[37],inprinciplesurface-dwelling fisharecapableofinvadingthesubterraneanenvironmentremarkablyquickly[8].SingleversusmultipleoriginsSingleoriginhypothesisHowmanyinvasionsoftheundergroundenvironment ledtotheextantdistributionof Astyanax cavepopulations?BrederandRasquin(1947)proposedthatsurface formfishenteredsubterraneancavesviatheRíoTampaónintotheChicacave,anddispersednorthwardvia cavernoussubterraneanconnectionsbelowtheVallede AntiguoMorelostotheSabinosandPachóncaves[38] (reviewedin[5]).Thishypothesiswaspopularized,and largelyacceptedintheliterature,despitethefactthat theauthorsprovidedtwoalternatives,including:1)originofeachcaveformneartheirpresentgeographical position(i.e.,multipleorigins),and2)asingleoriginat thenortherncavesfollowedbysouthernmigrationto theChicalocality[38]. Alvarez(1946)testedthesingle-originhypothesis throughassessmentofanumberofmorphologicalfeatures(e.g.,finraynumber,bodyproportionmeasurements)intheChica,SabinosandPachóncaves.This workdemonstratedoverlappingpopulation-levelmeasurementsthatbridgefromtheChicatotheSabinosto thePachóncaves[39].Later,ahybridanalysisby adolu(1957)demonstratedthatcrossesbetweensurface formsandChica,SabinosandPachóncavefishresulted inprogressivelysmallereyesinoffspring.Thisexperimentsupportedthenotionthatmoreisolatedcave forms(i.e.,PachóncomparedtoChica)demonstrate moresignificantregressiveloss[40].However,asubsequentanalysisbyWilkens(1970)failedtoobservenoticeabledifferencesineyesizeorstructureinPachón andSabinoscavefish[41].Moreover,classicalstudiesdid notaccountforthefactthatChicarepresentsapossible “ hybrid ” population[25]thatexperiencedrecentgene flowfromthenearbyepigeanfish[5]. Thesingleoriginhypothesiswassupported,inpart,by thefindingsofAviseandSelander(1972)whofoundlow levelsofheterozygosityinthreepopulationsofcavefish (averageheterozygosity=~7.7%)comparedtosixpopulationsofsurfacefish(averageheterozygosity=~11.2%). Basedonthisevidence,theauthorsremarked, “ Theeyeless,unpigmentedconditionisbelievedtohaveevolved inwholeorpartpriortothepresent-daysubdivisionof thepopulations, ” (p.16[16]).Whilethenumberofindividualssampledfromeachcavewaslarge,onlythreeof thenow29knownpopulationswererepresentedinthis analysis[7]. Todaythesingle-originhypothesishasbeendismissed foranumberofreasons.First,asMitchelletal.(1977) argued,ancestralsurfaceformslikelyinvadedthelimestonecavesseveraltimes.Thisassumptionwasbasedon thepresenceofgeographicbarriersbetweenvarious caves(e.g.,mountainousareasbetweentheMicosandEl Abraregions)thatrendersasingleoriginunlikely [5,7,42].Secondly,contemporarypopulation-levelanalyseshaveofferedverystrongsupportformultipleinvasionsintothecaveenvironmentthroughextensive intra-andinter-populationvariationanalyses(seebelow) [2,7-11,21,23,24].Finally,complementationcrossescarriedoutbetweenmembersofdifferentcavepopulations demonstratethatseveralcave-associatedtraitsarise throughdifferentgeneticlociindifferentpopulations.If geographicallydistinctcaveformsarosefromthesame eyelessfounderpopulation,thenalloffspringwouldbe predictedtoharboreyelessphenotypesgovernedbytheGross BMCEvolutionaryBiology 2012, 12 :105 Page5of12 http://www.biomedcentral.com/1471-2148/12/105

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samegeneticloci.Thisisnotthecase – ananalysisof eyephenotypesinF1individuals[15,43];demonstrates distinctgeneticbasesaccountforthelossofthevisual systemindifferentcavefishpopulations[43,44].Since distinctgeneticchangesaccountfortheeyelossphenotypeindifferentcavepopulations,thissuggestseach cavepopulationevolvedthistraitindependently.MultipleoriginhypothesisThenotionthatcaveformsarosethroughmultiplecolonizations,eachfollowedbyshort-rangedispersals, gainedfavorasmorecavepopulationswerediscovered. Fromthemid-1940sthroughthe1970s,caveexpeditions innortheasternMexicoledtodiscoveryof26additional caves – manyseparatedbydistinctgeologicalbarriers (Figure2).Categoricaldistinctionsintheliteraturecontrasted “ phylogeneticallyyoung ” and “ phylogenetically old ” populationsbasedonthedegreeoftroglomorphy evidentwithincavepopulations(seeFigure2).Cave formsdemonstratingmildcave-associatedphenotypes (e.g.,reducedbutnotabsentpigmentation,visualsystem regressionbutnottotalloss)wereregardedas “ young ” populations[42].Moreancientcavefishpopulations werethosedemonstratingmoreextremephenotypic changes,e.g.completelossofthevisualsystemstructuresandsignificantlyenhanced “ constructive ” morphologiestofacilitateidentificationofnutritioninthe trophic-poorcontextofthecave. Wilkens(1988)categorizedcertaincavepopulations as “ phylogeneticallyold ” populations – including Pachón,Sabinos,Piedras,YerbanizandPichijumo.Other caves,includingtheMicoscavepopulations(Otates, Subterráneo,Lienzo),represent insensu or “ phylogeneticallyyoung ” cavepopulations(Figure2)[42].ThedesignationsadoptedbyWilkens(1988)werebasedon population-levelcharacteristics,e.g.,sizeoftheeyerudimentis~10-20%ofthesizeofsurfaceformfish.An additionalcategoricaldescriptionof ‘ hybridfish ’ refers toarecentsurfacexcaveformintrogressionevent,e.g. theChicacave[24]. Phylogenetically “ old ” versus “ young ” categorieswere basedonthenotionthattheaccumulationofneutral mutationsistheprinciplemechanismdrivingregressive evolution,andthereforealongertimesinceisolationis predictedtoleadtotheaccumulationofmorephenotypicregression[3].However,theprecisemechanism(s) drivingtheevolutionofregressivetraitsremainsunknown.Arecentstudyarguesthatcave-associatedphenotypesareselectedforinthecaveenvironment,given thelevelofgeneflowthathasbeenobservedbetween caveandsurface-dwellingpopulations[8].Inlightofthis geneflow,thecavephenotyperemainshighlystablein differentpopulationsimplyingthatcave-associated allelesareunderstrongnaturalorsexualselection[8]. Irrespectiveofthemechanismthatgovernsevolutionary lossofphenotypiccharactersinthecaveenvironment, theoriginof Astyanax caveformsfrommultipleinvasionsandcolonizationeventsisnowwellaccepted. Supportingevidencefrompopulationgeneticstudiesis presentedbelow.PopulationlevelanalysesRecentpopulationlevelanalyseshaveprovidedclarityto earlyclassificationsof “ phylogeneticallyold ” versus “ young ” cavepopulations.Thesestudiesestimatetheage ofdifferentcavepopulationsutilizinggeneticratherthan strictlymorphologicalcharacteristics.Thisisparticularly importantgiventhatwidespreadconvergenceonthe cavephenotype,throughdiversegeneticmechanisms, rendersmorphologyalesspowerfultoolindefiningrelationshipsamongdifferentcaveforms[11]. ThefirstanalysisofmitochondrialDNA(mtDNA)sequencevariationwascarriedoutusingthe NADHdehydrogenase2 ( ND2 )locus[7].Thisstudyfoundthe levelofmtDNAdivergenceobservedin Astyanax isboth high,andcomparabletootherfreshwaterfishspecies [7];seealso[21].Combininggeneticsequenceanalysis withmorphologicalvariationdata(ribnumber),the authorsdiscoveredthatcaveformsclusterintotwo lineages(AandB;Figure2).Bothlineagesharboreyeless fishpopulations,providingsupportforeyelosshaving evolvedmultipletimes(Figure2),afindingconsistent withresultsofcomplementationstudies(seeabove). ThemajorityoflineageAfishhave12ribs,including fishfromtheSubterráneo,PachónandChicacaves alongwithsurroundingsurfacefish.Theauthorssuggestedthatthesecaveformsoriginatedmorerecently fromsurroundingepigeanpopulations[7]. ThemajorityofindividualsinLineageB(Sabinos, CurvaandTinajacavefish)harbor11ribs[7].These populationsdemonstratednosimilaritytoothercave formsorthesurroundingepigeanpopulations.The authorsarguedthatthesecaveformswereestablished byanancestralsurface-dwellingformthatiseitherextinct,ornolongerpresentinthisregionofMexico[7]. Indeed,theselineagesaremoresimilartoanotherspeciesfromsouthernMexicoandCostaRica( Astyanax aeneus )thantheyaretoeachother.Theauthorsfound thatsurfacefishharbormoregeneticvariationcompared tocaveforms.However,morethanonehaplotypewas observedinboththeTinajaandSubterráneocaves.This maybeduetothefactthatTinajarepresentsthelargest cave,andSubterráneoharborsanopeningthrough whichepigeanformsmayenter,andthereforecouldbe explainedbyintrogressionwiththesurfacefish.The authorsconcludedthatthedivergentND2haplotypes observedbetweenlineagesisconsistentwithmultipleGross BMCEvolutionaryBiology 2012, 12 :105 Page6of12 http://www.biomedcentral.com/1471-2148/12/105

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origins,followedbyconvergenceontheeyelossphenotype[7]. Asubsequentanalysisofgeneticvariationwithincave formswascarriedoutusingnuclear(microsatellite)loci aswellasmtDNA[23].Thisstudyreportedlittlegene flowbetweencaveandsurfaceforms,andthatevery cavepopulationexcepttheChicapopulationdemonstratedextremelylowmicrosatellitevariability,likely causedbyperiodicbottleneckeventsinthepast[23]. TheseauthorsfirstnotedimportantdisagreementsbetweenmtDNAandmicrosatellitedata.Basedonmicrosatellitedata,Pachón,TinajaandSabinoscluster togetheras “ phylogeneticallyold ” populations.Incontrast,basedonmtDNAsequences,TinajaandSabinos grouptogether – whilePachónandChicagrouptogetheralongwithsurroundingsurfacepopulations(Figure2)[23]. Theseresults,largelycongruentwiththefindingsof Dowlingetal.(2002),impliedacommonoriginforthe Pachón,SabinosandTinajacaves.Pachóncavefish, whichharborverylowlevelsofgeneticdiversity,may havebeenisolatedfromancestralsurfaceformsfora muchlongerperiodoftimecomparedtotheothercavefishpopulations[23].TheChicacave,incontrast,was concludedtobeanancientcavepopulationthatsubsequentlyhybridizedwithasurfacepopulation.Thisrecentgeneflowisdemonstratedbyreduceddivergence betweenChicaandsurfaceformfish[23].Insum,these tworeportsfirstsuggestedmultipleoriginsfordistinct cavepopulations,viaatleasttwoindependentinvasions [7,23]. Thisanalysiswasfollowedbyaphylogeographicanalysisofsurfaceandcaveformsbasedonvariationinthe mitochondrialgene cytB .Thisextensivestudy,which sampled174individualscomprising9caveand26surfacefishpopulations,identifiedsevenmajorclades.The observedcladedistributionislikelyduetoepisodesof invasionfollowedbyextinctionofsurfaceforms,as reportedalsobyDowlingetal.,2002.Fourcavelocalities (Piedras,Curva,SabinosandTinaja)werecategorizedas “ stronglyeye-andpigment-reducedpopulations ” (SEP) thatfailtoclusterwithsurfacefishnearsurrounding