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How to Breed Tomatoes for Organic Agriculture www.seedalliance.org�
How to Breed Tomatoesfor Organic Agriculture
Organic Seed AllianceAdvancing the ethical development and stewardship of the genetic resources of agricultural seedPO Box 772, Port Townsend, WA 98368
This publication was made possible through a grant from Organic Farming Research Foundation and Seed Matters
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org2
Section I: Introduction...................................................................................................................................................3 Section II: Introduction to Breeding........................................................................................................................3Section III: Breeding Goals...........................................................................................................................................4 Agronomictraits.......................................................................................................................................................................4 Defensivetraits.........................................................................................................................................................................5 FruitQuality................................................................................................................................................................................5Section IV: Sourcing Germplasm................................................................................................................................7
Section V: Reproductive Biology.................................................................................................................................8
Section VI: Pollination Techniques............................................................................................................................9
Section VII: Breeding Methods.................................................................................................................................11Glossary............................................................................................................................................................................18
References and Resources.........................................................................................................................................19
Table of Contents
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org3
Tomatoes(Solanum lycopersicum)areanexcellentcroptodoplantbreedingworkwithandcanbeusedtoclearlydemonstratethestepsofbreedingwithaself-pollinatedcropspecies.Notonlydotomatoesreadilyself-pollinate,performinghandpollinationsforcontrolledcrossesisrelativelyeasy.Tomatoesarealsocapableofproducingprolificamountsofseedaseachindividualfruitcanyieldseveralhundred.Tomatovarietiesalsospananabundantarrayofcolors,shapes,sizes,andflavors,creatinganexcitingpalettefortheaspiringbreedertoworkwith.
Althoughtomatoesarenotimpressivelyhighinnutrients,theymakeupasignificantamountoftheU.S.diet.WhileNorthAmericansconsumetoma-toesmainlyinprocessedform,mostofteninsaucesorpastes,theconsumptionandpopularityoffreshmarkettomatoesisontherise.Thismaybeduetoimprovedvarietiesandanincreasinginterestinthewiderangeoftomatoesavailable,suchasgrape,specialty,heirloom,andgreenhousetypes.
Moderntomatoesspananumberofdistinctmarketclasses.Theseclassesincludecherrytypeswithround,grape,andpearshapes;saladettetypesthataresimilartocherrytypesbutlargerandround;large,roundslicertypes;pasteorplumtypesthathavethickwallswithhightotalsolids,mainlyusedforprocessing;andheirloomtypes.Therearealsowildspecies,suchasSolanum pimpinellifolium,thatwillreadilycrosswithdomesticatedtypes.
Breedingprojectscanbedonewithinorbetweenanyofthesemarketclasses.Forexample,ifim-provingflavorinaslicingtypeisthegoal,thenoneapproachistocrossamodernslicerandanheirloomtypeknownforsuperiorflavor,suchas‘Brandywine.’Ifthegoalistocreatesomethingnovel,makingacrossbetweentwodistinctlydif-ferenttomatotypesfromtwodifferentclasseswillprovideabundantgeneticvariationfromwhichtoselect.Ingeneral,ifyouareseekingtoimproveavarietyforonlyoneorafewtraits,makingcrosseswithinaclasswillbemosteffectiveandefficient.Ontheotherhand,ifyouwanttocreatesomething
novelorimproveanumberoftraits,youmaywanttoconsidermakingcrossesbetweentwodifferenttomatoclasses.Severaldecadesago,itwascommonforasingletomatovarietytoservemultiplepurposesandmarkets.Today,tomatoesarebredforspecificmarkets,suchasprocessing,freshmarket,green-house,orhomegardenuse.Thischangeoccurredlargelybecausethecombinationofqualityandculturalrequirementsforeachofthesefourmar-ketshasbecomequitedistinct,drivingarelativelynewandunprecedentedspecializationwithinthetomatobreedingindustry.Todevelopnewvarietiesthatthriveunderthepressuresandchallengesofspecificconditions,andtoincorporatetraitsmostimportanttoeachmarket,itisvitalthatbreedingworkusethesameculturalpracticesandenviron-mentalconditionsinwhichthefinishedvarietywillbeproduced.
Allgoodbreedingprojectsneedafoundationofin-timatefamiliaritywiththecrop.Andgoodbreedersstartwithgoodmaterial.Taketimetoinvestigatewhichvarietiesandothergermplasmresourcesareavailabletoyoubeforebeginningabreedingproject.Alegacyofbreedingprecedesyou,anditwillbenefityoutotakeadvantageoftheproductsofthisexperienceandexpertise.Findinggoodbreedingmaterialwillsaveyoutimeandpotentialfrustration.Inbrief,thestepsinvolvedinbreedingwillbeto:
DevelopanddefineyourbreedinggoalsIdentifyparentmaterialorvarietiesyou’dliketoworkwithorcrossMakecrossesGrowoutandselectovermultiplegenerations,potentiallymakingmorecrosses
On-farmplantbreedinginvolvesmakingthebestuseoflimitedresources.Thebreedingplanforanycropwillbebasedonafewkeyfactors:
Determininghowmuchtime,space,andre-
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I. Introduction
*Definitionsfortechnicalwordsthatareitalicizedandboldedcanbefoundintheglossary
II. Introduction to Breeding
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org�
sourcesyouhavetodevotetobreedingworkWhatyourbreedinggoalsareandhowlikelytheyaretobeachievedHowmuchgeneticdiversityisavailableorcre-atedatthebeginningoftheproject
Thecombinationofthesefactorswilldeterminehowmanyplantsyouwillneedtoevaluate,particu-larlyintheearlystagesofyourwork.Forexample,acrossbetweentwogeneticallysimilarvarietieswiththegoaltoenhanceeatingqualitymayre-quirerelativelysmallpopulations.Ontheotherhand,largerpopulationswillbeneededforcrossesbetweentwogeneticallydistinctvarietieswiththegoaltocreateanovelvarietywithgoodagronomicandculinaryqualities.
Below,intheBreeding Methodssection,weofferexamplesofOrganicSeedAlliance’s(OSA)ap-proachtobreedingself-pollinatedcropsinaddi-tiontoacommercialpedigreebreedingscheme.Youcanperformaninterestingandsuccessfulbreedingprojectwithoutfollowinganestablishedcommercialbreedingprocedure.OSA’sapproachallowsyoutoincorporateagreaterdegreeofgeneticdiversityintoyourbreedingworkwhilestillachievingstabilityandanacceptablelevelofuniformityinanewvariety.Incorporatingandretainingdiversityinyourbreedingworkisgoodinsuranceagainstfailureandenvironmentalchal-lenges.Retainingdiversityisanespeciallygoodideaifyouarenewtoplantbreeding,becauseitkeepsyoufromnarrowinggeneticmaterialtoomuchandtooquickly,andbuffersagainstmakingpoorchoicesduetoinexperience.
