Chapter18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureJ - PDF document

Chapter18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureJustinD.Faris18.1Introduction..............................................................44018.2TheEvolutionofWheat....................................................44318.3ThePlaceandTimeofEinkornandEmmerWheatDomestication.................44518.3.1DomesticationofEinkornWheat......................................44618.3.2DomesticationofEmmerWheat......................................44618.4OriginofFree-threshingTetraploidWheats....................................44718.5OriginofHexaploidWheat.................................................44818.6GeneticsofDomesticationLoci..............................................45018.6.1BrittleRachis......................................................45118.6.2TenaciousGlume...................................................45218.6.3TheQLoci........................................................45418.6.4TheEvolutionofFree-threshingWheats................................45618.7WheatEvolutionUnderCultivation..........................................45718.7.1CaptureofGeneticVariability........................................45718.7.2FurtherDomesticationUnderCultivation...............................45818.7.3GenerationofNewGeneticDiversity..................................45918.8FutureNeeds.............................................................460....................................................................460Thedomesticationofwheatwasinstrumentalinthetransitionofhumanbehaviorfromhunter-gathererstofarmers.Itwasakeyeventintheagriculturalrevo-lutionthatoccurredabout10,000yearsagointheFertileCrescentoftheMiddleEast.Transitionsofformswithnaturalseeddispersalmechanismstoformswithnon-brittlerachisesledtothedomesticationofdiploideinkornandtetraploidemmerwheatinsoutheastTurkey.Theseearlydomesticateswerestaplecropsofearlyfarmersforseveralthousandyearsbeforebeingreplacedbyfree-threshingwheats.Allopoly-ploidization,mutationsingenesgoverningthreshabilityandotherdomesticationrelatedtraits,andinterspeciÞcgeneßowledtotheformationoftodayÕseconomi-callyimportantbreadwheat.Genetics,genomics,andarchaeobotanyhavetogether J.D.Faris(USDA-AgriculturalResearchService,CerealCropsResearchUnit,1605AlbrechtBLVD,Fargo,ND58102-2765,USAe-mail:justin.faris@ars.usda.govR.Tuberosaetal.(eds.),GenomicsofPlantGeneticResources,DOI10.1007/978-94-007-7572-5_18,©SpringerScience+BusinessMediaDordrecht2014 J.D.Farisprovidedstrongevidenceandinsightsregardingthetime,place,andeventsinvolvedintheevolutionanddomesticationofmodernwheat,butnumerousquestionsre-mainunanswered.Here,Ireviewhistoricalandrecentndingsthathaveshapedourcurrentunderstandingofwheatdomestication.Whole-genomesequenceanaly-sis,additionalgeneticstudies,andadvancesinarchaeologywilllikelyaddressourunansweredquestionsinthefuture.Athoroughandcomprehensiveunderstandingofwheatevolutionanddomesticationwillprovidecriticalknowledgetothespawningofanewagriculturalrevolution,whichwillbenecessarytoprovidesustenanceforarapidlyincreasingworldpopulationunderglobalclimatechange.KeywordsEinkornTriticumAegilopsEvolutionFertilecrescentBrittlerachisTenaciousglumeQgene18.1IntroductionBefore10,000yearsago,manlivedanomadiclifestyleasahunter-gathererrelyingonthehuntingofwildanimalsandcollectingwildplantsforhisfood.Then,theNeolithicrevolutiontookplacewherethehunter-gathererwayoflifewasreplacedbyanagrarianlifestyle.Thiswasacrucialturningpointinhumanhistoryandhadaprofoundeffectonlifethereafter.TheNeolithicrevolutiontookrootintheLevantineCorridorandspreadthroughtheFertileCrescent,whichislocatedintheMiddleEastandencompassesaregionextendingfromJordan,Israel,Lebanon,andSyriathroughsoutheastTurkeyandalongtheTigrisandEuphratesriversthroughIraqandwesternIran(Fig.).This“cradleofagriculture”wasthecenterofdomesticationofeinkorn(TriticummonococcumL.)andemmer(T.turgidumL.)wheat,whichwerestaplecropsofearlycivilizationandcloserelativesofmoderndaywheat.ThesecerealsweredomesticatedalongsideotherimportantcropsincludingbarleyHordeumvulgareL.),pea(PisumsativumL.)Lentil(LensculinarisMedikus),andchickpea(CicerarietinumL.),aswellasanimalssuchassheep(Ovisaries),goatsCaprahircus),cattle(Bostaurus),andpigs(Susscrofa)(Zeder),andtheyledthewayforanagriculturalrevolution.Nesbitt()describesdomesticationas“theprocessbywhichhumanstakereproductivecontrolofplantsoranimals,modifyingthemfortheirownpurposes.”Inwheatandothercerealcrops,therstandmostcriticalmodicationwastheacqui-sitionofanon-brittlerachis,whichlimitedthenaturalseeddispersalmechanismsofthewildformsandallowedearlyfarmerstoharvestthegrainmuchmoreefcientlywithoutspikeletsdroppingtothegroundprematurelyandbeinglost.Othermodi-cationsincludedlargerseeds,lossofseeddormancy,thefree-threshingcharacter,enhancedgrainquality,andothers(Harlanetal.).Thesechangesresultedindomesticatedformsthatreliedonfarmersfortheirpropagationandalsoallowedmechanizedcultivationonalargescale. 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuture Fig.18.1MapoftheancientMiddleEastshowingtheFertileCrescent(green).TheSouthernandNorthernLevantregionsareindicatedbythebrownshadedareas.Archaeologicalsitesmentionedinthetextareindicatedbyblackcircles(villages)andtrapezoids(mountainranges).Thedistri-butionsofwildwheatsareindicatedbythedottedlines(seelegendatthebottomofthegure).Thebasicmapwasobtainedfromd-maps.com.(http://d-maps.com/carte.php?lib=fertile_crescent_Today,about430milliontonnesofthefullydomesticatedfree-threshinghexaploidandtetraploidwheatsknownascommon,orbread,wheat(T.aestivumL.)anddurum,ormacaroni,wheat(T.turgidumL.),respectively,areproducedannuallyandprovideaboutafthofthecaloriesconsumedbyhumansworldwide(http://faostat.fao.org).Breadwheataccountsforabout95%ofthetotalwheatcropandisusedtomakebread,cookies,cakes,crackers,pastries,andnoodles,whereasdurumwheataccountsfortheremaining5%andisusedtomakepastaandothersemolinaproducts.Duetotherateoftheworld’spopulationgrowth,thedemandforwheatisexpectedtoincreaseby40%by2030(Dixonetal.Inordertomeetthisdemand,anannualincreaseinyieldof2%isneededandtheamountofagriculturallandneedstobestabilized.Thesegainswillneedtocomebywayofgeneticimprovementsandenhancedunderstandingofplantbiology.Advancingourknowledgeandunderstandingofwheatevolutionandthegeneticmechanismsunderpinningthecoredomesticationeventsthatshapedtoday’swheatplantmayprovidenewcluesastohowthegeneticdiversityavailableinthewildwheatprogenitorsandrelativescanbetappedintoandexploitedtoinitiateamodernagriculturalrevolutionunderachangingglobalclimate. J.D.Faris Fig.18.2TheevolutionarylineagesinvolvingTriticumwheatspecies.Thediploid,tetraploid,andhexaploidspeciesareseparatedbybluebarsOrangegreen,andcolorsindicatespecieswithbrittlerachisandhulledseed,specieswithnon-brittlerachisandhulledseed,andspecieswithafullytoughrachisandfree-threshingseed,respectively.Redarrowsindicateoccurrencesoftransitionsinvolvingoneormoreofthemajordomesticationgenes,or.Genotypesofmajordomesticationgenesareindicatedinboldbelowthetaxonomicalnamesandtheirgenome 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuture18.2TheEvolutionofWheatItwasdeterminednearlyacenturyagothatthecultivatedwheatspeciesoftheTriticumhavechromosomenumbersof214,28,and42.ThisindicatedthatthebasicTriticeaegenomewasorganizedintosevenchromosomes(1andthevariousTriticumspeciesconsistedofdiploids(214),tetraploids28),andhexaploids(242)(Sax;KimberandSearsThediploidprogenitorsandcloserelativesofmodernwheatradiatedfromacom-monancestorabout3millionyearsago(MYA)andgaverisetotheTriticumAegilopstaxa(Fig.).TheTriticumgroupconsistedoftheA-genomediploidsT.urartuTumanianexGandylian(214,AA(thecapitallettersrepre-sentthegenomeconstitution))andT.monococcumaegilopoides(Link)Thell.14,AA).JohnsonandDhaliwal()determinedthattheyarevalidbi-ologicalspecies.Alsoevolvingfromthecommonseven-chromosomeancestorwerenumerousdiploidAegilopsspeciesincludingAe.tauschiiCoss.