regions(Figure2).Theancestorsofthispopulationwere arguedtohavesurvivedas “ thermophilicrelics ” during climaticchangesofthePleistocenewhensurfaceancestorswentextinct[10]. Thisanalysisalsodescribeda “ variableeye-and pigment-reduced ” (VEP)cladebasedonmtDNA.This clade,similartoLineageA[7],clusterswithsurfacefish fromthesurroundingarea[10].Basedonthehighdegreeofgeneticdivergenceamongthedifferentclades, theseauthorsconcludedthatsurfacefishre-invadedrivers,streamsandcavesfromthesouth.TheonlyphenotypicdeparturefromthesedescriptionsisPachónwhich groupswithVEP,howeverdemonstratestroglobiticphenotypesmoresimilartotheSEPclade.Theauthorsexplainthisasduetointrogressivehybridizationwith surfaceformsinthepast[10]. Insum,theSEPcavepopulationsareolder,having beenderivedfromthefirstepigeanfishinvasion[10].In contrast,theVEPpopulationclusterwithsurfaceforms fromthesamegeographicregion,suggestingamorerecentinvasion.Thisreportusedadivergenceestimate, basedon cytB sequencevariation(1.5%/MY;[45]),toestimateatimeofdivergencebetweensurfaceandcave formsof1.8 – 4.5MYa(average=~3.1MYa;Figure1D solidredline).Thistimingfollowstheclosureofthe Panamanian-Colombianlandbridge,allowingnorthward migrationfromSouthAmericaintoCentralAmerica (Figure1C)[27].Followingtheinitialinvasion,surface formsarehypothesizedashavinggoneextinctasaconsequenceofsurfacecoolingduringthePleistocene/late Pliocene[46].TheSEPpopulationssurvivedinwarm subterraneanwaters[46]inrefugia.Asecondinvasion, seedingthemorerecentlycolonizedcaves,thenlikely spreadnorthward~1.8 – 3.0MYa(average=2.1MYa; Figure1E). Apopulationanalysisthatfollowedthisreport,however,arguedforanearlierinvasionofsurfaceformfish intonortheasternMexico[11].Theseauthorsperformed anextensivepopulationgeneticanalysisoftheentire Astyanax genusinMesoamericabyexaminingDNA sequencesofthreemitochondrialgenes( cytB , COI , 16S ) andanucleargene( rag1 )using208individualsfrom 147localities[11].Thisanalysisyieldedsixmajorphylogeneticgroups– Astyanax caveformsfellintoGroupI, subdividedintofourclades.Ofthefourlineageswithin CladeI,themore ‘ recent ’ troglobiticformsareinlineage Ia( “ Panuco-Tuxpan ” ,comprisingcavepopulationsfrom theHuastecanregion:Chica,Molino,Micos(i.e.,Subterráneo),Pachón,Yerbaniz,Japonés).MoreancientpopulationsareclusteredwithincladeII(LineageIe, “ Sabinos-Aguanaval-Mezquital ” ),includingcavepopulationsfromtheSabinoscluster,comprisingCurva,Tinaja, Sabinos,andPiedras(Figure2).ThisfindingisconsistentwiththeconclusionthatSabinosandTinajacavefish areremnantsofanearly Astyanax invasionintoMexico [10,21]. Oneoftheprincipalfindingsofthisreportwastheobservationofahighdegreeoftaxonomicdiversityin Astyanax fishcomparedtootherstudies.Theauthors attributedthisdiversitytoamuchearliercolonizationof Astyanax intoMesoamerica.Congruentwithotherstudies,thisreportarguedformultiple(atleasttwo)independentcolonizationsofepigeanformsintoupper Mesoamerica.Thefirstcolonizationofepigeanforms waspredictedtobequiteancient(~7.8 – 8.1MYa; Figure1Ddashedredline).ThisearliertimingimpliesGross BMCEvolutionaryBiology 2012, 12 :105 Page7of12 http://www.biomedcentral.com/1471-2148/12/105

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thepresenceofan ‘ incipient ’ landbridgethatpermitted migrationof Astyanax fishnorthward prior toclosureof thePanamanian-Colombianlandbridge(Figure1C,D dashedredline)[11].Thistimingoforiginforcave formsismuchearlierthanpreviouslysuggested (~3.1MYa)[10].Whiletheseauthorsestimateddivergencetimesusing cytB ,thediscrepancybetweentheir estimatesandothers(see[10])isaconsequenceofdifferentcalibrationpoints(i.e.,risingoftheSierraofPerija andMeridaMountainsandfinalclosureoftheTVMB) anddatingestimates(usinganaverageof~0.8%/million years;see[11,47,48]).Theirearlierestimateof colonizationintonortheasternMexico,however,isconsistentwiththeproposedtimingofinvasionofother speciesintoMesoamerica.Forinstance,thecatfish Rhamdialaticauda alsoreportedlyinvadedthisregion priortoclosureoftheColombian-Panamanianland bridge[49]. Amorerecentstudy,evaluatingbothmtDNAand microsatellitedataincaveandsurfacepopulationsof Astyanax ,furthercharacterizedthereporteddiscordance betweennucleargenotypesandthemtDNAcladespreviouslydefinedfromotherstudies.Hausdorfetal., (2011)surveyedvariationatsixmicrosatellitelociin25 populationscomprisedof4caveand21surface-dwelling localities.TheresultsoftheirmtDNAanalysiswere largelycongruentwithpriorfindings[11,23],however nucleargenotypicdatarevealedtwoclusters.ClusterI comprisedmostofthesurfacepopulationsintheregion ofnortheasternMexicoandmostoftheindividualsfrom theChicacave.ClusterII,the “ old ” cavecluster, includedcavefishfromPachón,Tinaja,andSabinosas wellastheremainingfishcollectedfromtheChicacave (~14%oftheircollectedsamples). Admixtureanalysesindicatedthattherehasbeenrecentgeneflowbetweendistinctmitochondrialclades, mostnotablyfromthePachóncavelocality[21].The authorsconcludedthatthePachóncavewascolonized bythe “ older ” invasionofnortheasternMexico,andthat thedisagreementbetweenthemtDNAcladesandnucleargenotypicclusterscanbeexplainedbymitochondrial “ capture ” thatoccurredbetweenPachónanda neighboringsurfacepopulation[21].Theseauthorsalso suggestthattheevolutionof “ constructive ” traitsin Astyanax cavefish(e.g.,enhancedtasteandlateralline senses),thathaveevolvedindependentlyinmultiplecave populationsmayhaveresultedinpre-matingisolation, representinganinitialsteptowardsparallelspeciation [21]. Todetermineif Astyanax cavefishrepresentaninstanceofparallelevolution,Streckeretal.