Itisimportanttosetclearandattainablegoalsforyourtomatobreedingwork.Doyouwanttocreateatomatovarietythatcombinesyourfavoritequali-tiesfromtwovarieties?Doyouwanttoenhanceorcreatedisease,pest,orinsectresistanceinyourfa-voritevariety?Doyouwanttoimproveeatingqual-ityinanexistingvariety?Ordoyouwanttocreatesomethingentirelynew?Areyoubreedingforyourownuseordoyouintendtoshare,sell,orevenlicenseyourcreation?Thegoalsandintendedout-comesofyourworkwillhelpyouidentifywhereto
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startandhowtodevelop,create,andimplementasuccessfulbreedingplan.Whileitisnotnecessarytobeatrainedgeneticisttodogoodplantbreeding,itisextremelyvaluabletounderstandbasicgeneticconceptsandhowtheyaffectyourbreedingwork.Foranoverviewandin-depthdiscussionofthege-neticbasisofplantbreeding,pleaserefertoOSA’sIntroduction to On-Farm Organic Plant Breeding.
Belowisalistoftraitscommonlyincludedinto-matobreedingwork.AchartofhowthesetraitsareinheritedandwhentomakeselectionsisincludedattheendoftheBreeding Methodssection.
Agronomic traitsGrowth habit:Indeterminatevarietiesgrowcon-tinuouslywiththreeleafnodesoccurringbetweeneachinflorescence.Determinatevarietiesexpressafiniteamountofgrowthwithonlyoneleafnodebe-tweeninflorescences.Semi-determinatevarietiesexpressintermediategrowthwithtwoleafnodesbetweeneachinflorescence.
Plant stature: Tallplantsaremostcommon,dwarfvarietiesarelesscommon.
Leaf shape:Heirloomtypesoftenexpresspotatoleafshapewhereasmostmodernstandardvariet-ieshaveregularshapedleaves.Potatoleaftypescommonlyhaveexertedstigmas.
Canopy cover:Minimalleafcoveragecancontrib-utetosunscald.However,minimalleafcoveragecanalsoenhancecolorinpurpletypesthathavelightresponsivecompoundsintheskin.
Maturity: Maturityisrelatedtotheintendedpro-ductionenvironment.Forexample,earlymaturingvarietiesareneededincolderclimateswithshortgrowingseasons.
Pedicel attachment:Fruitbreaksoffatthecalyxinjointlesspediceltypes.Inplantswithjointedpedicels,fruitbreaksoffwithabitofstem.Thisisofparticularsignificanceforprocessingtoma-toeswhereattachedstemscanpuncturefruitandreducequality.
III. Breeding Goals
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org�
Defensive traitsDisease resistance:Somevarietieshavesinglegeneresistance,alsoknownasverticalresistance,forcertainracesofthefollowingdiseases:leafmold,Septoria,lateblight,Alternaria,Stemphyli-um,andverticilliumandfusariumwilts,Dominantresistancetotobaccomosaicvirusandnematoderesistanceisalsoavailable.Otherformsofcomplex,polygenic(multi-gene)resistance,alsoknownashorizontalresistance,existandareextremelybeneficialtorobustbreedingwork.Wildtomatotypesareoftenusedinbreedingworkassourcesofdiseaseresistance.
Drought tolerance:Complexandpolygenic,littleisknownaboutthespecificcomponentsofdroughttolerance.Selectionforthiscomplextraitneedstobedoneunderdroughtconditions.
Fruit qualityFruit color: Thistraitinvolvesthecombinationofskinandfleshcolor.Skiniseitheryelloworcolor-lessandfleshisred,yellow,orange,green,orpur-ple/brown.Colorisanindicationofthedominantcompoundsinthefruit,withlycopenedominantinredtypes,beta-caroteneinorangetypes,delta-car-oteneinyellowtypes,andphytoeneingreentypes.Thereareanumberofgenesthatcontrolandcon-tributetofleshcolorintomatoes.Modifyinggenesmaycausegradationsinexpressionandproductionofcolorcompounds.Theparticularcombinationofskinandfleshcolorresultinthefollowingcolorsoftomatofruits:
RedtypeshaveredfleshandyellowskinPinktypeshaveredfleshandcolorlessskinYellowtypeshaveyellowfleshandskin(color-lessskinandyellowfleshresultsincreamtopaleyellowfruits)Orangetypeshaveorangefleshandeitheryel-loworcolorlessskinGreentypeshavegreenfleshandeitheryelloworcolorlessskinBlackandbrowntypeshavered/greenfleshwithyellowskinPurpletypeshavered/greenfleshwithcolor-lessskin
Fruit stripe:Thistraitreferstodarkgreenradialstripesthatoccuroppositethe locules.Stripestend
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tofadeasthefruitmatures.
Green striping:Thistraitreferstoirregularlon-gitudinalgreenstripingonthefruit.Stripeswilleitherstaygreenorturnyellowatfullmaturity.
Skin texture:Mosttomatovarietieshavesmoothskin,butpeachy,fuzzytexturedskinalsoexists.
Flavor: Flavorisacomplexandpolygenictrait.Loculesizeandnumber,aswellastexture,aresig-nificantcontributorstoatomato’sflavorprofile.
Texture:Heirloomtypesareknownfortheir“meaty”fleshtexture.Thisisoftenadesirabletraitintomatobreedingwork.
Shape and size:Therearenumerouspossibilitiesforfruitshapeandsizeasalargediversityexistsintomatogermplasm.Yourmarketwilllikelybethelargestdeterminingfactorinwhatsizeandshapeismostdesirable.
Fruit ripening:Greenshouldertypeshavedarkgreenshoulderswhenimmaturewhereasfruitofuniformripeningtypesareentirelylightgreenwhenimmature.Greenshouldertypeshaverecent-lybeenassociatedwithsuperiorflavortouniformripeningtypes.Uniformripeningtypesareassoci-atedtosomeextentwithcrackresistanceandaredesirableforpastetomatoes.
Parthenocarpy: Thistraitreferstoaplant’sabil-itytosetseedunderlimitedpollination,whichisimportantforsettingfruitunderhotorcoldcondi-tionswhenpollencannotfunctionnormally.
Obtainingaspecificcombinationoftraitscanbechallenginginself-pollinatingcrops.Eachgenera-tionofself-pollinationincreaseshomozygositywithintheplant(i.e.,thestateinwhichthetwoallelesatagenelocationarethesame).Oncethetwoallelesbecomethesameatagivengeneloca-tion,thetraititcontrolsorcontributestoiscon-sideredtobefixedandwillnolongersegregateinsuccessivegenerations.Refertofigure1foranexplanationofthisphenomenon.Asthegenecombinationssortoutandthedegreeofhomozy-gosityincreases,specificcombinationsoftraitsbecomemoresettled.Ifthedesiredcombinationof
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org6
traitsisnotachievedafterfiveormoregenerationsofself-pollination,thenadditionalcrossingandgeneticrecombinationmaybenecessary.Becauseofthisphenomenon,growingandretainingaslargeapopulationasispracticalinthefirstfewyearsiscriticaltooptimizingyoursuccess.