(214,DD)andaprogenitortotheAegilopsSitopsissection,whichgaverisetotheS-genomeAegilopsspeciesincludingAe.speltoidesTausch(214,SS).Theonlydomesticateddiploidwheatiseinkorn(T.monococcumL.,2),whichwasdomesticatedfromssp.aegilopoidesthroughtheacquisitionofanon-brittlerachis(Fig.).Theevolutionandforma-tionofthecultivatedformsofpolyploidwheatfollowedtwobasiclineages,bothofwhichinvolvedtwoamphiploidizationevents.Theseeventsresultedfromthehy-bridizationoftwodifferentspeciesfollowedbyspontaneouschromosomedoublingoftheFhybridthroughthefunctioningofmeioticrestitutiondivision(non-reduced)gametes.OnelineagebeganwithhybridizationofT.urartu(Dvoraketal.)andAe.speltoides,oracloserelativethereof(SarkarandStebbins;Rileyetal.),whichledtotheformationofthewildemmerwheatT.timopheeviiararaticumJakubz.(228,AAGG)containingapairofAgenomesfromT.urartuandapairofGgenomes,whichareconsideredtobeadivergentformoftheSgenomeoftheAegilopsprogenitor(Rodriquezetal.T.timopheeviiararaticumhasabrittlerachisconferredbythegene.Amutationintoanon-brittlerachisandthedomesticationofthisformtoT.timopheeviipheevii(Zhuk.)Zhuk(228,AAGG).T.timopheeviiwasnevercultivatedasasignicantcropandgrowsonlyinalimitedregionofGeorgia.Therefore,itwasprobablyasecondarydomesticate(NesbittandSamuelThehexaploidwheatbelongingtothislineageisT.zhukovskyiMenabdeetEr-iczjan(242,AAAGG),whichresultedfromahybridizationbetweenT.timopheeviitimopheeviianddomesticatedeinkornwheat(Jakubziner).Likessp.timopheeviiT.zhukovskyiisnotcultivatedandtendsto Fig.18.2constitutions.Homozygosityisinferredateachlocus,andgenotypesareindicatedonlyoncetosavespace.Genotypesinaresuggestedbutnoexperimentalevidenceisavailable.arrowsrepresenteventsthatoccurredtogiverisetohexaploidsubspeciesthatformedsubsequentT.aestivum.Durumandcommonwheat,thetwomodernwidelycultivatedformsofpolyploidwheat,arehighlightedwithyellowrectangles J.D.FarisbefoundinWesternGeorgiaasanadmixturewithT.timopheeviiandeinkornwheat(NesbittandSamuel).Althoughthisconstitutesaninterestingevolutionarylineageofwheat,itdidnotresultintheformationofanyoftoday’seconomicallyimportantwheats.Therefore,littleattentionwillbedevotedtothespeciesofthislineageintheremainderofthisreview.Liketherstlineage,thesecondalsobeganwithahybridizationeventbetweenurartu(Dvoraketal.)andacloserelativeofAe.speltoides(DvorakandZhang;Blakeetal.;Huangetal.;Chalupskaetal.;Salseetal.butoneofadifferentsubspeciesthantheoneinvolvedintherstlineage(Kilianetal.;Fig.).Thiseventledtotheformationofthetetraploidwildemmerwheatturgidum(Körn.)Thell(228,AABBgenomes).Althoughbothwildemmerwheatspecies(T.turgidumararaticum)ob-tainedapairofgenomesfromtheSgenome-containingAe.speltoides,signicantdivergencehassinceoccurredsuchthattheB,G,andSgenomes,whilestillrelated,arequitedistinct(Zhangetal.;Kilianetal.).Likessp.araraticum,ssp.hasabrittlerachis,andmutationsinlociledtothedomesticatedem-mersubspeciesT.turgidum(Schrank)Schübl(228,AABBgenomes),whichhasanon-brittlerachisandhulledseed(Fig.).Thesecondam-phiploidizationeventofthislineageresultedwhenthediploidgoatgrassAe.tauschiihybridizedwithaT.turgidumsubspecies(Kihara;McFaddenandSears).TheresultinghexaploidwashulledduetothepresenceoftheAe.tauschii(McFaddenandSears).Thissubspeciesmayhavebeensim-ilartoAsianspelta(T.aestivumL.,2AABBDD)(Figs.).Evolutionofthisspeciesthroughtheacquisitionofthefree-threshingcharacter(seebelow)resultedinthefree-threshinghexaploidbreadwheatT.aestivumL.,242,AABBDD)(Figs.),oneofthemosteconomicallyimportantcropsintheworldtoday.OtherhexaploidT.aestivumspp.include(Host)MacKey,sphaero-(Percival)MacKey,machaDekapr.etMenabde,andEuropean(clubwheat)andsphaerococcum(shotwheat)arebothfree-threshinganddifferfrombysinglegenesthatarosethroughmutation.carriesthegene,whichconfersacompactspikeandsphaero-carriesthegeneforsphericalgrains.Ssp.isgrowntodayinafewisolatedareasofEurope,theNearEast,andthenorthwesternU.S.,whereassphaerococcumisconnedmostlytoIndia.Spp.machaandEuropeanarenotfree-threshingandresembleprimitiveforms,butarenotprogenitorsto.Ssp.machalikelyarosethroughamorerecenthybridizationbetweenandtheemmerwheatT.turgidum(DvorakandLuo),andthereisnowmuchevidencedemonstratingthatEuropeanformedfromacrossbetweenT.aestivumanddomesticatedemmer(T.turgidum;MacKey;Blatteretal.;Yanetal.;Fig. 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuture A B C A B C A B C A B C A B C A B C Hybridization tg1Tg1 Brbrq Fig.18.3Thephenotypesofwheatspeciesinvolvedintheevolutionofmoderncultivatedwheatandmajortransitionsattheprimarydomesticationgenes(3A,3B,and).Thespike,resultofmoderatehandthreshing,andseedareshownforeachinpanelsA,B,andC,respectively.Al-thoughdirectevidenceislacking,themostlikelyscenarioinwhichafree-threshing(tetraploidhybridizedwiththeDgenomediploidAe.tauschiitoproducehexaploidwheatisshown.Also,Asianspeltaisshowntorepresenttheprimitiveprogenitortofree-threshingcommonwheatbecauseitislikelythattheprogenitorwasverysimilar.Thecommonnames,taxonomicalnames,genomeconstitutions,andgenotypesareindicatedbeloweachpanel18.3ThePlaceandTimeofEinkornandEmmerWheatDomesticationToday,ndingsfromarchaeologicaldigscombinedwithmoleculargeneticsexper-imentshaveprovidedmanyanswersregardingwheatdomestication.AgricultureoriginatedintheFertileCrescentapproximately10,000yearsago,buttheearliestgatheringofwildemmerwheathasbeendatedto19,000yearsbeforepresent(BP)intheMiddleEastindicatingthathumanscollectedwildgrainsforsome10,000yearsbeforedomesticationtookplace.Futhermore,TannoandWillcox()arguethatcerealswereactuallycultivatedforoverathousandyearsbeforetheemergenceofdomesticatesinwhatwouldbeconsideredtherstphaseofcultivation.Inthesec-ondphaseofcultivation,domesticatedformsconsistingofeinkornandemmerwheatweregrownbyearlyfarmers.Thesewheatsacquiredanon-brittlerachis,whichal-lowedearlyfarmerstoefcientlyharvestthegrainwithoutthespikesshatteringand J.D.Farisfallingtothegroundbeforeharvest.Later,furtherdomesticationoccurredthroughtheacquisitionofthefree-threshingcharacter.Thisledtocompletereplacementofthehulledeinkornandemmerwheatswiththefree-threshingdurumandbreadwheat,althoughreplacementwasnotrapid.18.3.1DomesticationofEinkornWheatWild(T.monococcumaegilopoides)anddomesticatedeinkornwheat(T.mono-)areverysimilarmorphologicallyexceptthatthattheformerhasabrittlerachisandthelatteranon-brittlerachis.ThedistributionofwildeinkornintheMiddleEastisratherwidespreadandisfoundfromtheBalkanstoIranprimarilyinthenorthernandeasternpartsoftheFertileCrescentgrowingasaweedalongroadsidesandelds(NesbittandSamuel;Fig.).Mancollectedgrainsofwildeinkornforsometimebeforecultivatingit,becausebrittle-rachiseinkornwheathasbeenfound,forexample,intheprehistoricsettlementofMureybitofthenorthernLevantineCorridordatedabout10,000BP(Renfrew).Then,themutationthatresultedinanon-brittlerachisledtothedomes-ticationofeinkornwheat.Usingampliedfragmentlengthpolymorphism(AFLP)DNAngerprinting,Heunetal.()locatedthesiteofeinkorndomesticationtotheKaracadagregionofsoutheasternTurkeyinthenorthernLevantineCorridor.Fromhere,wildanddomesticatedeinkornweregrownsidebysidethroughouttheFertileCrescentwiththenon-brittlerachistypegraduallyreplacingwildeinkorn.CultivationofdomesticatedeinkornthenspreadtootherpartsoftheMiddleEastandsouthernEurope,andeventuallycentralandwesternEurope(FeldmanToday,einkornwheatisareliccropthatisgrownsomewhatinpartsofTurkey,Italy,andtheformerYugoslaviaprimarilyforanimalfeed.18.3.2DomesticationofEmmerWheatWildemmer(T.turgidum)istheonlytruewildpolyploidwheatofthelineage,anditistheprogenitoroftoday’sdurumandbreadwheatcultivars.Therefore,thediscoveryofwildemmeranddocumentationofitsdistribution(Aaron-)contributedsubstantiallytoourunderstandingoftheeventsthatledtothedomesticationofmodernwheats.Unlikewildeinkornwheat,wildemmerdoesnotgrowasaweedbutonlyasatrulywildplantmostlyconnedtorelativelyundis-turbedhabitats.Therefore,wildeinkorngrowsoveramuchwiderareathanwildemmer,andthelatterhasnotspreadmuchoutsideoftheFertileCrescentgrowinginaregionextendingfromthesouthernLevantacrossIsraelandLebanontosouth-easternTurkeyandacrossnorthernIraqandnorthwesternIran(Fig.).However,theregionofwildemmerhabitatisnotcontinuousandcanbedividedintonorthernandsouthernsubpopulations. 