(2012)performedapopulation-levelanalysisofsevencaveand sevensurfacepopulations.Theseauthorsreportedsignificantpopulationdifferentiationbetween Astyanax caveandsurfaceformsthroughanalysesofbothnuclear (microsatellite)DNAandmtDNA.TheirmtDNAanalysisalsoagreeswithfindingsofothergroupsindicating epigean Astyanax invadednortheasternMexico,andthe caves,atleasttwice[7,10,11,21,23]. Inthisstudy,theoldinvasionwasdistinguishedon thebasisofmtDNA(cladeG=old)[23],whereasthe newinvasioncorrespondstocladeA,whichismore similartosouthern Astyanax surfacefish.Twonuclear genotypicclustersweredistinguishedfromtheseoldand newinvasions[21,23].Asobservedfrompriorstudies, Pachóncavefishgroupwiththeoldinvasionbasedon nucleargenotypicdata,butgroupwithneighboringsurfacepopulationsandtheChicacavebasedonmtDNA sequencesimilarity(CladeA)[21,23],furthersupporting mitochondrialcaptureofthesurroundingsurfacefishby the “ old ” Pachóncavepopulation[21,23].Interestingly, inthisstudydisagreementsbetweenmtDNAcladeassignmentandnucleargenotypicclusterswereobserved inmore Astyanax cavepopulations(Yerbaniz,Molino, Pichijumo,CaballoMoro)implyingmitochondrial“ capture ” maybeafrequentphenomenonin Astyanax cavefish[9]. UsingaSTRUCTUREanalysisofmicrosatellitedata [9],theauthorsassignedtheirgroupstoeachoffive clusters.ClusterIincludesallsurfacefishanalyzedfrom northernMexicoalongwithsurfacefishcollectedfrom theYerbanizcaveandasingleindividualfromeachof theMicosandChicacaves.ClusterIIincludesallofthe fishfromthe “ old ” caveinvasion,includingTinajaand Pachóncavefish,allblindfishcollectedfromYerbaniz cave,andallbutonefishfromtheSabinosandChica caves.ClusterIIIincludesonefishfromtheMicoscave, twofishcollectedfromRíoCoy,andmostfishfromtwo surfacepopulationsinsouthernMexico.ClusterIV includesalloftheCaballoMorocavefish.ClusterV includesmostfishfromtheMicosandChicacaves,one surfacefishfromnearthePachóncaveentry,foursurfacefishfromneartheMicoscavelocality,onefishfrom Sabinoscave,andthreesurfacefishfromtheRíoCoy andMalatengolocalities.Basedontheseanalyses,the authorsconcludedthatlittle(ifany)geneflowoccurs betweenthecavepopulationsandtheirneighboringsurfacepopulations. Incontrast,averyrecentstudybyBradicetal.,2012, analyzing26differentloci(microsatellites)in11cave and10surfacepopulations,arguesthatmeasurablegene flowhasoccurredbetweenbothcaveandsurfaceand fromcavetocavepopulations.Thispopulationanalysis agreeswithpriorreportssuggestingcavepopulations originatedfromtwo( ‘ old ’ and ‘ new ’ )wavesofancestral surface-dwellingforms[7,9-11,21,23]).The “ old ” caves arethoseoftheElAbraregion(Pachón,Yerbaniz, Japonés,Arroyo,Tinaja,Curva,ToroandChica).TheGross BMCEvolutionaryBiology 2012, 12 :105 Page8of12 http://www.biomedcentral.com/1471-2148/12/105

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youngcavesincludethosefromtheGuatemalaregionto thenorth(MolinoandCaballoMoro),andthewestern Micoscaves(Subterráneo).Certaincavepopulations (Pachón,Curva,MolinoandCaballoMoro)demonstratedthehighestlevelsofgeneticdifferentiationfrom surface-dwellingfish,likelyasaconsequenceofisolation sincethesecavesareperchedpopulationsorisolated fromnearbysurfacestreams[8]. Thisstudyfoundcavefishpopulationsaremoregeneticallydifferentiatedthantheirsurfaceformcounterparts, despitesurfacefishbeingmoregeographicallyseparated. Thiswassupportedbythefindingoflowerallelicdiversityincaveversussurfacepopulations.Theauthorsalso arguethattheobservedlowergeneticdiversity,which correlateswithsmallereffectivepopulationsizes,likely aroseasaconsequenceoflimitedfoodandspaceinthe subterraneanenvironmentand/orperiodicbottleneck events[8,23]. Insum,accordingtoBradicetal.(2012),theentire geographicalextentofnortheasternMexicoharboring blind Astyanax cavefishpopulationswerecolonizedby anoldandnewstockofsurface-dwellingancestors.The olderpopulationinvadedtheElAbracavesatleastthree timesandthenwentextinct(Figures1Dand2).The youngerepigeanstockcolonizedtheMicosandGuatemalaregionsmuchlater,throughatleasttwoinvasions (Figures1Eand2).Thismorerecentinvasionwasestablishedfromawaveofancestralsurface-dwellingfishthat arecloselyrelatedtothesurface-dwellingformsfound intheregiontoday.Basedontheindependentacquisitionandstabilityofcave-associatedtraits,despitesignificantgeneflow(seebelow),theseauthorsconcludethat thesetraitsarelikelymaintainedundernaturalorsexual selection[8]. Theconsensusview,drawnfromseveralrecentpopulationgeneticanalyses,isthat Astyanax cavepopulations originatedfromtwowavesofancestralepigeanforms fromthesouth.TheolderwavegaverisetotheElAbra populations,whiletheyoungerwavegaverisetothe GuatemalaandtheMicospopulations(Figure1D,Eand 2)[8].Ratherthanacolonizationtimeontheorderof tensofthousandsofyearsago,asestimatedfromearlier studies[3,5],thecaveswerecolonizedseveralmillionsof yearsago(Figure1and2)[8,10,11].Further,ratherthan asingleinvasionoriginatingfromoneancestralstock, extantcaveformsoriginatedfroman “ old ” and “ new ” stocksofsurface-dwellingancestors[9,10],eachthrough multiple(i.e.,atleastfive)independentinvasions[8].TheoriginofChicacavefishThepreciseoriginofthesouthernChicacaveisinteresting.Thispopulationdemonstratesahigherlevelofgeneticdiversitycomparedtomostcaves,probablyasa resultofahigheramountmigrationfromsurrounding surfacefish[8].Haudorfetal.