Obtaining Traits
Repeatedgenerationsofself-pollinationintomatoesandotherself-pollinatingcropsleadstoincreasedgeneticuniformity.Witheachgenerationofself-pollination,thegenesinatomatobecome50%morehomozygous;meaning,halfoftheallelecombinationsateachgenelocationbecomethesame.Asanex-ample,let’slookatacontrolledcrossbetweenvarietyAandvarietyB.Thefirstgenerationfollowingthecrosswillbe100%heterozy-gous,witheverygenelocationhavinganallelefromparentAandonefromparentB(AB).Therearefourwaysfortheseallelestorecom-bineafterself-pollination,twoofwhichretainaheterozygouspairing(ABandBA)andtwoofwhicharehomozygous(AA,BB).
Allele A BA AA ABB BA BB
This50%chanceofhomozygosityoccursateverylocus(genelocation)ineverygenerationofself-pollination.Onceanallelepairbecomeshomozygous(AAorBB),itbecomes“fixed”andwillremainthatwaythroughfuturegenera-tions(untilamutationoranoutcrossoccurs).
Allele A AA AA AAA AA AA
Therefore,thegeneticmakeupofmaterialselectedfromacrossbecomesverygeneticallystableafterfiveorsixgenerations.
Generation %Heterozygosity
%Homozygosity
F1 100 0F2/S1 50 50F3/S2 25 75F4/S3 12.5 87.5F5/S4 6.25 93.75F6/S5 3.125 96.875
F=filial-a,thegeneraltermforagenerationfollowingtheparentgeneration(F1isthefirstgenerationfollow-ingacross,F2isthesecond,F3thethird,andsoon).S=selfed-a,thegenerationfollowingtheparentgenerationthathasbeencreatedbyanactofself-pollination.
Thisphenomenoniswhyitisimportanttoretainasmuchdiversityaspossibleinearlygenerations,andthatyoualsomakestringentevaluationsandselectionsseveralgenerationsafterthecrossismade.Thisprocessensuresyouwillbeselectingonastablearrangementofgeneticmaterial.
Ashortdiscussiononyield:Althoughitistempt-ingtomakeselectionsforhigheryield,selectingdirectlyforyieldisseldomveryeffective.Yieldisstronglyinfluencedbyenvironmentalfactorsandculturalpractice.Instead,itisadvisedtobaseyourselectionsoncomponentqualitiesandcharacteris-ticsthatcontributetorobustyields.Thismaymeanplacingemphasisonothertraits,suchasdiseaseresistance,maturity,growthhabit(determinate,semi-determinate,orindeterminate),concentra-tionoffruitset,resistancetocracking,andotherfactorsthatcontributetoahighpercentageofqual-ity,marketablefruitratherthanfocusingsolelyontherelativeabundanceoffruit.
Itisimportanttorecognizeandmakeselectionsbasedonfavorablegeneticcombinationsthatareheritable,andthuscanbepassedonfromonegen-erationtothenext.Justbecauseaplantlooksexcel-lentinthefielddoesnotmeanthatithasexcellentgenetics.Theplantmaylookgoodbecauseitwasplacedinasuperiorlocationthathadaddedfertil-ity,orwassubjectedtoveryfewstressors,suchasdisease,pestpressure,orlackofwater.Incorporat-ingfamily-basedselection,alsoknownasprogenyselection,isthebestwaytominimizethechance
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org7
ofselectingplantsthatlookgoodbutmayinfactbeharboringunfavorableorweakgeneticcom-binations.RefertoOSA’sIntroductiontoOn-farm Organic Plant Breeding fordetailsandsuggestionsonincorporatingfamily-basedselectionintoyourbreedingwork.
Fortunately,thereisnorightorwrongwaytodoplantbreeding.Thereexistsboundlessgeneticvariationandpotentialbothinnatureandinourworktointentionallycrossonetomatotoanother.Thereismoregeneticvariabilityandpotentialthanyoucouldpossiblycaptureinasinglelife-time.Youwillcertainlymissordiscardexcellentgeneticcombinations,butyouwillcaptureothers.Plantbreedingrequiresacertainamountofruth-lessnessandalotofdecisiveness.Combinedwithanartisticeyeandanopennesstounexpectedresults,plantbreedingcanbecomeadynamicandengaginglifelongpassion.
Thereareessentiallytwowaystobeginatomatobreedingproject:eithercreategeneticvariabil-ityorbeginwithgeneticallydiversematerial.Tocreategeneticvariability,youwillneedtomakeacontrolledcross,createtheopportunityforanatu-ralcrosstooccur,orwaitforanoff-typeormutantvarianttoarise.Whattypeofmaterialyoustartwithandwhereyousourceitfromwilldependonyourobjectivesandhowmuchtimeandenergyyouarewillingtoinvest.
Oneplacetobeginisbyacquiringapopulationofmaterialthatisknowntobegeneticallydiverse.Thismaterialissometimesreferredtoas“breed-ers’mixes”orcollections.Ifyoustartwithadiversepopulation,besuretoacquirethematerialfromareputablebreeder,seedproducer,orseedcompany.Somegeneticallydiversematerialisavailableinanunimprovedorwildstate.Theremaybesomegoodmaterialinthesecollectionsbutitwilllikelytakemanyyearstoteaseitout.
Youcanalsostartbysavingseedofavarietyyouknowandlove.Thiscanbedonewitheitherastan-dard(non-hybrid)orahybridvariety.Olderstan-dardvarietiesandheirloomsoftencontainsurpris-
ingamountsofgeneticvariation.Standardvarietieswillgiveyoulessgeneticvariabilityanddiversitytoworkwiththanahybridvariety.Hybridvarietiesoftomatoesarehighlyheterozygous(havingdif-ferentallelesatanumberofgenelocationsacrossthegenome),whereasstandardvarietiesaremuchmorehomozygous(allelesarethesameatmostgenelocations).However,natureisdynamic,andnewgeneticvariationisalwaysarising.Bypayingattentiontothisgeneticvariation,youcanidentifyvariationthatmaybeusefulinabreedingproject.
Acommonwaytoinitiateabreedingprojectistocreategeneticvariabilitybymakingacross.Controlledcrossesaremadebyhandpollination,transferringthepollenfromonedesirableplanttotheflowerofanother(describedinthePollination Techniquessectionbelow).Plantingtwovarietiesinverycloseproximitytoeachothercangeneratenaturalcrosses.Naturalcrossingwillmostreadilyoccurwitholderstandardandheirloomvariet-iesorcherrytypeswithexserted stigmasthatfacilitateandencouragecross-pollination(seetheReproductive Biologysectionformoreinforma-tion).Nearlyallcommercialtomatobreedersbasetheirprogramsoncontrolledcrossesfollowedbypedigreeselection.Pedigreebreedingfollowsasinglelinederivedfromasinglecrosstocreateanewvariety.Anexampleofbreedingbythepedi-greemethodisoutlinedintheBreeding Schemes and Methodssectionbelow.
YoucanalsobeginabreedingprojectbymakingacontrolledcrossbetweenoneortwoqualityF1hy-brids,whichwillcreateaplethoraofgeneticvari-abilitytoselectfrom.However,itwillbedifficultatbesttopredictwhatkindofvariabilityyouwillgetandifanyofitwillbeusefulorvaluable.Ifyouchoosetostartbycrossinghybrids,werecommendthatyougrowoutandobservetheprogenyofthehybridsyouwanttouseasparentsforatleastoneortwogenerationsbeforemakingthecrosses.Thiswillallowyoutoseethevariabilityandthegeneticpotentialencompassedineachhybrid.Itisnotuncommonthatthephenotypesoftheparentsofahybridarequitedifferentfromthephenotypeofthehybriditself.Knowingthegeneticmakeupandpotentialofyourbreedingmaterialisimportanttoanysuccessfulbreedingproject.