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureAswitheinkornwheat,wildemmerwheatwascultivatedforsometimebeforemutantswithanon-brittlerachisappeared.Then,wildemmerwaslikelycultivatedforseveralhundredmoreyearsasamixturewiththenon-brittlerachisformknownasdomesticatedemmer(T.turgidum)(Kislev).Thearchaeologicalrecordindicatesthat,aswithdomesticatedeinkornwheat,domesticatedemmerrstappearedinthesouthernLevantandinsoutheasternTurkeyabout9,500–9,000BP(NesbittandSamuel),butthequestionofwhereemmerwasrstdomesticatedhasbeendebatable.ThefactthatdomesticatedemmershowedupinboththenorthernandsouthernLevantalmostsimultaneouslysuggeststhatemmerwasdomesticatedineitherthenorthernorsouthernpartandthenrapidlyspreadtotheother.UsingAFLPanalysis,Ozkanetal.()showedthatdomesticatedemmerwasmorecloselyrelatedtothenorthernwildemmerpopulationsthantothesouthernpopulations.Onthecontrary,Morietal.()evaluatedchloroplastmicrosatellitevariationinamorecompletesetofwildemmeraccessionsandconcludedthatemmerwasdomesticatedintheKartalDagimountainsofsoutheasternTurkey,butalsosuggestedemmermayhavebeendomesticatedasecondtimeelsewhere.UsingtheaccessionsofMorietal.(),Ozkanetal.()conductedAFLPanalysisandshowedthattheKartalDagiregionwasnotthesiteofemmerdomestication,andinsteadsuggestedthatemmerwasdomesticatedintheKaracadagregion,theSulaimaniyaregion,orbothregionsindependently.Luoetal.()usedRFLPngerprintingtoshowthatemmerwasunlikelydomesticatedintheSulaimaniyaregion,butwaslikelydomesticatedintheKaracadagregion.SubstantialgeneowbetweenthenortherndomesticatedemmerpopulationandthesouthernwildemmerpopulationresultedinhighlevelsofdiversityinthesouthernLevantandledtothedevelopmentofnorthernandsouthernsubpopulationsofdomesticatedemmer(Luoetal.).Therefore,botheinkornandemmerweremostlikelyofmonophyleticoriginandbothweredomesticatedinessentiallythesameplace.Free-threshingderivativesofdomesticatedemmer,suchastheextincttetraploidT.turgidum,appearinthearchaeologicalrecordshortlyafterdo-mesticatedemmer(Kislev).Inspiteofbeingnon-free-threshing,domesticatedemmerwasthemostabundantwheatcropintheMiddleEastduringthePrepotteryNeolithicBperiod,andcontinuedtobeamajorcropforseveralthousandyears.About8,000BPitspreadfromthenorthernFertileCrescent,presumablyalongT.turgidum,southtoMesopotamiaandwesttoAnatolia,theMediterraneanbasin,andEurope(Feldman).ItarrivedinEgypt,centralAsia,andIndiaabout6,000BPandwasthedominantcerealcropinalltheseregionsupuntilabout3,000BPwhenitwaslargelyreplacedbyfree-threshingdurumwheat.Today,domesticatedemmerisareliccropgrownonlyinlimitedareasoftheMiddleEastandsouthAsia.18.4OriginofFree-threshingTetraploidWheatsThearchaeologicalrecordindicatesthatfree-threshingtetraploidwheatsappearedabout8,000–9,000BPinthePrepotteryNeolithicBperiod,aboutthesametimeasdomesticatedemmer.TheseearlyndsoccurredinTellAswadandotherSyrian J.D.FarissitesaswellasCanHassanIIIinsouthernTurkey(Kislev).Thisfree-threshingtetraploidwithverysmallgrainsandcompactspikeswasconsideredtobeanex-tinctsubspeciesandgiventhenameT.turgidum.Itwasassumedthatthiswheatwasderivedfromdomesticatedemmer(T.turgidumandthatdomesticatedemmerwasgrownforsometimebeforessp.appeared(Feldman).Ssp.mayhavebeengrownasanadmixturewithdomesticatedemmerwheat,oritmayhavebeengrownasaseparatecrop.Ineithercase,itspreadalongwithdomesticatedemmerthroughouttheFertileCres-centandwasgrownforseveralmillenniaintheMiddleEast.However,somehavequestionedtheexistenceofssp.duetoalimitednumberofcompletelycharacterizedsamples(NesbittDurumwheat(T.turgidum)evolvedfromdomesticatedemmerwheatpossiblybywayofssp..TherstdurumwheatwasfoundatCanHassanIIIanddatedabout6,500–7,500yearsBP(Hillman),butdurumwheatwasnotestablishedasaprominentcropuntilabout2,300yearsBP(FeldmanToday,durumisamajorcropwelladaptedtodryclimatesandusedformacaroniandsemolinaproducts.ItisprimarilygrownintheGreatPlainsregionoftheU.S.andCanada,Russia,India,Italy,andtheMiddleEast.Mostothertetraploidwheatsubspeciesarefree-threshingandprobablyaroserelativelyrecently.Theseincludespp.turgidumturanicumandothersallofwhicharequitesimilartossp.anddifferbyonlyafewtraits.Some,forexamplessp.,probablyarosebyhybridizationbetweenthefree-threshinghexaploidT.aestivumandanothertetraploid(Kuckuck18.5OriginofHexaploidWheatBothfree-threshingandnonfree-threshingformsofcultivatedhexaploidwheatex-ist,butwildprogenitorsofcultivatedhexaploidwheatdonot.HexaploidwheatoriginatedasaresultofhybridizationbetweenanABgenome-containingtetraploidandthediploidgoatgrassAe.tauschii,whichcontributedtheDgenome(Kihara;McFaddenandSears).AnumberofstudieshavepointedtoAe.tauschiistrangulataasbeingthedonoroftheDgenomeasopposedtossp.tauschii(Nishikawa;Nishikawaetal.;Jaaska;Dvoraketal.Ae.tauschiistrangulataisdistributedintworegions:TranscaucasiaandanareaofIransoutheastoftheCaspianSea(Fig.).Therefore,itwasthoughtthatthehybridizationeventthatformedhexaploidwheatmusthaveoccurredinoneofthesetwoareasandseverallinesofresearchinvolvingtheevaluationofcollectionsAe.tauschiipopulationspointedtotheareaofIransoutheastoftheCaspianSeaasthemostprobablebirthplaceofhexaploidwheat(Jaaska;Nakai;Dvoraketal.),whichmostlikelyinvolvedmultipleamphiploidizationeventsbetweenT.turgidumAe.tauschiiwithsubsequentintercrossingthatledtotheformationofasinglegenepool(Dvoraketal.;Lelleyetal. 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureT.turgidumwaslargelyconnedtotheFertileCrescentandthedistribu-tionofAe.tauschiiprimarilyoccupiesnorthernIran,Transcaucasia,andAfghanistan,thehybridizationevent(s)thatresultedinhexaploidwheatprobablydidnotoccuruntilexpansionofdomesticatedemmersubpopulationsinthenorthernLevantover-lappedwiththeprimarydistributionareasofAe.tauschiiinTranscaucasiaandsouthoftheCaspianSeatherebyprovidingthebirthplaceofhexaploidwheat.However,theSouthCaspianbeingthebirthplaceofhexaploidwheatdoesnotagreewiththearchaeologicalrecord(NesbittandSamuel),becausetheearliestrecordsofhexaploidwheatdateto8,800to8,400BPidentiedfromseveralareasincludingCanHassanIIIinsouthernTurkeyandAbuHureyrainSyria(Hillman;Mooreetal.;deMoulins;Fairbairnetal.;Fig.).Inlinewiththis,GilesandBrown()reportedndingancientAe.tauschiipopulationsinSyriaandTurkey,andobtainedresultssuggestingthatthersthybridizationeventbetweenT.turgidumAe.tauschiithatgaverisetohexaploidwheatcouldhaveoccurredinsoutheasternTurkeyornorthernSyria,withintheFertileCrescentneartherstarchaeologicalndings.Therefore,whilemoststudiesagreethathexaploidwheatisofpolyphyleticorigininvolvingmorethatoneAe.tauschii,theexactsiteoftheoriginofhexaploidwheatisyetuncertain.T.turgidumparentinvolvedintheformationofhexaploidwheatisalsoyetamatterofdebate.Itisgenerallyacceptedthatssp.wasnottheAB-genomedonorbecause,ifitwere,theresultinghexaploidwouldhaveabrittlerachis,andthereforelittlechanceofbeingselectedbyfarmers(KimberandSearsThemostprobabletetraploidprogenitorstohexaploidwheataredomesticatedemmer)oranextinctfree-threshingsubspeciessuchasssp.whichappearaboutthesametimeinthearchaeologicalrecord.ItisinterestingtonotethatKerber()extractedtheABgenomecomponentsfromhexaploidwheat,andtheresultingAB-tetraploidswereverysimilarinspikemorphologytossp..However,somegeneticevidencebasedongenesforwaxinesspointstodomesticatedemmerastheABprogenitor(Tsunewaki),whileotherresearchbasedonmeioticrestitutionsuggestsfree-threshingdurumwheat(T.turgidum)couldhavebeeninvolved(MatsuokaandNasudaRegardlessofthesubspeciesinvolved,itiscertainthatT.turgidumwasthedonoroftheABgenomesandAe.tauschiidonatedtheDgenometohexaploidwheat(McFaddenandSears).Thersthexaploidhadhulledseedduetothe(tenaciousglume;seebelow)geneacquiredfromAe.tauschii,andthereforewouldhavebeenverysimilartoT.aestivum(McFaddenandSears;KerberandRowland).Today,twoformsofT.aestivumexistandareclas-siedasEuropeanandAsianspelta.EuropeanspeltarstappearedinEuropeintheEarlyBronzeAge(4200–3500BP)neartheSwisslakedistrictandelsewhere(Nes-)severalthousandyearsaftertheappearanceoffree-threshinghexaploidwheat.ItwasoncethoughtthatEuropeanspeltacouldhavebeentheprogenitortofree-threshinghexaploidwheat,butitisnowknownthatitarosemorerecentlyasaresultofhybridizationbetweenT.aestivummer(T.turgidum)(Bertsch;MacKey;Blatteretal.;Yanetal. J.D.FarisKuckuck()foundaspelta-likeformofwheatgrowinginnorthIran,andotherpopulationsofthisAsianspeltaarenowknowntogrowinAfghanistan,Tad-shikistan,Armenia,andotherareasoftheMiddleEastregion.GeneticstudieshaveindicatedthatAsianandEuropeanspeltahaveseparateorigins(MacKey;LiuandTsunewaki;Luoetal.),leavingopenthepossibilitythatAsianspeltacouldbetheprimitiveformofcommonwheat.However,thereisnoreliablearchae-ologicalevidencefortheexistenceofspeltaintheregionswherehexaploidwheatisthoughttohaveoriginated,anditisfairlycertainthatcultivationoffree-threshingcommonwheatprecededthecultivationofhulledhexaploidwheatmakingitunlikelythatAsianspeltaisancestraltocommonwheatandmorelikelythatit,likeEuropeanspelta,isasecondaryderivativeofcommonwheat.Neithergeneticnorarchaeologicalevidenceprovideanswersastowhenandwhereeithertypeofspeltaoriginated,butitisclearthatthersthexaploidmusthavebeenspelta-like(McFaddenandSears;KerberandRowland).Thearchaeologicalrecordwouldsuggestthatneitherisancestraltocommonwheat,andgeneticanalysisfurtherrulesoutEuropeanspeltaasaprimitiveform.Therefore,thersthulledhexaploidmayhavebeenshort-livedandnowextinct.Thiswouldsuggestthatthetransitionfromhulledtofree-threshingwheatoccurredveryrapidly,andwasprobablytheresultofamutationof(seebelow).Theearliestndingsoffree-threshingssp.arefromCanHassanIIIabout8,500BP,whichagreesfairlywellwiththegeneticevidencefortheoriginofhexaploidwheatoccurringabout8,000BP(Huangetal.).LaterndswereunearthedinwesternIran,northernIraq,andwesternAnatoliafollowedbyndsintheMediterraneanbasinandMesopotamia.Free-threshingcommonwheatthenspreadfromtheseareasabout6,000BPtotheNileBasin,centralandwesternEurope,andAsia.18.6GeneticsofDomesticationLociTransitionsinthreemajorgenesduringwheatevolutionultimatelyyieldedfree-threshingfullydomesticatedbreadwheat.Thosethreemajorgenesare,and,which,intheirprimitiveform,conferabrittlerachis,tenaciousglume,andthenonfree-threshingcharacter,respectively.Mutationsinthelocionchromosomes3Aand3B,on2D,andon5Ahadthemostprofoundimpactsondomesticationcharacters(Fig.),buthomoeologouscopiesofeachofthesegenes,andinsomecasesalternategeneticloci,havealsobeenshowntogovernand/orinuencedomesticationtraits.Therefore,arelativelydetailedsynopsisofourunderstandingofthesethreemajorgenes,theirhomoeologs,andotherrelevantlociisprovidedbelow. 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuture18.6.1BrittleRachisAmechanismofnaturalseeddispersalisahallmarktraitofawildplantspeciesbecauseitisessentialtoensurethespreadandpropagationofthespecies.Asessentialasitistowildplants,itisasdetrimentaltocultivatedplantspeciesbecausethefruitsorseedsfalltothegroundatmaturityandarelost.Therefore,thelossoftheabilitytonaturallydisperseseed,i.e.thechangefromabrittlerachistoanon-brittlerachis,wasoneoftherstandmostessentialdomesticationtraitsacquiredbythecultivatedwheatforms.Seeddispersalsystemsdependontheformationofabscissionzonesatparticularsitesthatallowbreakageandsubsequentdispersaloffruitsorseeds.Thetwobasictypesofdisarticulationfoundinwheatarespiketype,wherebreakageoccursatthebaseofthespikeandthewholespikeisdispersedasasingleunit,andspikelet-type,whichisfurtherclassiedintoeitherbarrel-(B)orwedge-shaped(W)disarticulation,dependingonthedisarticulationproducts.AbscissionatthebottomofthespikeletbaseleavinganadjacentrachisfragmentattachedbehindthespikeletisconsideredB-typedisarticulation.W-typedisarticulationiswhenabscissionoccurssuchthatarachisfragmentisleftattachedbeloweachspikelet.Wildeinkornwheat(T.monococcumaegilopoides)undergoesW-typedis-articulationwhereasdomesticatedeinkorn,T.monococcumanon-brittlerachis.SharmaandWaines()showedthatnon-brittlenessinssp.wascontrolledbytwocomplementaryrecessivegenes.Tomyknowl-edge,thechromosomallocationsofthesegeneshavenotbeendeterminedandthereforetheirrelationshipswiththeothermorecharacterizedbrittlerachisgenesinwheat(seebelow)areunknown.Whiletherachisofssp.isnotasfragileasthatofssp.aegilopoides,itisnotverytough,andmoderatepressurecausesbreakageoftherachisleadingtospikeletsegmentsresemblingthoseofwildeinkorn.Consequently,thethreshingofbothformsleadstohulledgrainsintheformofspikeletswithW-typedisarticulation.Therefore,thedomesticationofeinkornwheatmaybeconsideredincomplete,orpartial,becauseitinvolvedonlyoneorafewoftheseveralimportantstepsneededtowardcompletedomestication.Similartransitionsoccurredinthedomesticationofcultivatedemmer(T.turgidum)fromwildemmer(T.turgidum).Studiesusingge-neticstockswhereindividualpairsofwildemmerchromosomesweresubstitutedforhomologouspairsofdurumchromosomeshaveshownthatwildemmerchro-mosomes3Aand3Bharborgenesconferringthebrittlerachistrait(WatanabeandIkebata).MolecularmappinganalysisusingrecombinantinbredchromosomelinesderivedfromthesamestocksevaluatedbyWatanabeandIkebata()indi-catedthatthegenesarelocatedontheshortarmsofchromosomes3A()and3B()andthattheyarelikelyhomoeologous(Nalametal.;TableBothgenesleadtoW-typedisarticulation(Fig.).Thisandotherstudieshavealsoindicatedthatthelociintetraploidwheatarehomoeologouswiththeloci,whichconferabrittlerachisinbarley(Nalametal.;LiandGillThesestudiesindicatethatwasderivedfromT.urartufromtheB-genomedonor,andthatmutationsatbothlociwereneededtoconferthenon-brittle J.D.Farisrachisofdomesticatedemmer.However,likedomesticatedeinkornwheat,domesti-catedemmerdoesnothaveaverytoughrachisanddisarticulationissimilartowildemmerwhensufcientpressureisapplied.Therefore,threshingleadstohulledseedintheformofspikeletsforbothwildanddomesticatedemmer(Fig.Atleasttwostudiesregardingthegeneticsandmappingofrachisbrittlenessinwildemmerhavereportedageneonthelongarmofchromosome2A(Pengetal.;Pelegetal.;Table).Whileneitherstudyreportedthetypeofdisarticulationconferredbythe2Alocus,thiswouldsuggestthatthegeneticsysteminvolvedincontrollingspikeletdisarticulationisundercomplexregulationorperhapsthe2Alocusrepresentsanindependentgeneticpathway.Themutationsinthewildemmerlocihappenedbeforetheamphiploidizationeventthatgaverisetohexaploidwheat.Therefore,withtheexceptionofT.aestivummacha(whichwaslikelyformedsecondarily)noneofthehexaploidsubspecieshavetheprimitiveallelesandthereforedonothavebrittlerachisesconferredbythesegenes.Ahexaploidsemi-wildwheatlandracewasfoundinTibetandreportedtohaveafragilerachisandW-typedisarticulation(Caoetal.Itwaslaterdeterminedthatrachisbrittlenessinthislinewasduetoa)ontheshortarmof3D,whichwaslikelyderivedfromAe.tauschiihomoeologousto(Chenetal.;Table).However,Ae.tauschiihasB-typedisarticulationandsoitwassuggestedthatlikelyconfersB-typedisarticulationinAe.tauschiibutW-typeinahexaploidbackground.FurtherworkbyLiandGill()indicatedthatB-typedisarticulationinAe.tauschiiconferredbyagene()onthelongarmofchromosome3D.TheyconcludedthatB-andW-typedisarticulationsarecontrolledbydifferentgeneswith,andcontrollingW-typeandcontrollingB-type.Ineithercase,gene(s)presentintheAe.tauschiiprogenitormusthaveundergonemutationverysoonafterthehybridizationeventthatgaverisetohexaploidwheat,oriftheAe.tauschiiprogenitorcarriedonly,itseffectsaregreatlydiminishedinahexaploidbackground.18.6.2TenaciousGlumeTheprimitivenon-domesticatedwheatformshadtoughglumesthattightlyenvelopedtheseedinordertoprotectitduringnaturalseeddispersal.Domesticatedeinkornwheat,theonlycultivateddiploidwheat,isnotfree-threshingbecauseithastoughadherentglumesthatdonotallowtheseedtobeeasilyseparatedfromthespikelets.Investigationofaspontaneousfree-threshingmutantofdomesticatedeinkornwheat,referredtoasT.sinskajae,indicatedthatasinglerecessivegenedesignatedsogcontrolledthesoftglumetraitandmappedtotheshortarmofchromosome2A(Taenzleretal.;Soodetal.Theglumesofwildanddomesticatedemmeraretough,holdthekernelstightly,andprohibitthefree-threshingtrait(Fig.).Simonettietal.()evaluatedatetraploidmappingpopulationderivedfromacrossbetweenT.turgidum 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureTable18.1Thethreeprincipaltraitsaffectedbymutationleadingtowheatdomesticationandtheirassociatedgeneticloci TraitGeneChrom.armSpeciesthatacquiredmutationtodomesticform(ploidy) BrittlerachisT.turgidumWatanabeandIkebata(Nalametal.