(2011)explainthevariabilityineyesizeamongindividualswithintheChica caveascausedbyperiodichybridizationeventsinthe pastbetweenChicacavefishandsurroundingsurface populations.However,basedonthehighFSTvalues observedbetweencavefishandsurroundingsurfacefish populations,theauthorsarguethatnocontinuousgene flowoccursfromsurfacefishintothecave[21].The authorsalsoreportthatdespitethepresenceofhundredsofsurfacefishthatgetsweptintotwoothercave localities(MicosandYerbaniz)duringspringflooding, noappreciablehybridizationoccurs[21]. Basedonthisevidence,Hausdorfetal.(2011)concludedthatChicacavefishprobablydidnotarisefrom thesameinvasionastheolderSabinos,Tinajaand Pachóncavessincemostofthefishatthislocality groupedwithnucleargenotypicclusterI.Itshouldbe notedhowever,that~14%ofthefishfromChicadid groupwiththeoldercaves.Theseauthorsarguethat Chicawascolonizedbyayoungercavepopulationthat laterintrogressedwithanoldercavepopulation[21,23]. Incontrast,Bradicetal.(2012)concludedthatChica wasseededbytheoldstockofepigeanfish.Thiswas basedonthefindingthatthegeneticdistanceforChica cavefishwasequidistantbetweentheolderandnewer populations[8].TheresultsoftheirFSTanalysisdemonstratedthatthispopulationissignificantlydifferentfrom theothercaveandsurfacepopulationssurveyedintheir study.Moreover,theauthorssuggestthat – duetoits southerngeographicposition – Chicamayhavebeen oneofthefirst Astyanax cavepopulationsseededfrom theolderwave.ThismaybeduetothefactthatChica harborsalargebatroostthatprovidesasignificantnutrientbase.Thishighernutrientbasemayhaveallowed periodicsurfacemigrantstosurviveintheotherwise nutrient-poorenvironmentofthecave[8].Geneticvariabilitywithincavesandgeneflowbetween populationsBeyondtheChicacave,thelevelofgeneticvariability withinandbetweenothercavefishpopulationshaslong beenofinterestto Astyanax researchers.Generally,cave organismsareassumedtoharborlowlevelsofgenetic variability[16].Classicaltheoriesaimedtodetermine whateventsleadtoreducedlevelsofgeneticvariability incavepopulations.Soulé(1971)arguedthatrelatively lowlevelsofgeneticvariabilityshouldbeobservedand maintainedincave-dwellingorganisms[50];reviewedin [16].Ontheotherhand,Barr(1968)arguedthatlow levelsofheterozygositywouldbepredictedfollowinga caveinvasion,butthatheterozygositywouldsubsequentlyre-expand[51].PoulsonandWhite(1969) arguedthatlowlevelsofvariabilitywouldremainconstant – lessasaconsequenceofpopulationbottlenecksGross BMCEvolutionaryBiology 2012, 12 :105 Page9of12 http://www.biomedcentral.com/1471-2148/12/105

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orinbreeding-butduetostrongstabilizingselectionfor cave-adaptivetraits[52]. Toassessthevalidityofthesecompetingtheoriesin Astyanax ,AviseandSelander(1972)firstsetouttodetermineifcaveformsharboredreducedheterozygosity comparedtosurfaceformfish.Theydiscoveredthat caveformsdemonstratedreducedlevelsofheterozygosity(e.g.,all17lociinPachóncavefishweremonomorphic)comparedtosurfaceforms.Thecommon assumptionthatall Astyanax cavefishpopulationsharborlowlevelsofgeneticdiversity,however,mayneedto berevised.BasedonananalysisofRAPDsand19 microsatellitemarkers,Panarametal.(2005)demonstratedalow,butnotabsent,levelofgeneticdiversityin cavepopulations.Aftertestingtheeffectsofavarietyof causes,onlythepresenceofeyedfishwithinthecave populationwasassociatedwithincreasedgeneticdiversity.Theauthorsconcludedthatthisintra-cavediversity likelyreflectsrecentgeneflowbetweensurfaceand cavefish.Microsatelliteanalysesprovidedfurthersupport forthenotionofmeasurablegeneflowbetweencave andsurfaceforms[2]. Afascinatingcasestudyofintra-cavephenotypicvariabilityoccursattheCaballoMorolocalitythatharbors botheyedandeyelessfishpopulations.Thiscavehasa karst “ window ” ,originatingfromacollapsedroofthat illuminatesacavelakeduringcertaintimesofday[5]. EspinasaandBorowsky(2000)observedthedistribution withinthecaveisnon-randomwitheyedandeyelessfish remainingdistinctlywithintheilluminatedanddarkportionsofthelake,respectively.Thisdivisionistheresult ofaggressiontowardseyelessfishswimmingintothe lightedregionofthecave[53].Theseattacksmayprovideamechanismbywhicheyedandeyelessindividuals remainsubdividedwithinthesamepopulation.RAPD geneticanalysesrevealedtheeyedCaballoMorofishare morecloselyrelatedtotheeyelessfishthantheyareto surroundingsurfacefishpopulations.Thisleadstothe intriguingpossibilitythateyedformswerere-established fromstandingvariationintheeyelesspopulation,after thekarstwindowwasestablished[53]. Inamorerecentstudy,Streckeretal.,2012observed