III. Sourcing Germplasm
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org8
Therearemanysourcesforobtainingvarietiesorothergermplasmtobeginyourbreedingwork.TheUSDAGermplasmResourcesInformationNetwork(GRIN)systemisagreatresourceforsourcingbothwildandcultivatedtomatomaterial.Itisasearch-abledatabasehousedwithintheUSDAAgricultureResearchService.TomatocollectionsarehousedattheGeneva,NewYork,andUCDavis,California,locationsandmaterialfromthemisavailableforfreebyrequest.TheSeedSaversExchangehousesalargecollectionofheirloomandolderstandardva-rietiesthathaveawealthofpotentialforbreedingwork.Publicuniversities,particularlylandgrantinstitutions,oftenhaveadvancedbreedingmaterialthatcanbelicensedorsharedforbreedingwork.NorthCarolinaStateUniversity,CornellUniversity,andOregonStateUniversityhaveexcellenttomatobreedingprograms.Local,regional,andnationalseedcompaniesandseedlibrariesarealsogoodplacestosourcevarietiesforbreedingwork.
Onceyouhavesourcedpotentiallyusefulgerm-plasm,conductingvarietytrialscanassistyouinidentifyingpromisingbreedingmaterial.Varietytrialsareawayofobservingandevaluatinganum-berofdifferentvarieties,orsourcesofasinglevari-ety,inaside-by-sidecomparison.Spendingatleastoneormoreseasonsconductingtrialsisagreatwaytogettoknowacropandtoseethevariationavailableforbreedingwork.Avarietytrialcanbesetupaseitheranunreplicatedobservationtrialorasareplicatedtrialthatwillallowyoutodeter-mineifthedifferencesyouseebetweenvarietiesareduetochanceortothevarietiesthemselves.Formoreinformationonvarietytrialsandhowtoconductthem,refertoOSA’sOn-Farm Variety Tri-als: A Guide for Organic Vegetable, Herb, and Flower Producers,whichisavailableforfreedownloadatwww.seedalliance.org.
Awordofcautiononintellectualpropertyprotec-tions:Unfortunately,therearesomeveryrestric-tiveintellectualpropertyprotectionsonagricul-turalseed.Itiscriticalthatyouknowwhat,ifany,restrictionsexistforthematerialyouwanttouse.YouneedtobeawareofwhetheravarietyisprotectedthroughtheuseofacertificategrantedthroughunderthePlantVarietyProtectionAct(PVPA),aMaterialTransferAgreement(MTA),orautilitypatent.Theseformsoflegalprotectionall
havevaryingdegreesofrestrictionsthatmayaffectyourabilitytousecertainvarietiesorotherformsofcropgermplasm,sobesuretoresearchanypos-siblelegalrestrictionsonyourbreedingmaterialbeforeyoubegin.
Grownunderoptimalconditions,tomatoeshavea95to115daylifecycle.Thefirstflowerswillappearandopen7to8weeksafterseedingandmaturefruitswillfollow6to8weekslater.
Tomatoeshaveperfectflowersthatcontainbothfunctionalmaleandfemaleparts.Themalepartsareknownasthestamensandareeachmadeupofanantherborneonafilament.Thefemalepartsareknownasthepistilsandaremadeupofastyleandstigmapositionedaboveanovary.
Floweringcanoccurforweeks,afeatureofto-matoesthatfacilitatestheabilitytomakecrossesbetweenvarietieswithdistinctlydifferentmaturi-ties.Normallyatleast4to8flowersareborneoneachinflorescence,andasingletomatoplantmayproduceasmanyas20ormoreinflorescencesoveraseason,givingyouampleopportunitiesformakingcrosses.
Mostmoderntomatovarietiesarealmostentirelyself-pollinating.Thisisfacilitatedbythefloral
IV. Reproductive Biology
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org9
structureinwhichatight,protectiveantherconeformsaroundthestigma,greatlyreducingthein-cidenceofnaturalcross-pollination.Inolderstan-dardandheirloomvarieties,andoftenincherrytypes,thedegreeofnaturalcross-pollinationcanbemuchhigherthantheoccurrenceinmodernstandardvarieties.Olderstandardandheirloomvarietiesoftenhavelongerstyles,pushingthestigmaeitherflushwithorsometimesbeyondthetipoftheanthercone.Thisarrangementfacilitateshigherratesofnaturalcross-pollination.Thephotobelowillustratesthisfloralstructuredifference.
Beeandinsectactivitycanalsoincreasenaturalcross-pollination,whichtendstobehigheronorganicallymanagedlandduetodiversecroppingsystemsandalackofpesticides.Ifyouarework-ingwithtwoolderstandardorheirloomvarieties,plantingthemrightnexttoeachothermayresultinanaturalcross.
Domesticatedtomatoes,includingbothmodernandheirloomtypes,arebiologicallycompatiblewithmanyoftheirwildrelatives.Unlikethemod-ern,domesticatedtomato,manywildrelativeshaveself-incompatibilitysystemsthatenforcecross-pol-lination.Wildspecieshaveplayedanimportantroleintomatoimprovementastheyhavebeen,andcontinuetobeusedas,criticalsourcesofdiseaseresistanceinmostmodernbreedingprograms.
Followingisadescriptionofhowtoperformcontrolledcrossesusingsimplehandtoolsandtechniques.Makingacontrolledcrossentails
emasculatingimmatureflowersbyremovingtheandroecium,ormalestructures,ofthechosenfemaleparent(pollenacceptor)andcollectingandtransferringpollenfrommatureflowersofthecho-senmaleparent(pollendonor).
Thefirststepinmakingahandpollinationcrosswithtomatoesistoidentifywhichflowersareatthecorrectmaturitytoserveasthemaleandfemale.Sincetomatoflowersareperfect,theageandmaturityoftheflowerwilldictatewhetheritcanbeusedasamaleorfemaleintheintendedcross.Flowersopenprogressivelyalongtheinflo-rescence,withtheoldestandmostmaturefloweronthebasalendandtheyoungestontheterminalend.Youcanexpect1to2flowerstoopeneachday.
Flowersinthelatebudstagearemostappropriateforemasculation.Atthisstagethecorolla(petals)andanthersremaintightlyclosedaroundthepistil,thesepalswillhavebeguntoopen,andthepetalswillbechangingfrompaletobrightyellow.Ifthepetalshavealreadyopened,orhavebeguntoopen,itistoolate.Thepollenwillhavealreadydehisced(beenreleasedfromtheanthers)andlikelypol-linatedthestigma.Therewilloftenbemorethanonebudonaninflorescencethatisappropriateforemasculation.Thehighestsuccessratesforhandpollinationareachievedwhenusingbudsnearthebasalendoftheinflorescence.Inthephotobelow,flowersA,B,andCaretoooldforemasculationbutareappropriateforpollencollection.FlowersD
V. Pollination Techniques
How to Breed Tomatoes for Organic Agriculture www.seedalliance.org�0
andEareappropriateforemasculationandflowersFandGaretooyoung.