(LiundGill(T.turgidumWatanabeandIkebata(Nalametal.(LiundGill(T.aestivumChenetal.(T.aestivumLiundGill(T.turgidumPengetal.(TenaciousglumeT.aestivumJantasuriyaratetal.Nalametal.(Soodetal.(T.turgidumSimonettietal.T.turgidumT.aestivumFarisetal.(Simonsetal.( Thissubspeciesisnotknownforcertainandcouldhavebeen,orperhapsanothertetraploidsubspeciesforquantitativetraitloci(QTLs)associatedwiththefree-threshingtrait.OneQTLcorrespondedtothefree-threshinglocusonthelongarmofchromosome5A(seebelow)andanotherwithmajoreffectsmappedtotheshortarmofchromosome2B.ThelatterQTLwaslocatedinapositionapparentlysyntenicwiththetoughglumegeneonchromosome2D(seebelow).Itispossiblethatthegeneunderlyingthe2BQTL()and(Table)arehomoeologous,orthatishomoeologuswithSogon2Aineinkornwheat,butappropriatecomparativemappingexperimentstoaddressthesemattershavenotbeenconducted.Anothermattertobeaddressediswhetherornotdomesticatedemmer(turgidum)possesses,orifthetoughglume,non-free-threshingtraitofdomesticatedemmerisduetothefactitcarriesthealleleon5A.ThendingthatT.turgidumvar.—avarietyofdomesticatedemmerwithadensespike—carriesthealleleon5Ayetisnon-free-threshingduetotoughadherentglumes(Muramatsu;Simonsetal.)wouldsuggestthatdomesticatedemmerprobablycarriestheallele.Nogenehasbeen J.D.FarisdescribedontheAgenomeofpolyploidwheatorfromtheA-genomeprogenitorurartuKerberandDyck()rstdescribedthetenaciousglumetraitinwheatandattributedthecharactertoanincompletelydominantgene.Earlycytogeneticworkplacedontheshortarmofchromosome2D(KerberandRowlandandmorerecentmolecularmappingexperimentshavevalidatedthepositionofonchromosomearm2DS(Jantasuriyaratetal.;Nalametal.;Soodetal.;Table).Soodetal.()demonstratedthatsogarenothomoeologoussuggestingthatSogarosefromindependentmutationsatnon-orthologousloci.Therefore,itispossiblethatishomoeologouswithSogon2Afromeinkornwheator,butnotboth.AdditionalanalysisbyNalametal.()suggestedthattwocloselylinked,possiblyparalogous,lociexiston2DS.FurtherworkisnecessarytovalidatethisworkandtoclarifythegeneticrelationshipsamongtheTg/sog18.6.3TheQLociInadditiontotheloci,thelocusonwheatchromosome5Aalsocontrolsthefree-threshingcharacter.Amutationintheprimitivealleleledtotheformationofthepartiallydominantallele,whichresultsinfree-threshingseed(Fig.Tableisepistatictobecauseplantsthathavearenotfree-threshing(KerberandRowland),butboththeallelesarenecessarytoconferthefree-threshingtrait(Fig.).Thelocushasbeenanintriguingsubjectofstudyforthepast100yearsduetothefactthatitaffectsarepertoireoftraits.Plantsthathaveallelesbutlacktheallele(possessallele)arenon-freethreshing,haveasemi-brittlerachis,aspikethatislaxandprimitiveinappearance(speltoid),somewhattenaciousglumesthatadheretotheseed,andaretaller,owerearlier,anddifferinyieldcomparedtoplantsthatharborallele(Watkins;Mackey;Sears;Muramatsu;Singh;Katoetal.;FarisandGill;Farisetal.;Simonsetal.;Zhangetal.).Therefore,affectsnumerousdomestication-relatedandagronomicallyimportanttraits.Ofthemajordomesticationgenesinwheat,istheonlyonethathasbeenclonedsofar(Farisetal.;Simonsetal.isamemberoftheAP2familyoftranscriptionfactors.RelatedmembersincludeAPETALA2,whichcontrolsowerandseeddevelopmentinArabidopsis(Jofukuetal.)andspikelet1),whichgovernsspikeletmeristemfateinmaize(Chucketal.Homologsalsoexistintheothergrassessuchasrice,barley,Brachypodium,andsorghum(Simonsetal.;Farisetal.),butfunctionshavenotbeenascribedtothegeneinthesespecies.Sofar,wheatistheonlyplantspeciesknownforwhich-likegenehasbeenrecruitedfordomestication.DNAsequenceanalysisofallelesfromvariouswheatspeciesofdifferentploidylevelsvalidatedthenotionthatisthemoreprimitivealleleand 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureformedonceastheresultofamutation(Simonsetal.).ThisworkalsodemonstratedthattheA-genomediploidsT.monococcumurartuaswellastheABtetraploidsT.turgidum,andhexaploidT.aestivummacha(European)allpossesstheprimitivealleles.Thefree-threshingwheatsincludingthetetraploidsT.turgidum,and,thefree-threshinghexaploidT.aestivum,andalsothenonfree-threshingT.aestivum(Asian)shownbyLuoetal.()tohave,wereallveriedtohavetheallele.ThelatterndingfurthersupportsthenotionthatAsianspeltahasanorigindifferentfromthatofEuropeanspeltaandthatitprobablypossesseseither,whichwouldmasktheeffectsofthefree-threshingComparativesequenceanalysisrevealedonlyoneconservedstructuraldifferenceallelesattheproteinlevel:allallelesharboredavalineataminoacidposition329whereasallalleleshadanisoleucine(Simonsetal.).TheVtoImutationwasfoundtoleadtoanabundanceofhomodimerformationbytheprotein.Transcriptionlevelsofwerealsofoundtobemorethantwicethelevelof,anditishypothesizedthatproteinhomodimerizationmaybeamechanismofself-regulation.Indeed,increasedtranscriptionlevelsofanditseffectsonthedomestication-relatedphenotypeswereclearlydemonstratedintransgenicplants(Simonsetal.),whichconrmedthereportsofMuramatsu)regardingthedosageeffectsofusingcytogeneticstocks.AnotherpotentialmechanismofregulationisamicroRNA172bindingsiteinexon10,whichcouldmimictheregulationofAPETALA2Arabidopsisattheleveloftranslation.Clearlymoreworkisneededtounderstandthemechanismsresponsibleforgeneregulation,anditsinteractionswithothergenes.Forexample,Jantasuriyaratetal.()evaluatedapopulationofrecombinantinbredlinesderivedfromacrossbetweenahexaploidwheatvarietyandasynthetichexaploidwheatline,whichwascreatedbycrossingT.turgidumAe.tauschiifollowedbychro-mosomedoublingtherebyessentiallyrepeatingthehybridizationeventthatledtoformationofhexaploidwheat(apracticethatisroutinelydonefortheexploitationofdesirabletraitsfromtheprogenitorspecies).QTLanalysisofthethreshabilitytraitrevealedalocuswithstrongeffectsontheshortarmofchromosome2Dinthevicinityof,aswasexpectedbecausethesyntheticparentisnonfree-threshingduetofromtheAe.tauschiiparent.However,aQTLon5Anearthewasalsodetected,whichwasunexpectedduetotheassumptionthatbothparentspossessedtheallele.Theeffectsofthelocusweretherebyattributedtopossibleallelicvariationwithin.Similarworkusingapopulationofdoubledhaploidlinesderivedfromdifferentsyntheticandcultivatedwheatparentsandsub-sequentanalysisofQTLassociatedwiththreshabilityrevealedessentiallythesameresults(FarisJD,ChuCG,FriesenTL,XuSS,unpublished).But,inthiscasewesequencedtheallelesfrombothparentsandfoundnovariationwithinthegenecodingsequences.Thismightsuggestthattheeffectsofthelocusonthreshabilityinsynthetichexaploid-derivedpopulationscouldbeduetovariationingeneex-pression,possiblyinuencedbyorothergenes.Workiscurrentlyunderwaytogainfurtherunderstandingofthisphenomenon. J.D.Farislocionchromosomes5Band5DwerelongthoughttoexistandtheworkofSimonsetal.()proveditso.Zhangetal.()conductedstudiestobetterunderstandtheevolution,organization,andfunctionofhomoeologousallelesandtheirrelationshipswithalleles.TheiranalysisrevealedthatgenesequenceswerehighlyconservedamongA,B,andDgenomesinhexaploidwheat,theAandBgenomesintetraploidwheat,andtheA,S,andDgenomesinthediploidprogenitors,butaduplicationofthegenepriortoradiationofthediploidprogenitorssome5.8millionBPwasfollowedbytheselectivelossofonecopyfromtheAgenomeprogenitorandtheothercopyfromtheB,D,andSgenomes.Ae.tauschiiaswellashexaploidwheatpossessintactandfunctionalalleles,andfunctionalandphenotypicanalysisindicatedthatcontributestothesuppressionofthespeltoidsyndromejustas,buttoalesserdegree.Ae.speltoidesanintactandfunctionalallele,butbecameapseudogeneuponformationoftheABtetraploid.However,stillcontributestodomestication-relatedtraitsthroughmechanismsofhomoeoalleleco-regulationinarathercomplexmanner.Therefore,whilethemutationthatgaverisetowasamajorfactorindomesticatingwheat,thecontributionsofthroughpolyploidizationarealsorecognizedasrelevantcontributors.18.6.4TheEvolutionofFree-threshingWheatsAsetofratherprofoundcircumstancesoccurredintheformationoffree-threshingpolyploidwheat,especiallygiventhattheevolutionarystepsinvolvedmutationsatthreemajorloci(,and)andtwoallopolyploidizationevents.Someoftheeventsandtransitionsarewellunderstood,whileothersarenot,butclearlybothgeneticsandthearchaeologicalrecordindicatethatalltheseeventshappenedduringan“evolutionaryburst”thatoccurredrelativelyquicklyoveraperiodoflessthanafewthousandyears.Thersttransitionwasofcoursetheacquisitionofanon-brittlerachisinT.turgidum.Thisrequiredmutationsinbothinwildemmerleadingtotheformationofdomesticatedemmer(Figs.).Althoughdomesticatedemmerhasanon-brittlerachis,itisnotfree-threshingbecauseitcarriesalleleandmostlikely).Free-threshingtetraploidwheatevolvedfromdomesticatedemmerbywayofthemutationinthatgaverisetoandalsolikelythusformingthefree-threshingtetraploid,whichisthegenotypeoftoday’smoderndurumwheat.Notonlydidthemutationtoresultinthefree-threshingcharacter,italsoconferredafullytoughrachis.Itismostlikelythatafree-threshingtetraploidwasinvolvedintheamphiploidiza-tioneventthatgaverisetothersthexaploid(Fig.).However,thersthexaploidwasnonfree-threshingduetoacquisitionofthegenefromAe.tauschii.ItalsolikelyacquiredgenesfromAe.tauschiiforbrittlerachis,possibly,orboth,andhadthegenotype.Therefore,mutationsingene(s)obtainedfromAe.tauschiiwerenecessaryinthetransitionofthis 