thatallofthefishattheCaballoMorolocalitygrouped withanucleargenotypiccluster(IV)thatismoregeneticallysimilartosurfacefishpopulationssouthofthe TVMBthanneighboringsurfacepopulationsinnorthern Mexico.TheauthorscategorizedthesefishasaVEP (variableeyeandpigmentation)population.Basedon theirmicrosatellitedataanalysis,theseauthorsconcludedthatCaballoMorocavefisharosefromarelativelyrecentcolonizationacrosstheTMVBintothecave environment.Thus,CaballoMoroislikelyayoungcave populationthathasundergonedivergentselection withinthecavethathasledtothefirststagesofgenetic separation,representinganexampleofincipientspeciation[9]. Inothercaves,intra-cavevariationcouldbeexplained, inprinciple,bydifferencesintrophicdiversityor hybridizationwithsurfaceformfish[10].Theconsensus viewamongseveralreportsisthathybridizationwith surfacefishlikelyexplainsphenotypicandgenotypicdiversityattheChicacavelocality(seeabove).Butwhat aboutothercavelocalitiesinhabitedby Astyanax cavefish?Hausdorfetal.(2011)arguethat,despiteflooding eventsthatbringhundredsofsurfacefishintothewesternMicoscaveandtheYerbanizcaves,thereisnoappreciablelevelofhybridization.Indeed,inYerbaniz, hundredsoffisharereportedlysweptintothecavesfollowingspringflooding,howeveronlytwospecimens havebeenreportedthatbearintermediatephenotypes [5,9].Theauthorssuggestthatthisfailuretohybridize likelyoccursasaconsequenceofcompetitionforlimited foodsupplyamongsurfaceformsthatgetsweptintothe cave.AsimilarcircumstanceexistsattheMicoscavelocalitywheresurfacefishpresentinthecavehavebeen observedtobeinastarvationstate[54]. PastexpeditionstothePachóncavehavesimilarly reportedchangesinpopulation-levelphenotypes(e.g., frompredominantlypigmentedtopredominantlyalbino fish)suggestingthatcontactwithsurfacefishisarandomeventthatoccursunevenlyovertime[55].Recent QTLstudiescarriedoutbyProtasetal.,(2006)and Grossetal.(2009)offerevidenceofgeneflowbetween caves.Bothstudiesreportedloss-of-functionallelesassociatedwithpigmentationphenotypes(albinismand brown ,respectively)fromthePachóncavepopulations werepresentinthegeographicallydistantJaponésand Yerbanizcaves[6,56].Theseobservationssuggestthat recentgeneflow,orpresenceofthesameallelesinthe ancestralepigeanformsthatoriginallycolonizedthe threecavepopulations,mayexplainwhythesamealleles areobservedingeographicallydistantpopulations.A clearunderstandingofthelevelofgeneflowbetween caveandsurfaceforms,orbetweendifferentcavepopulations,iscriticalforunderstandinghowandwhyalleles maybesharedamongdifferentpopulations. Streckeretal.(2012)reportednogeneflowbetween thestronglyeye-andpigment-reduced(SEP)cavepopulations(Sabinos,TinajaandPachón)andsurrounding surface-dwellingforms.Geneflowwasnotobserved evenincaseswheresurfacefishwerepresentwithinthe samecave(e.g.,theYerbanizlocality).Theauthorsalso reportedlittlegeneflow,ifany,betweenpopulations demonstratingvariabilityineyesizeandpigmentation (VEP=Micos,ChicaandCaballoMoro;Figure2)and neighboringsurface-dwellingfish.Basedonthis,the authorsarguetheVEPcavefishmorelikelyevolvedfrom arecentcolonization,ratherthanfromanancientcaveGross BMCEvolutionaryBiology 2012, 12 :105 Page10of12 http://www.biomedcentral.com/1471-2148/12/105

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populationthatsubsequentlyhybridizedwithsurfacedwellingforms. Incontrast,Bradicetal.(2012)reportedasignificant levelofgeneflowamong Astyanax fish.Ingeneral,surfacepopulationsdemonstratedhigherratesofmigration withothersurfacepopulationsinasymmetricalfashion. Migrationratesbetweencaveandsurfacepopulations weregenerallyfoundtobeasymmetricalwithmoresurfacemigrationintothecaveratherthanfromthecave intothesurface[8].TheMicoscavepopulation,however,demonstratedanearlyequalamountofmigration withthesurroundingsurfacefishpopulation.Migration ratesamongcaveswerefoundtobeverylowexceptfor cavesthatweregeographicallyclosetooneanother[8]. Thisfindingsuggeststhattheremaybesubterranean connectionsthatunitedifferentcavestooneanother, allowingfortheobserved(albeitlow)frequencyofgene flow.Anadditionalfactorthatmayplayintothismigrationisaltitudesincetheauthorsobservedatrendinmigrationratesthatfollowedgeneflowfromhigher populations(e.g.,N2at175mabovesealevel)tolower populations(e.g.,N1at125mabovesealevel)[8].ConclusionsInrecentyears,throughnumerousgeneticandmolecularanalyses,aclearerpictureof Astyanax caveformoriginshasemergedfromtheliterature.Thereare importantchallengesassociatedwithgeneratingaconclusivephylogenyofcaveformssincewidespreadconvergenceamongcaveformsrendersidentificationof preciselineagesproblematic[11].Theconsensusview, basedonallestimates,indicatesthattheoriginsofcave formsarefarolderthaninitiallyassumed.Ratherthan evolvingfromasinglecolonizationofthesubterranean biotope,severalinvasionsstretchingbackseveralmillionsofyearsledtotheextantdistributionof Astyanax cavefishpopulations.Severalindependentinvasionsfrom each “ stock ” oftheseepigeanancestorshaveseededmultiplecaves.Forinstance,boththePachónandSabinos cavepopulationsarederivedfromthesame “ old ” stock [8],butthesepopulationslikelyinvadedeachcaveatdistincttimesinthepast. Inaddition,geneticstudiesindicatealargeramountof geneticvariabilitywithincavesexistscomparedtoearly estimates.Thismayindicatealargerrelativeamountof geneflowbothbetweencavesandbetweensurfaceand cavefish[8].Thisobservationissupported,inpart,by recentQTLanalysesthatdemonstratethepresenceof identicalhaplotypesfor Oca2 and Mc1r ingeographically distantcaves(PachónandYerbaniz/Japonéscaves; [6,56]). Thegrowinginterestin Astyanax cavefish,asamodel forunderstandingregressivephenotypicevolution, underscorestheimportanceofahigherresolution phylogenyforinterpretingexperimentalstudies.In addition,studiesinvestigatingthegeneticbasisfortrait evolutionincavefishcanbeusefulforinformingtheextenttowhichallelesaresharedbetweengeographically distinctcaveforms.Competinginterests Theauthordeclaresnocompetinginterests. Acknowledgements TheauthorwishestothankmembersoftheGrosslabforhelpfuldiscussions andfeedbackonthisarticle.Thismanuscriptisdedicatedtothememoryof Dr.RobertWhitton(DavidsonCollege). Authors ’ contributions JBGwrotethemanuscript. Received:31May2012Accepted:30June2012 Published:30June2012 References1.CulverDC,KaneTC,FongDW: Adaptationandnaturalselectionincaves:The evolutionof Gammarusminus.Cambridge,Mass:HarvardUniversityPress; 1995. 2.PanaramK,BorowskyR,QuattroJM: Geneflowandgeneticvariabilityin caveandsurfacepopulationsoftheMexicantetra, Astyanaxmexicanus (Teleostei:Characidae). Copeia 1985:409 – 416. 3.CulverDC: Cavelife:Evolutionandecology .Cambridge,MA,USA:Harvard UniversityPress;1982. 4.BrederCMJr: DescriptiveecologyofLaCuevaChica,withespecial referencetotheblindfish, Anoptichthys . Zoologica 1942, 27 :7 – 15. 5.MitchellRW,RussellWH,ElliottWR: MexicanEyelessCharacinFishes,Genus Astyanax:Environment,Distribution,andEvolution .Lubbock,Texas:Texas TechPress;1977. 6.ProtasME,HerseyC,KochanekD,ZhouY,WilkensH,JefferyWR,ZonLI, BorowskyR,TabinCJ: Geneticanalysisofcavefishrevealsmolecular convergenceintheevolutionofalbinism. NatGenet 2006, 38 :107 – 111. 7.DowlingTE,MartasianDP,JefferyWR: Evidenceformultiplegeneticforms withsimilareyelessphenotypesintheblindcavefish, Astyanax mexicanus . MolBiolEvol 2002, 19 :446 – 455. 8.BradicM,BeerliP,Garcia-deLeonFJ,Esquivel-BobadillaS,BorowskyRL: GeneflowandpopulationstructureintheMexicanblindcavefish complex( Astyanaxmexicanus ). BMCEvolBiol 2012, 12 :9. 9.StreckerU,HausdorfB,WilkensH: Parallelspeciationin Astyanax cavefish (Teleostei)inNorthernMexico. MolPhylogenetEvol 2012, 62 :62 – 70. 10.StreckerU,FaundezVH,WilkensH: Phylogeographyofsurfaceandcave Astyanax (Teleostei)fromCentralandNorthAmericabasedon cytochromeb sequencedata. MolPhylogenetEvol 2004, 33 :469 – 481. 11.Ornelas-GarciaCP,Dominguez-DominguezO,DoadrioI: Evolutionary historyofthefishgenus Astyanax Baird&Girard(1854)(Actinopterygii, Characidae)inMesoamericarevealsmultiplemorphological homoplasies. BMCEvolBiol 2008, 8 :340. 12.StehliFG,WebbSD: TheGreatAmericanbioticinterchange .NewYork:PlenumPress;1985. 13.Aguayo-CamargoJE: ThemiddleCretaceousElAbraLimestoneatitstype locality(facies,diagenesisandoilemplacement),east-centralMexico. RevistaMexicanadeCienciasGeológicas 1998, 15 :1 – 8. 14.Armstrong-AltrinJ,MadhavarajuJ,SialA,Kasper-ZubillagaJ,NagarajanR, Flores-CastroK,RodríguezJ: Petrographyandstableisotopegeochemistry ofthecretaceousElAbraLimestones(Actopan),Mexico:Implicationon diagenesis. JGeolSocIndia 2011, 77 :349 – 359. 15.WilkensH: Geneticinterpretationofregressiveevolutionaryprocesses: Studiesonhybrideyesoftwo Astyanax populations(Characidae,Pisces). Evolution 1971, 25 :530 – 544. 16.AviseJC,SelanderRK: Evolutionarygeneticsofcave-dwellingfishesofthe genus Astyanax . Evolution 1972, 26 :1 – 19. 17.MariaPonsJ,VicensE,PichardoY,AguilarJ,OviedoA,AlencasterG,GarciaBarreraP: AnewearlyCampanianrudistfaunafromSanLuísPotosíin Mexicoanditstaxonomicandstratigraphicsignificance. JPaleontol 2010, 84 :974 – 995.Gross BMCEvolutionaryBiology 2012, 12 :105 Page11of12 http://www.biomedcentral.com/1471-2148/12/105

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18.MuirJM: GeologyoftheTampicoRegion,Mexico.SpecialVolumeed .Tulsa, Oklahoma:AmericanAssociationofPetroleumGeologists;1936. 19.BonetF: BiostratigraphicnotesontheCretaceousofeasternMexico .In GeologyofPeregrinaCanyonandSierradeElAbra .MexicoCorpusChristi: GeologicalSociety;1963:36 – 48. 20.JansonX,KeransC,LoucksR,MarhxMA,ReyesC,MurguiaF: Seismic architectureofaLowerCretaceousplatform-to-slopesystem,Santa AguedaandPozaRicafields,Mexico. AAPGBull 2011, 95 :105 – 146. 21.HausdorfB,WilkensH,StreckerU: Populationgeneticpatternsrevealedby microsatellitedatachallengethemitochondrialDNAbasedtaxonomyof Astyanax inMexico(Characidae,Teleostei). MolPhylogenetEvol 2011, 60 :89 – 97. 22.HüppopK: Oxygenconsumptionof Astyanaxfasciatus (Characidae, Pisces):Acomparisonofepigeanandhypogeanpopulations. EnvirBiol Fishes 1986, 17 :299 – 308. 23.StreckerU,BernatchezL,WilkensH: Geneticdivergencebetweencave andsurfacepopulationsof Astyanax inMexico(Characidae,Teleostei). MolEcol 2003, 12 :699 – 710. 