Emasculationcanbeaccomplishedwiththeuseofforceps(recommended)orapairofneedlenosedtweezers.Thecorollaandstamenconearetightlyfoldeduparoundthepistilatthelatebudstage,formingatwo-layeredconethatcanberemovedinasinglestep.Insertonepointofyourforcepsintothetopofthecone,betweenthepistilandtheanthers,andworkitinallthewaytothebot-tomofthecone.Placetheotherpointbetweenthecalyx(sepals)andthecorolla.Graspandfirmly,butgently,pullstraightup,removingtheentireconeandleavingthepistilexposedandunharmed.Iftheconedoesnotdetachinonepiece,carefullyremoveanyremaininganthersandpiecesofcorolla.Itisalsopossibletopulloffeachlayeroftheflower,startingwiththecorollaandthenremovingtheanthersindividually.Thismethodwillwork,thoughitmaybemoredifficulttoavoiddamagingthestyleandstigma.Successfulemasculationtakespractice.Don’tbediscouragedifittakesseveralattempts.
Forpollencollection,chooseafullymatureflowerfromtheplantyouhavedesignatedtobethemaleparent.Removingthepetalscanbehelpful,butisnotnecessary.Lookfortheseriesoflongitudinalslitsthatrunalongtheanthercone,workyourtwee-zersintooneoftheslits,andprytheconeopen.
Therearethreewaystotransferpollen:1)removeasetofanthersandrubthemagainstthestigmaonyouremasculatedflower;2)pullthestyleoutoftheemasculatedflowerandrubtheremainingantherconeoveryouremasculatedstigma;or3)collectpollenusingabellorotherresonanttoolheldclosetotheflowerandrungtoshakepollenlooseontoa
collectingplateorslide,thendipyourforcepsintothepollenandpaintitontotheexposedstigmaofyouremasculatedflower.Multiplepollinationstothesameflowercanimprovefruitset,andinvolvescollectingandtransferringpollenfromseveralma-tureflowersofthemaleparenttothesingleemas-culatedflowerofthefemaleparent.Tomatopollenislong-livedandstoreswellintheeventyouneedtocollectitforfutureuse.Thepollencanremainviableforweeksatroomtemperature(outofdirectsunlight),andifdrieddownandrefrigeratedcanlastforseveralmonths.
Labeleachcrosswithatagidentifyingthefemaleandmaleparentsandthedatethecrosswasper-formed.Smallwhitemerchandisetagsareeffectiveandinexpensiveforlabelingcrosses.Itisalsoad-visabletoremoveallotherflowersfromtheinflo-rescencebearingthecontrolledcross.Thisstepeliminatesconfusionatfruitsetandfocusesalloftheplantresourcesforfruitdevelopmentintothatonefruitonthatparticularinflorescence.
Itisimportanttorinseyourforcepsinrubbingalcoholwhenyoumovefromworkingwithonevarietytoanother.Thisshouldbedonebothwhen
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youaredoingemasculationandpollencollection.Rubbingalcoholwillkillanypollenremainingonorstickingtoyourtool.Thisstepisveryimportantwhenyouaremakingcontrolledcrossessoastoavoidunintendedself-pollinationorcontamina-tion.Makesuretoallowtimeforyourforcepstofullydrybeforecollectingpollensothatwhatyoucollectisnotrendereduselessbythealcoholstillonyourtool.
Emasculatedflowersdonotneedtobecoveredtopreventcontamination.Atthispoint,thecolorfulandshowypetalsandanthershavebeenremoved,sothereisnothinglefttoattractpotentialpollina-tors.Ifyouareconductingcontrolledpollinationsoutsideinthefieldyoumayneedtocoveremascu-latedflowerswithglassinebagstopreventthemfromdryingoutanddesiccatingunderhot,dry,orwindyconditions.Glassinebagsarequiteinexpen-siveandcaneasilybelocatedwithaquickInternetsearch.Ifpossible,itisidealtomakecrossesout-doorswhentheweatheriscoolandovercastwithlittletonowind.
Controlledpollinationscanbeeffectivelydoneinalldaylighthours;however,somestudiessuggestade-clineineffectivenessinthelateafternoon.Eachseedrepresentsaseparateactofpollinationandfertiliza-tionandmanyvarietieswillsetfruitifonlyasingleseedisformed.Somevarietiesareparthenocarpic,meaningtheycansetseedwithoutfertilization.Makingcrosseswiththesetypesofvarietiesispos-siblebutmayrequiremultiplepollinationstoensuresuccessfulfertilizationandseedset.
Whetheryoubeginwithaqualityhybrid,ageneti-callydiversepoolofmaterial,oracontrolledornaturalcross,yourbreedingworkwillfollowasimilarcourse.Ingeneral,youwillneedtogrowoutthelargestpopulationsyoucanforthefirstsev-eralyearsfollowedbyafewyearsofsingleplantselectionsandfamilyrowevaluations.Youwillthenbulktogethertheseedofdesirableplantsthatmeetyourneedsandgoals.Thereissomedebateabouttheminimumandidealnumbersofindividualsperfamilythatwillmaximizeyourchancesofsuc-cess.Youwillbebetteroffplantingfewerindividu-
alsfromagreaternumberoffamiliesratherthanplantinglotsofindividualsfromonlyafewfamilies.Plantingfewerthanfiveindividualsperfamilyisnotrecommended.Themoreindividualsyouplantperfamilythemoreyouwillseethebreadthofge-neticpotentialandexpressioneachfamilyiscapa-bleof.Alwayskeepsomeremnantseedinreserveeachyearsothatallisnotlostintheeventofacropfailureorunintendedcross-pollination.Thecloseryoucanreplicatetherecommendationsinthisguide,themoregroundedyourworkwillbeinthegeneticfoundationsofplantbreeding.Thatsaid,plantbreedingisanartaswellasascience,andthebestplansarenotimmunetochangeandadaptation.Strivetomaintainflexibilityinyourbreedingworksoyoucanadaptandchangecourseinresponsetowhatyouseeinthefield.
Thepedigreemethodofplantbreedinginvolvesmakingacontrolledcrossfollowedbyseveralsuc-cessivegenerationsofsingleplantselections.Thegoalistodevelopatleastonenewvarietyfromasinglecross.Breedersandcompaniesthatemploythismethodaregenerallymakingmanycrosseseachyearandoftenhavelargeprograms.Thefactthattheyarealwaysworkingonmultiplebreedingprojectssimultaneouslyhelpstoensuretheirsuc-cess.Thefollowingexampleofapedigreebreed-ingmethodisbasedontheprogramatapublicuniversitythatproducesnewtomatovarietiesforboththefreshmarketandtheprocessingindustry.Thisparticularpublicprogramisrelativelysmallinscale,sothenumberspresentedbelowmaybesmallerwhencomparedtoothercommercialto-matobreedingprograms.