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuture“Asianspelta-like”wheattomodernfree-threshingT.aestivum.IftheABgenomedonorweredomesticatedemmer,thersthexaploidwouldhavehadtoundergomutationsinandprobablyinadditiontoandpossiblytogiverisetoT.aestivum.Underthisunlikelyscenario,thefreethreshingtetraploidswouldhaveevolvedlaterthanfree-threshinghexaploidsthroughtheacquisitionofT.aestivumThearchaeologicalrecorddoesnotshedmuchlightontheunknownsofthesescenarios.Free-threshingtetraploidsandhexaploidsappearatthesametime,shortlyaftertheappearanceofdomesticatedemmer,sotheexactABtetraploidprogenitorandtheanswertowhetherrstaroseinthetetraploidsorthehexaploidsisyetun-known.Also,noprimitivenonfree-threshinghexaploidsprecedethefree-threshinghexaploidsinthearchaeologicalrecord.GeneticstudieshavediscoveredtheoriginofEuropeanspelta,andindicatethatitisaderivativeratherthanaprogenitorof,butnogeneticevidenceallowsconclusionsonwhetherornotAsianspeltaisanancientwheatortheresultofarecenthybridization.IfthehybridizationeventthatgaverisetohexaploidwheatoccurredwhenAe.tauschiicameintocontactwithaeldoffree-threshingtetraploidwheat,theresultwouldhavebeenahulledhexaploidgrowinginaeldoffree-threshingtetraploidwheat.AsNesbitt()pointsout,earlyfarmerswouldhaveprocessedhulledwheatsdifferentfromfree-threshingwheats,andasaresult,hulledwheatgrowninafree-threshingeldwouldhavebeenunderstrongselectionpressuretobecomefree-threshingitself.Thus,thetransitionwouldhaveoccurredrapidly,whichwouldexplainlargeabsenceofspeltwheatfromtheMiddleEast.IfdomesticatedemmerhybridizedwithAe.tauschiitoformthersthexaploid,mutationswouldhavehadtooccurat,twoloci,andpossiblytwolociatthesametimetoexplaintheabsenceofspeltfromthearchaeologicalrecord.And,underthisunlikelyscenario,afree-threshingtetraploidwouldnothaveyetexistedbecauseweknowthatthemutationformingoccurredonlyonce,indicatingthat(andpossiblywouldhavebeenacquiredbyanAB-tetraploidviageneowfromthehexaploid.Thisgene-oweventwouldhavehadtooccurveryrapidlytoexplaintheappearanceoffree-threshingtetraploidsandhexaploidsatthesametime.Whateverthescenario,thetransitionsnecessaryforfree-threshingtetraploidandhexaploidwheattoevolvemusthaveoccurredveryrapidly.Cloningandfurthergeneticanalysisoflocialongwithgenesshouldprovidemoredenitiveanswerstotheseunknownsinthefuture.18.7WheatEvolutionUnderCultivation18.7.1CaptureofGeneticVariabilityDomesticatedemmerwheatarosethroughveryfewmutationsintheirwildpro-genitorsubspecies,makingtherstdomesticatedformgeneticallyverysimilartothecorrespondingwildform.Eventhoughdomesticatedemmerisbyandlargea J.D.Farisself-pollinatedspecies,hybridsbetweenwildanddomesticatedtetraploidsarefullyfertileandtheirchromosomesreadilypairandrecombine.Earlyfarmersgrewdo-mesticatedemmerinmixtureswithwildemmerforalongperiodoftimeallowingampleopportunityforgeneowtooccurleadingtoincreasedgeneticvariability,formationofsub-populations,andreductionofthefoundereffect(Luoetal.Geneowhascontinuedtooccurevenafterdomesticatedemmerlargelyreplacedwildemmerandothertetraploidwheatshavecometoexistsuchastoday’seconom-icallyimportantdurumwheat.Manyofthesubspeciescomeintocontactwitheachotherattheedgesofeldsofcultivateddurumorevenhexaploidwheatandformhybridswarmsresultingingeneowfromwildtocultivatedformsandviceversa(Dvoraketal.;Syoufetal.ThefoundereffectforhexaploidwheatismuchlargercomparedtoemmerwheatbecauseveryfewhybridizationeventsbetweenT.turgidumAe.tauschiiresultinginalargermagnitudeofgeneticdrift.Thisandthefactthathexaploidwasrelativelyisolatedgeneticallybecauseithybridizeslessfrequentlywithitsprogen-itorsresultedinamuchnarrowergeneticbaseandreducedvariabilitycomparedtodomesticatedtetraploidwheats.However,thelevelofdiversityislessrestrictedintheAandBgenomescomparedtotheDgenome,andinsupportofthis,studieshaveshownthathybridswarmsinvolvingwildemmerandcommonwheatexistandthatgeneowfromtheformertothelatterhasoccurred(Dvoraketal.).Inlinewiththis,thereisstrongevidencethatthehexaploidT.aestivummacha(European),whichhaveprimitivetraits,originatedfrominterspeciccrossesinvolvinghexaploidsandtetraploids(Bertsch;MacKey;DvorakandLuo;Blatteretal.;Yanetal.),andthetetraploidT.turgidumprobablyarosethroughgeneowinvolvinghexaploidwheat(Kuckuck).Therefore,thesenewsubspeciesmayhaveresultedfromhybridswarmsas18.7.2FurtherDomesticationUnderCultivationIncreasedseedsizewasalsoanimportantdomesticationtrait,andwasselectedforveryearlyintherstyearsofwheatcultivation,probablybeforetheacquisitionofanon-brittlerachis(Fuller).SeedsizeisunderpolygeniccontrolandgenesandQTLsgoverningthistraithavebeenlocatedonmanywheatchromosomes(Pengetal.).Followingthetransitiontoanon-brittlerachis,othertraitsacquiredbynewlydomesticatedemmerwheatincludednon-dormantseedswithuniformgermination,yield,andprobablymoreerectplants(Feldman).Aswithseedsize,manyofthesetraitsareunderpolygeniccontrolandmultipleQTLsassociatedwiththesetraitshavebeenreported(Pengetal.).ThespreadofwheatcultivationthroughoutEurope,Africa,andAsiaalsorequiredwideadaptationtothedifferentenvironments,whichincludedalterationsinoweringtimeandgrowthhabit.AccordingtoFeldman(),asecondphaseofevolutionundercultivationinvolvedalengthyperiodoftimewherecontinuousselectionoccurredforvarious 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuturetraitsineldsconsistingofmixturesofdifferentgenotypesandlikelyevendifferentspeciesofdifferentploidylevels.Thesemixturesofdifferentlandracesprovidedsomeprotectionagainstdiseaseepidemicsandprobablysomeenvironmentalhazardssuchasdrought,extremeheat,andoodingaswell.Therefore,thecultivationofthesemixturesprovidedsomelevelofcropsecurityandensuredyieldstability.Thesemixturesalsoprovidedampleopportunityforinterspecichybridizationsandgeneow,thusincreasinggeneticvariability.However,theyalsocreatedacompetitiveenvironmentwhereplantsthathadmoretillersandweretallerwithhorizontalleaveswouldshadethecompetitorsandthushaveaselectiveadvantage.Thedawnofmodernbreedingpracticesbeganaboutacenturyago.Sincethattime,individualgenotypeshavebeentheunitofselectionratherthanmixtures,whichhasledtowheateldsconsistingofsinglegenotypeswithnoopportunityforinterspecicgeneticexchangetoincreasevariability.However,plantbreedingpracticesinthemid-20thcenturyallowedthedevelopmentoftherstuniformhigh-yieldingcultivarsthroughtheintrogressionofeliteagronomiccharacters.Aprofoundexampleofthiswasthereplacementoftallvarietieswithsemi-dwarfanddwarfvarietiesthoughtheintroductionofthegenes.Thisoccurredduringthe“greenrevolution”undertheleadershipofNormanE.BorlaugattheInternationalMaizeandWheatImprovementCentre(CIMMYT)inMexico.Breedingpracticestodayallowextensiveandintricatemanipulationsofthewheatplanttobemadewithcertaintyaswellasratherprecisemonitoringofintrogressionsandgeneticexchangesresultingfromarticialhybridizationsthroughtheuseofmoderngenomicstoolsand18.7.3GenerationofNewGeneticDiversityAlthoughhexaploidwheatcapturedagoodamountofgeneticdiversityfortheAandBgenomes,thegeneticbottleneckscreatedbydomesticationresultedintheincreasedfrequencyofmanyadaptedallelesbutalsothelossofotherpotentiallyusefulones.Thislimitedamountofdiversityreducesthepotentialforwheattofur-theradapttochangingenvironments.However,genomicanalysesofwheatanditsrelativesoverthepastdecadehasrevealedthewheatgenomesinapolyploidstatearedynamicandundergothegenerationofnewvariationintheformofdeletions,insertions,andpointmutationscreatedbyrepetitiveelementsaffectinggenesandregulatoryelements(DubcovskyandDvorak).Suchalterationsarewelltoler-atedinwheatduetoitspolyploidbufferingcapacity.Thesealterationsingenecodingregionscreateintergenicpolymorphisms,whichfurtherincreasesgeneticvariabilityamongthegenomesandmayleadtosubtlechangesinexpressionduetogenedosagedifferences.Alternatively,homoeologoussetsofgenesinapolyploidstatearegiventheopportunitytobecomealtered,forexample,throughsub-functionalization(be-cominglimitedinfunction)orneo-functionalization(acquiringanewfunction).Agoodexampleofthisisanditshomoeoallelesbecauseunderpolyploidization,becamehyper-functional,underwentpseudogenization,and