24.EspinasaL,BorowskyR: Originsandrelationshipofblindcave populationsoftheBlindMexicantetra, Astyanaxfasciatus ,intheSierra deElAbraregionofMexico. EnvirBiolFishes 2001, 62 :233 – 237. 25.WilkensH: Evolutionandgeneticsofepigeanand caveAstyanaxfasciatus (Characidae,Pisces):Supportfortheneutralmutationtheory .In EvolutionaryBiology .EditedbyHechtMK,WallaceB.NewYork,NY:Plenum PublishingCorporation;1988:271 – 367. 26.MyersGS: DerivationofthefreshwaterfishfaunaofCentralAmerica. Copeia 1966, 1966 :766 – 773. 27.BussingWA: TheGreatAmericanbioticinterchange.Volume4 .In Patterns ofdistributionoftheCentralAmericanichthyofauna .EditedbyStehliFG, WebbSD.NewYork:PlenumPress;1985:453 – 473. 28.BerminghamE,MartinAP: ComparativemtDNAphylogeographyof neotropicalfreshwaterfishes:Testingsharedhistorytoinferthe evolutionarylandscapeoflowerCentralAmerica. MolEcol 1998, 7 :499 – 517. 29.BarrTC: The Pseudanopthalmus oftheAppalachianValley(Coleoptera: Carabidae). AmerMidNat 1965, 73 :41 – 72. 30.BarrTC: Asynopsisofcavebeetlesofthegenus Pseudanopthalmus of theMitchellPlaininSouthernIndiana(Coleoptera,Carabidae). AmerMid Nat 1960, 63 :307 – 320. 31.MoreauRE: ThebirdfaunasofAfricaanditsislands .NewYork:Academic;1966. 32.HafferJ: SpeciationinAmazonianforestbirds. Science 1969, 165 :131 – 137. 33.MitchellRW: Acomparisonoftemperateandtropicalcavecommunities. SouthwestNat 1969, 14 :73 – 88. 34.HobbsHHJr,BarrTCJr: Theoriginsandaffinitiesofthetroglobitic crayfishesofNorthAmerica(Decapoda,Astacidae).I.Thegenus Cambarus . AmerMidNat 1960, 64 :12 – 33. 35.KosswigC: ÜberdasTempoevolutorischerProzesse. ZoolBeitrNF 1967, 13 :441 – 450. 36.WilkensH: Regressiveevolutionandphylogeneticage:Thehistoryof colonizationoffreshwatersofYucatanbyfishandCrustacea. AssocMex CaveStudBull 1982, 8 :237 – 243. 37.FitzsimmonsK: TilapiaaquacultureintheAmericas.Volume2 .In Tilapia aquacultureinMexico .EditedbyCosta-PierceB,RakocyJ.BatonRouge:The WorldAquacultureSociety;2000:171 – 183. 38.BrederCM,RasquinP: Comparativestudiesinthelightsensitivityofblind characinsfromaseriesofMexicancaves. BullAmerMusNatHist 1947, 89 :325 – 351. 39.AlvarezJ: Revisióndelgénero Anoptichthys condescipcióndeuna especienueva(Pisces,Characidae). AnEscNacCienBiolMéxico 1946, 4 :263 – 282. 40. ado luP: MendelianinheritanceinthehybridsbetweentheMexican blindcavefishesandtheirovergroundancestor. VerhDtschZoolGesGraz 1957, 1957 :432 – 439. 41.WilkensH: BeiträgezurDegenerationdesAugesbeiCavernicolen, GenzahlundManifestationsartUntersuchungenanmexikanischen Höhlenfischen. JZoolSystEvolRes 1970, 8 :1 – 47. 42.WilkensH,BurnsRJ: Anew Anoptichthys cavepopulation. AnnSpéléol 1972, 27 :263 – 270. 43.WilkensH,StreckerU: Convergentevolutionofthecavefish Astyanax (Characidae:Teleostei):Geneticevidencefromreducedeye-sizeand pigmentation. BiolJLinnSoc 2003, 80 :545 – 554. 44.BorowskyR: Restoringsightinblindcavefish. CurrBiol 2008, 18 :R23 – R24. 45.ZardoyaR,DoadrioI: Molecularevidenceontheevolutionaryand biogeographicalpatternsofEuropeancyprinids. JMolEvol 1999, 49 :227 – 237. 46.JuberthieC: SubterraneanEcosystems.Volume30 .In Conservationof subterraneanhabitatsandspecies .EditedbyWilkensH,CulverDC, HumphreysWF.Amsterdam:Elsevier;2000:691 – 700. 47.DíazdeGameroM: ThechangingcourseoftheOrinocoRiverduringthe Neogene:Areview. PalaeogeogrPalaeoclimatolPalaeoecol 1996, 123 :385 – 402. 48.FerrariL,Lopez-MartinezM,Aguirre-DiazG,Carrasco-NunezG: Space-time patternsofCenozoicarcvolcanismincentralMexico:FromtheSierra MadreOccidentaltotheMexicanVolcanicBelt. Geology 1999, 27 :303 – 306. 49.PerdicesA,BerminghamEAM,DoadrioI: Evolutionaryhistoryofthegenus Rhamdia (Teleostei:Pimelodidae)inCentralAmerica. MolPhylogenetEvol 2002, 25 :172 – 189. 50.SouléM: Thevariationproblem:Thegeneflow-variationhypothesis. Taxon 1971, 20 :37 – 50. 51.BarrTC: Caveecologyandtheevolutionoftroglobites. EvolBiol 1968, 2 :35 – 102. 52.PoulsonTL,WhiteWB: Thecaveenvironment. Science 1969, 165 :971 – 981. 53.EspinasaL,BorowskyR: Eyedcavefishinakarstwindow. JCaveKarst Studies 2000, 62 :180 – 183. 54.WilkensH,HüppopK: Sympatricspeciationincavefishes? JZoolSystEvol Res 1986, 24 :223 – 230. 55.LangeckerTG,WilkensH,JungeP: Introgressivehybridizationinthe PachónCavepopulationof Astyanaxfasciatus (Teleostei:Characidae). IchthyolExplorFreshwaters 1991, 2 :209 – 212. 56.GrossJB,BorowskyR,TabinCJ: Anovelrolefor Mc1r intheparallel evolutionofdepigmentationinindependentpopulationsofthecavefish Astyanaxmexicanus . PLoSGenetics 2009, 5 :e1000326. doi:10.1186/1471-2148-12-105 Citethisarticleas: Gross: Thecomplexoriginof Astyanax cavefish. BMC EvolutionaryBiology 2012 12 :105. Submit your next manuscript to BioMed Central and take full advantage of: € Convenient online submission € Thorough peer review € No space constraints or color “gure charges € Immediate publication on acceptance € Inclusion in PubMed, CAS, Scopus and Google Scholar € Research which is freely available for redistribution Submit your manuscript at www.biomedcentral.com/submit Gross BMCEvolutionaryBiology 2012, 12 :105 Page12of12 http://www.biomedcentral.com/1471-2148/12/105


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