PO Box 772, Port Townsend, WA 98368 How to Breed · PDF filehow many plants you will need to evaluate ... Tall plants are most common, dwarf varieties are less ... How to Breed Tomatoes

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


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


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


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


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


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


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


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

How to Breed Tomatoes for Organic Agriculture www.seedalliance.org��

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


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.


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


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.


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


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)


Trait Inheritance When to Select

Canopycover Complex Lategenerations(F4andbeyond)

Maturity Complex Earlygenerations(F2andbeyond)

Pedicelattachment Jointlessrecessive Earlygenerations(F2andbeyond)

Defensive Diseaseresistance Somedominanceandsomerecessive-nessdependingondisease


Droughttolerance Complex Lategenerations(F4andbeyond)

Fruit quality Skincolor Yellowdominant,colorlessrecessive Earlygenerations(F2andbeyond)

Fleshcolor

Reddominant.Redcombinedwithgreenfleshproducespurple/brownflesh.Greenripegeneresultsinbrightgreenflesh.


Fruitstripe Dominant Earlygenerations(F2andbeyond)

Greenstriping Recessive.Colorcombinationdependsonallelesatredcolorgenelocation.


Skintexture Smoothdominant,peachrecessive Earlygenerations(F2andbeyond)

Flavor ComplexEarlyandlategen-erations(F2andbeyond)

Texture Complex Lategenerations(F4andbeyond)

Shapeandsize Complex Earlygenerations(F2andbeyond)

Greenshoulderanduniformripening

Greenshouldersdominant,uniformripeningrecessive


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.


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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References and Resources


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