J.D.Farissub-functionalized(Zhangetal.).Therefore,tosomeextent,polyploidwheatisabletocreatealevelofitsowndiversityduetotheplasticityofitsgenomesandcooperationbetweengeneticmutationscreatedbyrepetitiveelementsandtolerancethroughpolyploidbuffering.Nevertheless,weneedtomakeconcentratedeffortstopreservealargereservoirofwildrelativesandlandracesthroughgermplasmcollec-tionsandgenebankssothatwehavetheabilitytointroduceadditionalvariabilitywhenneeded.18.8FutureNeedsToday’swheatvarietiesarefarsuperiorinyield,quality,bioticandabioticresistance,andoverallagronomicperformancetheneventhosethatwereconsideredelitejustafewdecadesago.However,moreprofoundadvancesintheseandothertraitsmustbemadeinthenearfutureifwearetofeedtheworld’sgrowingpopulation.Signicantadvancesinourunderstandingofthebiologyofthewheatplantmustbeachieved,andthiscanbeinitiatedthroughthecrackingofthegeneticcodeofwheatandcharacterizingthestructureandfunctionofcriticalgenes(seechapterbyC.Feuillet).Obtainingthegenomesequenceofpolyploidwheatanditsrelativeswillalsoallowscientiststoconductstudiesthatyieldimportantinformationthatmayhelpanswerunknownsregardingwheatevolutionanddomestication.Thisinturnwillhelpwheatscientiststobettercopewithglobalclimatechangebyallowingtherecoveryandexploitationofvaluableallelesthatexistinkeywheatrelatives.Underacontinuouslychangingclimate,theworldmaywellseeaneedforanotheragriculturalrevolutionandneo-domesticationofwheatinordertomeetfuturedemands.ReferencesAaronsohnA(1910)AgriculturalandbotanicalexplorationsinPalestine.BullPlantIndustry,USDeptAgriculture,Washington,DCNo.180:1–63BertschF(1943)DerDinkel.LandwJahrbuch92:241–252BlakeNK,LehfeldtBR,LavinM,TalbertLE(1999)PhylogeneticreconstructionbasedonlowcopyDNAsequencedatainanalloploid:theBgenomeofwheat.Genome42:351–360BlatterRHE,JacometS,SchlumbaumA(2002)Spelt-specicallelesinHMWgluteningenesfrommodernandhistoricalEuropeanspelt(TriticumspeltaL.).TheorApplGenet104:329–337BlatterRHE,JacometS,SchlumbaumA(2004)AbouttheoriginofEuropeanspelt(TriticumspeltaL.):allelicdifferentiationoftheHMWgluteninB1-1andA1-2subunitgenes.TheorApplGenetCaoW,ScolesGJ,HuclP(1997)Thegeneticsofrachisfragilityandglumetenacityinsemi-wildwheat.Euphytica94:119–124ChalupskaD,LeeHY,FarisJDetal(2008)homoeolociandtheevolutionofthewheatgenomes.ProcNatlAcadSciUSA105:9691–9696ChenQ-F,YenC,YangJ-L(1998)ChromosomelocationofthegeneforbrittlerachisintheTibetanweedraceofcommonwheat.GenetResCropEvol45:21–25ChuckG,MeeleyRB,HakeS(1998)ThecontrolofmaizespikeletmeristemfatebytheAPETALA2likegeneindeterminantspikelet1.GenesDev12:1145–1154 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFuturedeMoulinsD(2000)AbuHereyra2:plantremainsfromtheNeolithic.In:MooreAMT,HillmanGC,LeggeAJ(eds)VillageontheEuphrates.OxfordUniversityPress,Oxford,pp399–422DixonJ,BraunHJ,KosinaPP,CrouchJ(2009)Wheatfactsandfutures.CIMMMYT,MexicoDubcovskyJ,DvorakJ(2007)Genomeplasticityakeyfactorinthesuccessofpolyploidwheatunderdomestication.Science316:1862–1866DvorakJ,ZhangHB(1990)VariationinrepeatednucleotidesequencesshedslightonthephylogenyofthewheatBandGgenomes.ProcNatlAcadSciUSA87:9640–9644DvorakJ,diTerlizziP,ZhangH-B,RestaP(1993)Theevolutionofpolyploidwheats:identicationoftheAgenomedonorspecies.Genome36:21–31DvorakJ,LuoMC(2001)Evolutionoffree-threshingandhulledformsofTriticumaestivum:oldproblemsandnewtools.In:CaligariPDS,BrandhamPE(eds)Wheattaxonomy:thelegacyofJohnPercival.LinneanSociety,London,pp127–136(LinneanSpecialIssue3)DvorakJ,LuoMC,YangZL,ZhangHB(1998)ThestructureoftheAegilopstauschiiandtheevolutionofhexaploidwheat.TheorApplGenet97:657–670DvorakJ,AkhunovED,AkhunovARetal(2006)MolecularcharacterizationofadiagnosticDNAmarkerfordomesticatedtetraploidwheatprovidesevidenceforgeneowfromwildtetraploidwheattohexaploidwheat.MolBiolEvol23:1386–1396FairbairnA,AsoutiE,NearJ,MartinoliD(2002)Macro-botanicalevidenceforplantuseatNeolithicCatalhoyuk,south-centralAnatolia,Turkey.VegHistArchaeobot11:41–54FarisJD,GillBS(2002)Genomictargetingandhigh-resolutionmappingofthedomesticationgeneinwheat.Genome45:706–718FarisJD,FellersJP,BrooksSA,GillBS(2003)Abacterialarticialchromosomecontigspanningthemajordomesticationlocusinwheatandidenticationofacandidategene.GeneticsFarisJD,SimonsKJ,ZhangZ,GillBS(2005)Thewheatsuperdomesticationgene.WheatInfoServ100:129–148FarisJD,ZhangZ,FellersJP,GillBS(2008)Micro-colinearitybetweenrice,Brachypodium,andTriticummonococcumatthewheatdomesticationlocus.FunctIntegrGenomics8:149–164FeldmanM(2001)Originofcultivatedwheat.In:BonjeanAP,AngusWJ(eds)Theworldwheatbook.Ahistoryofwheatbreeding.LavoisierPublishing,Paris,pp3–56FullerDQ(2007)Contrastingpatternsincropdomesticationanddomesticationrates:recentarchaeobotanicalinsightsfromtheOldWorld.AnnBot100:903–924GilesRG,BrownTA(2006)allelevariationsinAegilopstasuchiiTriticumaestivumimplicationsfortheoriginsofhexaploidwheats.TheorApplGenet112:1563–1572HarlanJR,WetMJde,PriceEG(1973)Comparativeevolutionofcereals.Evolution27:3110–325HeunM,Schaefer-PreglR,KlawanDetal(1997)SiteofeinkornwheatdomesticationidentiedbyDNAngerprinting.Science278:1312–1314HillmanGC(1978)Ontheoriginsofdomesticrye—Secalecereal:thendsfromAceramicCanHasanIIIinTurkey.AnatolianStudies28:157–174HuangS,SirikhachornkitA,SuXetal(2002)Genesencodingplastidacetyl-CoAcarboxylaseand3-phosphoglyceratekinaseoftheTriticum/Aegilopscomplexandtheevolutionaryhistoryofpolyploidwheat.ProcNatlAcadSciUSA99:8133–8138JaaskaV(1978)NADP-dependentaromaticalcoholdehydrogenaseinpolyploidwheatsandtheirrelatives.Ontheoriginandphylogenyofpolyploidwheats.TheorApplGenet53:209–217JaaskaV(1980)Electrophoreticsurveyofseedlingesterasesinwheatsinrelationtotheirphylogeny.TheorApplGenet56:273–284JaaskaV(1981)Aspartateaminotransferaseandalcoholdehydrogenaseisozymes:intraspecicdifferentiationinAegilopstauschiiandtheoriginoftheDgenomepolyploidsinthewheatgroup.PlantSystEvol137:259–273JakubzinerMM(1958)Newwheatspecies.In:JenkinsBC(ed)Proceedingsoftherstinternationalwheatgeneticssymposium.Winnipeg,pp207–220 