Althoughthepedigreemethodisareliableandsuccessfulmeanstodevelopingnewtomatovari-eties,itcanbegeneticallyrestrictiveforon-farmanddiversesystembreedingwork.Afinishedva-rietyfromapedigreeprogramisknownasapure-line,meaningthecommerciallyreleasedvarietycomesfromasuccessionofsingleplantselectionsthatresultinahighdegreeofgeneticuniformity.Thedevelopmentofmultiline varietiesisamoreusefulandgeneticallyresilientapproachtocreat-ingnewvarietiesfororganicanddiversefarmingsystems.Avarietyreleasedasamultilineiscom-posedofseveralsimilarbutgeneticallydiverse
VI. Breeding Methods
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purelinematerials.ThiscanbeaccomplishedbyfollowingOSA’sapproachtobreedingself-pol-linatedcrops,referredtoasthe“OSAmethod”inthesectionbelow.
Ourrecommendedpopulationsizesandnumberofplantsallowyoutobetterobservethegeneticex-pressionoftraitsandmakeeffectiveselections.Asyoumodifynumbersintosmallerpopulationsizes,yourabilitytodistinguishbetweenthegeneticcon-tributiontoatraitversusthecontributionofenvi-ronmentaleffectsdiminishes.Thisdoesnotmeanyouwillnotbesuccessful,butratherthechanceyourselectionswillbebasedonnon-genetic,andthusnon-heritable,factorswillincrease.Strivetoplantthelargestpopulationsofplantsthatyoucanpracticallyhandle.
Itisalsohelpfultoknowifanyofthetraitsofinterestaredominantorrecessiveinnature.Oncearecessivetraitappearsinaself-pollinatedcropliketomatoes,youcanbeconfidentthatitisfixedandwillbecarriedonthroughfuturegenerations.Dominanttraits,ontheotherhand,requireprog-enyobservationtoidentifywhetherornotthetraitisfixedorstillinaheterozygousform.Inthiscase,ifthetraitisfixed,theoffspringwillallexpressthedominantformofthetrait;whereas,ifthetraitisnotfixedmostoftheoffspring(approximately75%)willexpressthedominantformofthetraitandafew(approximately25%)willexpresstherecessiveform.Formoreondominantandreces-sivetraitexpressionandinheritance,refertoOSA’sIntroduction to On-Farm Organic Plant Breeding.Seetable1attheendofthissectionforalistoftraitscommonlyselectedforandwhentoselectforthem,alongwiththeirgenesymbolsandthetypeofinheritancetheyexhibit.Foracomprehensivelistofknowntomatogenes,refertotheC.M.RickTomatoGeneticsResourceCenter’slistofgenenamesandsymbols,foundathttp://tgrc.ucdavis.edu/Data/Acc/Genes.aspx.
BelowweprovideinstructionsforboththeOSAmethodandpedigreemethodbybreakingdownthefirstsixyearsofatomatobreedingproject.
Year 0Defineyourbreedinggoalsandidentifypoten-tialparentmaterial.Usevarietytrialstohelpyou
evaluateanddeterminewhichvarietiesorothergermplasmtouseasparentmaterialinyourbreed-ingworktobestmeetyourbreedinggoals.
Year 1OSA MethodMakecrossesbetweenseveralpairsofvarieties,ideallybetweenasmanydifferentparentvarietiesasispracticalforyou(shootfor3to6).Increasingthenumberofparentcombinationswillincreasetherangeofpossiblenewgeneticcombinationstoselectfrom.Itismoreefficienttogrowoutandevaluatesmallerpopulationsofprogenyfromanumberofdifferentcrossesthantolookatalargepopulationofprogenyfromasinglecross.Thisapproachincreasestheprobabilityoffindingyourdesiredideotypeandcombinationoftraits,andassumesyouhavepromisingparentmate-rial.Makingcrosseswithinferiormaterialjusttoincreasethenumberofparentcombinationswillnotbebeneficial.
Therearetwoapproachestomakingthesecrosses.Thefirstistomakereciprocal crosses betweenthesamesetofparentvarieties.Thesecondistomakemultiplecrossesbetweendifferentsetsofparentvarieties.Areciprocalcrossusestwoparentvariet-iesasboththefemaleandmaleinthecross:varietyAcrossedtovarietyB(AxB,thefemaleplantisal-wayslistedfirstinacross)andvarietyBcrossedtovarietyA(BxA).Thesecondapproachistomakemultiplecrossesbetweenmultiplesetsofdifferentparents.Forexample,ifyouplantomakesixcon-trolledcrosses,youcouldeithermakereciprocalcrossesbetweenthreedifferentsetsofparentsormakecrossesbetweensixdifferentsetsofparents.Reciprocalcrossingrequiresfewerplantsandgenerallyresultsinanarrowerrangeofvariationtoselectfrom.
Makingmultiplecrossesbetweendifferenttypesoftomatoesisthemostefficientwaytocreateadi-versepoolofgeneticmaterialfromwhichtobeginyourbreedingwork.Forexample,ifyouareinter-estedincrossinganorangeslicerwitharedgrapetype,sourcethreevarietiesofinterestingorangeslicersandthreevarietiesofredgrapesandmakemultiplecrossesbetweenthem.Thesecrosseswouldgiveyouninedifferentcombinationoptionsand18potentialreciprocalcrosses.
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Toensuresuccess,makecontrolledcrossesbe-tweenatleasttwosetsofplantsforeachvarietycombinationyouareworkingwith.Makecrossesonatleast4to6flowersondifferent inflores-cences betweeneverysetofparents.Collectseedfromtwoormorefruitsfromeachcross.ItisOKtocombineseedcollectedfromcrossesbetweenmorethanonesetofthesameparents,butkeepseedfromcrossesbetweendifferentsetsofparentsseparate.Ifmakingreciprocalcrosses,keepseedfromeachcrossseparate(i.e.,donotcombineseedofAxBwithseedofBxA).Thefollowingdiagramillustratesthissetupwithcrossesmadebetweentwosetsofparentsforeachvarietycombination.Theboxesrepresentwhereitisacceptabletocom-bineseed.Thetwoboxesontherightrepresentareciprocalcross.
Pedigree MethodMakecontrolledcrossesbetweenthreepairsofplants,usingthesamevarietyforthefemaleandthesameforthemaleineachcross.Makecrossesonatleast4to6flowersondifferentinflores-cencesbetweeneachsetofparents.Collectseedfromatleast2or3fruitsfromeachfemaleparentplant.Forexample,sayyouhavethreeplants(1,2,
and3)fromvarietyAthatyouhavechosentobeyourfemaleplants,andthreeplants(1,2,and3)fromvarietyBthatareyourmaleplants.MakefourcrossesonfourdifferentflowersonplantA1usingpollenfromplantB1(A1xB1).FollowthesameprocedureusingplantsA2andB2inacross(A2xB2)andplantsA3andB3inacross(A3xB3).Whenyoucollect,process,andcleantheseedfromthesecrosses,keepalloftheseedfromeachcrosstogetherbutseparatefromtherest(i.e.,theseedfromA1xB1iscollected,processed,andcleanedseparatelyfromtheseedfromA2xB2andA3xB3).Thisisillustratedinthefollowingdiagram.