J.D.FarisJantasuriyaratC,ValesMI,WatsonCJW,Riera-LizarazuO(2004)Identicationandmappingofgeneticlociaffectingfree-threshinghabitandspikecompactnessinwheat(TriticumaestivumL.).TheorApplGenet108:261–273JofukuKD,denBoerBGW,VanMontaguM,OkamuroJK(1994)ControlofArabidopsisowerandseeddevelopmentbythehomeoticgeneAPETALA2.PlantCell6:1211–1225JohnsonBL(1968)ElectrophoreticevidenceontheoriginofTriticumzhukovskyi.In:FinlayKW,ShepherdKW(eds)ProceedingsoftheThirdInternationalWheatGeneticsSymposium.Canberra,Australia,pp105–110JohnsonBL,DhaliwalHS(1976)ReproductiveisolationofTriticumboeoticumTriticumurartuandtheoriginofthetetraploidwheats.AmJBot63:1088–1094KatoK,MiuraH,SawadaS(1999)QTLmappingofgenescontrollingearemergencetimeandplantheightonchromosome5Aofwheat.TheorApplGenet98:472–476KatoK,SonokawaR,MiuraH,SawadaS(2003)Dwarngeffectassociatedwiththethreshabilityonwheatchromosome5A.PlantBreed122:489–492KerberER(1964)Wheat:Reconstitutionofthetetraploidcomponent(AABB)ofhexaploids.Science143:253–255KerberER,DyckPL(1969)InheritanceinhexaploidwheatofleafrustresistanceandothercharactersderivedfromAegilopssquarossa.CanJGenetCytol11:639–647KerberER,RowlandGG(1974)Originofthefreethreshingcharacterinhexaploidwheat.CanJGenetCytol16:145–154KiharaH(1944)DiscoveryoftheDD-analyzer,oneoftheancestorsofTriticumvulgare.AgricultureandHorticulture19:13–14(Tokyo)KilianB,OzkanH,DeuschOetal(2007)IndependentwheatBandGgenomeoriginsinoutcrossingAegilopsprogenitorhaplotypes.MolBiolEvol24:217–227KimberG,SearsER(1987)EvolutioninthegenusTriticumandtheoriginofcultivatedwheat.In:HeyneEG(ed)Wheatandwheatimprovement.2ndedition.AmericanSocietyofAgronomy,Madison,pp154–164KislevME(1980)Triticumparvicoccum,theoldestnakedwheat.IsrJBot28:95–107KislevME(1984)Emergenceofwheatagriculture.Paleorient10:61–70(http://persee.fr/web/revues/home/prescript/article/paleo_0153-9345_1984_num_10_2_940)KuckuckH(1959)NeuereArbeitenzurEntstehungderhexaploidenKulturweizen.Z.PanzenzüchtKuckuckH(1979)OntheoriginofTriticumcarthlicumNeyski(=TriticumpersicumVav.).WheatInfServ50:1–5LelleyT,StachelM,GrausgruberH,VollmannJ(2000)AnalysisofrelationshipsbetweenAe.tauschiiandtheDgenomeofwheatutilizingmicrosatellites.Genome43:661–668LiWL,GillBS(2006)Multiplepathwaysforseedshatteringinthegrasses.FunctIntegrGenomicsLiuY-G,TsunewakiK(1991)Restrictionfragmentlengthpolymorphism(RFLP)analysisinwheat.II.LinkagemapsoftheRFLPsitesincommonwheat.JpnJGenet66:617–633LuoMC,YangZL,DvorakJ(2000)ThelocusofIranianandEuropeanspeltwheat.TheorApplGenet100:602–606LuoMC,YangZL,YouFMetal(2007)Thestructureofwildanddomesticatedemmerwheatpopulations,geneowbetweenthem,andthesiteofemmerdomestication.TheorApplGenetMacKeyJ(1954)NeutronandX-rayexperimentsinwheatandrevisionofthespeltoidproblem.Hereditas40:65–180MacKeyJ(1966)SpeciesrelationshipinTriticum.HereditasSupplement2:237–276MatsuokaY,NasudaS(2004)Durumwheatasacandidatefortheunknownfemaleprogenitorofbreadwheat:anempiricalstudywithahighlyfertileFhybridwithAegilopstauschiiTheorApplGenet109:1710–1717McFaddenES,SearsER(1946)TheoriginofTriticumspeltaanditsfree-threshinghexaploidrelatives.JHered37:81–89,107–116 18WheatDomestication:KeytoAgriculturalRevolutionsPastandFutureMooreAMT,HillmanGC,LeggeAJ(2000)ThesignicanceofAbuHureyra.In:MooreAMT,HillmanGC,LeggeAJ(eds)VillageontheEuphrates.OxfordUniversityPress,Oxford,ppMoriN,IshiT,IshidoTetal(2003)Originsofdomesticatedemmerandcommonwheatin-ferredfromchloroplastDNAngerprinting.In:PognaNE,RomanoM,PognaEA,GalterioG(eds)Proceedingsofthe10thInternationalWheatGeneticsSymposium,Paestum,Italy.IstitutoSperimentaleperlaCerealicoltura,Rome,pp25–28MuramatsuM(1963)Dosageeffectoftheofhexaploidwheat.Genetics48:469–482MuramatsuM(1979)Presenceofthevulgare,inadense-spikevarietyofTriticumdicoccumSchübl.ReportofthePlantGerm-PlasmInstitute,KyotoUniversity,No.4:pp39–41MuramatsuM(1985)SpiketypeintwocultivarsofTriticumdicoccumwiththecomparedwiththe-bearingvariety.JpnJBreed35:255–267MuramatsuM(1986)Thevulgaresupergene,:itsuniversalityindurumwheatanditsphenotypiceffectsintetraploidandhexaploidwheats.CanJGenetCytol28:30–41NakaiY(1979)IsozymevariationinAegilopsTriticum,IV.Theoriginofthecommonwheatsrevealedfromthestudyofesteraseisozymesinsynthesizedhexaploidwheats.JpnJGenetNalamVJ,ValesMI,WatsonCJWetal(2006)Map-basedanalysisofgenesaffectingthebrittlerachischaracterintetraploidwheat(TriticumturgidumL.).TheorApplGenet112:373–381NalamVJ,ValesMI,WatsonCJW,JohnsonEBetal(2007)Map-basedanalysisofgeneticlocionchromosome2Dthataffectglumetenacityandthreshabilitycomponentsoffree-threshinghabitincommonwheat(TriticumaestivumL.).TheorApplGenet116:135–145NesbittM(2001)Wheatevolution:integratingarchaeologicalandbiologicalevidence.In:CaligariPDS,BrandhamPE(eds)Wheattaxonomy:thelegacyofJohnPercival.LinneanSociety,London,pp37–59(LinneanSpecialIssue3)NesbittM,SamuelD(1996)Fromstaplecroptoextinction?Thearchaeologyandhistoryofhulledwheats.In:PadulosiS,HammerK,HellerJ(eds)Hulledwheats,promotingtheconservationanduseofunderutilizedandneglectedcrops4:proceedingsoftherstinternationalworkshoponhulledwheats.CastelvecchioPascoli,Tuscany,pp41–100NishikawaK(1974)Alpha-amylaseisozymesandphylogenyofhexaploidwheat.In:SearsER,SearsEMS(eds)Fourthinternationalwheatgeneticssymposium,vol1.UniversityofMissouri,Columbia,pp851–855NishikawaK,FurutaY,WadaT(1980)Geneticstudiesonalpha-amylaseisozymesinwheat.III.IntraspecicvariationinAegilopssquarrosaandbirthplaceofhexaploidwheat.JpnJGenetOzkanH,BrandoliniA,Schafer-PreglR,SalaminiF(2002)AFLPanalysisofacollectionoftetraploidwheatsindicatestheoriginofemmerandhardwheatdomesticationinSoutheastTurkey.MolBiolEvol19:1797–1801OzkanH,BrandoliniA,PozziCetal(2005)Areconsiderationofthedomesticationgeographyoftetraploidwheats.TheorApplGenet110:1052–1060PelegZ,FahimaT,KorolABetal(2011)Geneticanalysisofwheatdomesticationandevolutionunderdomestication.JExpBot62:5051–5061PengJH,RoninY,FahimaTetal(2003)DomesticationquantitativetraitlociinTriticumdicoccoidestheprogenitorofwheat.ProcNatlAcadSciUSA100:2489–2494PengJH,SunD,NevoE(2011)Domesticationevolution,geneticsandgenomicsinwheat.MolBreeding28:281–301RenfrewJM(1973)Palaeoethnobotany—theprehistoricfoodplantsoftheNearEastandEurope.MethuenandGo.Ltd,London,pp1–248RileyR,UnrauJ,ChapmanV(1958)EvidenceontheoriginoftheBgenomeofwheat.JHeredRodriguezJ,MaestraB,PereraE,DiezMetal(2000)PairingafnitiesoftheB-andG-genomechromosomesofpolyploidwheatswiththoseofAegilopsspeltoides.Genome43:814–819 J.D.FarisSalseJ,ChagueV,BolotSetal(2008)NewinsightsintotheoriginoftheBgenomeofhexaploidwheat:EvolutionaryrelationshipsattheSPAgenomicregionwiththeSgenomeofthediploidrelativeAegilopsspeltoides.BMCGenomics9:555SarkarP,StebbinsGL(1956)MorphologicalevidenceconcerningtheoriginoftheBgenomeinwheat.AmJBot43:297–304SaxK(1922)Sterilityinwheathybrids.II.Chromosomebehaviorinpartiallysterilehybrids.Genetics7:513–552SearsER(1956)Thesystematics,cytologyandgeneticsofwheat.HandbPanzenzücht,2ndEdition,2:164–187SharmaHC,WainesJG(1980)InheritanceoftoughrachisincrossesofTriticummonococcumT.aegilopoides.JHered71:214–216SimonettiMC,BellomoMP,LaghettiGetal(1999)Quantitativetraitlociinuencingfree-threshinghabitintetraploidwheats.GenetResCropEvol46:267–271SimonsKJ,FellersJP,TrickHNetal(2006)Molecularcharacterizationofthemajorwheatdomesticationgene.Genetics172:547–555SinghMP(1969)Someradiationinducedchangesat‘’locusinbreadwheat(TriticumaestivumL.).Caryologia22:119–126SoodS,KuraparthyV,BaiGH,GillBS(2009)Themajorthreshabilitygenessoftglume(sogandtenaciousglume(),ofdiploidandpolyploidwheat,tracetheirorigintoindependentmutationsatnon-orthologousloci.TheorApplGenet119:341–351SyoufM,Abu-IrmailehBE,ValkounJ,BdourS(2006)Introgressionfromdurumwheatlandracesinwildemmerwheat(Triticumdicoccoides(Körn.exAsch.etGraibner)Schweinf).GenetResCropEvol53:1165–1172TaenzlerB,EspostiRF,VaccinoPetal(2002)Molecularlinkagemapofeinkornwheat:mappingofstorage-proteinandsoft-glumegenesandbread-makingqualityQTLs.GenetResCambTannoK,WillcoxG(2006)Howfastwaswildwheatdomesticated?Science311:1886TsunewakiK(1966)Comparativegeneanalysisofcommonwheatanditsancestralspecies.II.Waxiness,growthhabitandawnedness.JpnJBot19:175–229WatanabeN,IkebataN(2000)Theeffectsofhomoeologousgroup3chromosomesongraincolourdependentseeddormancyandbrittlerachisintetraploidwheat.Euphytica115:215–220WatkinsAE(1940)TheinheritanceofglumeshapeinTriticum.JGenet39:249–264YanY,HsamSLK,YuJZetal(2003)HMWandLMWgluteninallelesamongputativetetraploidandhexaploidEuropeanspeltwheat(TriticumspeltaL.)progenitors.TheorApplGenet107:1321–ZederM(2008)DomesticationandearlyagricultureintheMediterraneanBasin:Origin,diffusion,andimpact.ProcNatlAcadSciUSA105:11597–11604ZhangP,FriebeB,GillBS(2002)Variationinthedistributionofagenome-specicDNAsequenceonchromosomesrevealsevolutionaryrelationshipsintheTriticumAegilopscomplex.PlantSystEvol235:169–179ZhangZC,BelcramH,GornickiPetal(2011)Duplicationandpartitioninginevolutionandfunctionofhomoeologouslocigoverningdomesticationcharactersinpolyploidwheat.ProcNatlAcadSciUSA108:18737–18742