Year 2OSA MethodThegenerationfollowingyourcrossisknownasthefirstfilial,orF1,generation.Alloftheseplantsshouldlookprettyidentical,assumingtheparentvarietieswerehighlyuniform.Plant3to6F1off-springfromeachcrossyouhavemade.Theprog-enyplantsshouldlookdistinctlydifferentfromtheparentvarieties.Plantingouttheparentvarietiesalongsidetheprogenycanbeveryhelpfulasavisualcomparison.Ifself-pollinationhasoccurred
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insteadofacross,theoffspringwillresemblethematernalparent.Collectandbulkseedfromseveralfruitofeachoriginalcross,keepingseedofeachcrossseparatefromtheothers.
Pedigree MethodPlantahalf-dozenplantsusingtheseedfromoneofyourcrosses.Keeptheseedfromtheothertwocrossesasremnant seedincaseofacropfailureorunforeseenproblem.Collectseedfromseveralfruitsfromeachplantandbulktheseed.
Year 3OSA MethodThisnextgenerationisknownasthesecondfilial(F2)orfirstselfed(S1).Thisyearyouwillbegintoseetheexpressionofgeneticvariabilitycreatedinthecross.However,thegeneticvariabilitywon’tbefullyexpressedinthisgeneration,sointensiveselectionshouldbeavoided.Anyselectionsshouldbebasedonhighlyheritabletraits,suchasgrowthhabit,fruitsize,shape,color,maturity,anddiseaseorpestresistance.
Plantseveralsmallpopulationsfromyourmultiplecrosses,aimingforatotalpopulationsizesome-wherebetween50to200plants.Plantasmanyplantsfromeachcrossasyouhavethetimeandspacefor,asevaluationandselectionwillbemini-malatthispoint.Forexample,ifyoustartedwiththreecrosses,growapopulationofatleast15to30plantsfromeachinthisgeneration.However,considerthatasyouprogresstofamily selectioninthenextseveralgenerationsyourtotalpopulationsizecansubstantiallyincrease.Ifyouareworking
withacrossbetweentwodistinctlydifferenttypesoftomatoes(i.e.,anorangeslicerandaredgrape)and/orareaimingtocombineasuiteofmultipletraits,youwillneedtomaintainlargepopulations.Ifyouareworkingwithacrossbetweentwosimi-lartomatoes(i.e.,tworedslicers)andwanttoim-proveonlyafewtraits,youcanworkwithsmallerpopulationsizes.
Discardplantsthatareobviouslyorfatallyflawedoreasilysuccumbtodisease.Tastetestfruitfromeveryplantanddiscardanyplantswithterribleorofftastingfruit.Strivetomaintainthebreadthofdiversityandvariabilityyouseeinthisgenerationinyourselections.Saveseedfromseveralfruitsfromasmanyoftheremainingplantsasyoucanpracticallyhandle,andkeepseedfromeachplantseparate.Inthenextseasonyouwillbeginfam-ilyobservation.Eachplantyousaveseedfrominthisseasonwillbecomeafamilywithfiveormoreplantsinthenextseason,soplanthenumberofyourselectionsaccordingly.Seethediagrambelow.
Pedigree MethodPlantaminimumof100plants,preferably200,fromtheF1seed.Selectionsaremadetoencompasstherangeofdiversityexpressedinthepopulation.Selectapproximately30%ofthepopulation:atleast30in-dividualplantsfromapopulationof100.Saveseedfrom2to3fruitsoneach.Keepseedfromeachplantseparate.Theseplantsnowrepresent30ormoredistinctfamilies.Dependingonresources,time,andfacilities,somepublicandprivatebreedingpro-gramswillselectmoreseverelyinthisgeneration,keepingonly1to10%ofplantsinthepopulation.
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ThefollowingdiagramillustrateshowtosetupandusefamilyselectionintheF2/S1throughF4/S3generations.SelectedplantsfromtheF2/S1gen-erationareplantedouttofamilyrowsthefollowingyeartocreatetheF3/S2generation.Eachselection(plantsthatarecircled)madefromeachofthesefamilieswillbecomeitsownfamilyinthefollowingyeartocreatetheF4/S3generation.Entirefamiliescanbeeliminatedinthisgenerationanddesirableindividualsareselected.Thiscycleiscontinuedforasmanygenerationsasnecessarytoarriveatmate-rialthatreflectsthedesiredideotypeandmeetsthebreedinggoalsoftheproject.
Year 4OSA MethodForthethirdfilial(F3)orsecondselfed(S2)gen-eration,planteachofyourpreviousgenerationse-lectionstosinglefamilyrowswithasmanyplantsperfamilyrowasispractical.Sinceyoupotentiallysavedseedfrom50ormoreindividualplantsinthepreviousgeneration,plant5to20individualsfrom
eachofthesefamilies.Atthispointyoustillwanttomaintainahighdegreeofdiversityandvariabil-ityinyourselections,andyoustillwanttodiscardplantsthatareobviouslyorfatallyflawed,tasteterrible,oreasilysuccumbtodisease.Again,youonlywanttoselectforhighlyheritabletraits.Col-lectseedseparatelyfromasmanyplantsasyoucanreasonablyworkwithasfamiliesinthenextgen-eration.Trytocollectseedfromthesamenumberofplantsasinthepreviousyearsothatyourpopu-lationsizedoesnotbecomeunmanageable.
Pedigree MethodPlant15to25plantsfromeachfamilyinto30separaterows.Thisgenerationwilldisplaythegreatestrangeofexpressionofgeneticdiversity.Inthisroundofselectiontheaimistomaintainthefullrangeofvariationinthepopulationwhilestriv-ingtomaintainthesameapproximatepopulationsize.Select30individualsandsaveseedfrom2to3
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fruits.Keeptheseedfromeachplantseparate.
Year 5OSA MethodInthisgeneration,F4/S3individualswithineachfamilyshouldstarttolooksimilaranddifferencesbetweenfamiliesshouldstarttobecomemoreobviousanddistinct.Discardentirefamiliesthatdonotmeetbreedinggoalsorareperformingpoorly.
FromeachF3/S2selection,againplantfamilyrowswith5to20individualplantsperfamily.Atthisstage,youcanbeginamorerigorousselectionprocess.Youcanstarttoselectformorequantita-tive traits,suchasconcentrationoffruitset,yield,complexdiseaseresistance,canopycoverandplantarchitecture,andqualitytraits,suchasflavor,tex-ture,andresistancetocracking.Selectplantswithfavorabletraitcombinationsandagainsaveseedseparatelyfromeach.Keepthenumberofselectedplantssimilartotheprevioustwoyears.
Pedigree MethodPlant10to20plantsfromeachplantselectedthepreviousyearintoseparatefamilyrows.Makean-otherseriesofsingleplantselectionsfromwithinthemostdesirablefamilies.Discardentirefami-liesthatdonotmeetbreedinggoalsorareper-formingpoorly.Keepseedfromeachindividualplantseparate.
Year 6 (and beyond)OSA and Pedigree MethodIntheF5/S4generation,beginreplicatedtrials.Plantingthreerandomizedreplicationsallowsforanalysisandevaluationofquantitativetraits
atthisstage.Measurementscanbetakenand/orscorescanbeassignedtodesirabletraits.Thesenumbersarethenaveragedoverthethreerepli-cationstoillustrateandteaseoutthedifferencesbetweenfamilies.Ifdesired,basicstatisticalanalysiscanbeperformedatthispointtodeter-minewhich,ifany,ofthefamiliesarestatisticallydifferent(ratherthandifferentduetochance)fromtheothers.Ananalysis of variance(ANOVA)isthemostcommonlyusedstatisticaltestforthistypeofevaluation.
Plantthreefamilyrowsof5to10plantsfromeachF4/S3singleplantselection.Discardundesirablefamiliesthatdonotmeetbreedinggoals.Basese-lectionsonquantitativetraits,suchasyield,pro-ductivity,fruitdistribution,setconcentration,andfruitquality.Familiesshouldbeuniformenoughtorougeoutanyunderperformingindividuals.Bulktheseedfromremainingindividualswithineachfamily.SometimessingleplantselectionsarecontinuedforafewgenerationspasttheF5/S4,dependingonthestabilityanddegreeofuniformityinthematerialatthisstage.ReplicatedtrialsarecontinuedintheF6/S5andF7/S6generationstodeterminethemostdesirableandrobustselectionforaneventualvarietyrelease.
Fromthispointonyoucaneithermaintainthevarietybycontinuingtopreservethefamiliessepa-ratelyandmakecompositemixtureswhenneededormaintainthevarietyasasinglepopulation.Ifyouhavenotachievedthedesiredcombinationoftraitsandqualitiesatthispoint,considermakingcon-trolledcrossesagainusingthemostdesirableplants.
Trait Inheritance When to Select
Agronomic GrowthhabitIndeterminantdominant,determinantrecessive.Modifierscanresultinsemi-determinance
Earlygenerations(F2andbeyond)
Plantstature Talldominant,dwarfrecessive Earlygenerations(F2andbeyond)
Leafshape Regulardominant,potatoshaperecessive Earlygenerations(F2andbeyond)
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Trait Inheritance When to Select
Canopycover Complex Lategenerations(F4andbeyond)
Maturity Complex Earlygenerations(F2andbeyond)
Pedicelattachment Jointlessrecessive Earlygenerations(F2andbeyond)
Defensive Diseaseresistance Somedominanceandsomerecessive-nessdependingondisease
Earlygenerations(F2andbeyond)
Droughttolerance Complex Lategenerations(F4andbeyond)
Fruit quality Skincolor Yellowdominant,colorlessrecessive Earlygenerations(F2andbeyond)
Fleshcolor
Reddominant.Redcombinedwithgreenfleshproducespurple/brownflesh.Greenripegeneresultsinbrightgreenflesh.
Earlygenerations(F2andbeyond)
Fruitstripe Dominant Earlygenerations(F2andbeyond)
Greenstriping Recessive.Colorcombinationdependsonallelesatredcolorgenelocation.
Earlygenerations(F2andbeyond)
Skintexture Smoothdominant,peachrecessive Earlygenerations(F2andbeyond)
Flavor ComplexEarlyandlategen-erations(F2andbeyond)
Texture Complex Lategenerations(F4andbeyond)
Shapeandsize Complex Earlygenerations(F2andbeyond)
Greenshoulderanduniformripening
Greenshouldersdominant,uniformripeningrecessive
Earlygenerations(F2andbeyond)
Parthenocarpicfruit Recessive Earlygenerations(F2andbeyond)
Table 1. Traits commonly selected for and when to select for them, along with their gene symbols and the type of inheritance they exhibit.
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GlossaryAllele:oneoftwoormoreformsofagene.Toma-toeshavetwoallelesateachgenelocation.
Analysis of Variance (ANOVA): acommonstatis-ticaltestusedtodeterminedifferencesbetweengroupmeans.
Dehiscence:releaseordischargeofpollen.
Emasculation:removalofthemalestructures(anthers).
Exserted:thrustout.
Family:agroupofgeneticallyrelatedplants.
Family selection:selectingindividualplantsorfam-iliesbasedontheoverallperformanceofafamily.
Filial:relatingtoagenerationorthesequenceofgenerationsfollowingtheparentalgeneration.
Germplasm:theentirecollectionofgeneticmate-rialforanygivencropspecies.
Heterzygous:havingdifferentallelesatagivengenelocation.
Heterozygosity:stateofhavingdifferentallelesatoneormoregenelocations.
Homozygous:havingthesameallelesatagivengenelocation.
Homozygosity:thestateofhavingthesameallelesatoneormoregenelocations.
Hybrid:theproductofacrossbetweengeneticallydistinctparents.
Ideotype:anidealizedvisionoftheperfectcombi-nationofalldesiredfeaturesforthegrowth,ap-pearance,productionandperformanceofacrop.
Inbred:avarietyproducedbysuccessiveinbreed-ingoveranumberofgenerations.
Inflorescence: agroupofflowersarrangedonastem.
Locule:achamberwithinthetomatofruitwheretheseedsandplacentaltissuearefound.Thenum-berofloculesintomatoescanvaryfrom2to25,butismostcommonlybetween5to9.
Multiline variety: mixtureofanumberofpurelinevarietiesorpopulationswithsimilarappearancesandqualities.
Parthenocarpy:thenaturalproductionoffruitwithoutfertilizationoftheovaries.
Pedicel:thestemthatattachesasingleflower(andeventualfruit)tothestemoftheinflorescence.
Phenotype: theobservable,outwardappearanceofthecrop.
Polygenic trait:relatingtoorcontrolledbymul-tiplegenes.
Pureline:geneticallyuniformvarietydevelopedbyselectingaseriesofsuperiorindividualplantsfromasinglecross.
Reciprocal cross: Twomirroredcrossesmadebetweenasinglesetofparentswhereeachparentservesasboththefemaleandthemale.
Remnant seed:seedreservedforplantingatalaterdate.
S1 family:afamilystructurewheretheplantsinthefamilyallresultedfromthesameself-pollination.
Quantitative trait:synonymouswithpolygenictrait(See polygenic trait definition).
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Allard,R.W.1960&1999.Principles of Plant Breeding.1stand2nded.JohnWiley&Sons,NewYork,NY.
Basset,M.J.1986.Breeding Vegetable Crops.AVIPublishingCompany,Westport,CT.
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Colley,M.andJ.Myers.2007.On-Farm Variety Trials: A Guide for Organic Vegetable, Herb, and Flower Producers.OrganicSeedAlliance,PortTownsend,WA.
Deppe,C.2000.Breed Your Own Vegetable Varieties: The Gardener’s and Farmer’s Guide to Plant Breed-ing and Seed Saving.ChelseaGreenPublishing,WhiteRiverJunction,VT.
Dyck,E.,S.Wakefield,R.White,andB.Connolly.2011.Breeding Organic Vegetables: A Step-by-Step Guide for Growers.NortheastOrganicFarmingAs-sociationofNewYork.
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References and Resources
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AuthorsCompleted 20��Laurie McKenzie, Organic Seed AlliancePictures courtesy of Laurie McKenzie (OSA) unless otherwise noted
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