ContentslistsavailableatScienceDirect
Field
Crops
Research
j o ur na l h o me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / f c r
Development
of
a
pearl
millet
Striga-resistant
genepool:
Response
to
five
cycles
of
recurrent
selection
under
Striga-infested
field
conditions
in
West
Africa
夽
Boubacar
A.
Kountche
a,b,d,e,
C.
Tom
Hash
b,
Harouna
Dodo
b,
Oumarou
Laoualy
a,b,
Moussa
D.
Sanogo
c,
Amadou
Timbeli
c,
Yves
Vigouroux
d,e,
Dominique
This
e,
Randy
Nijkamp
f,
Bettina
I.G.
Haussmann
b,∗aUniversityAbdouMoumouni,FacultyofAgronomy,BP10960,Niamey,Niger
bInternationalCropsResearchInstitutefortheSemi-AridTropics(ICRISAT),BP12404,Niamey,Niger cInstitutd’EconomieRural(IER),StationdeCinzana,BP214,Segou,Mali
dInstitutdeRecherchepourleDéveloppement(IRD),BP64501,34394Montpelliercedex5,France eMontpellierSupAgro,2PlaceViala,34060MontpellierCedex1,France
fCropandWeedEcologyGroup,WageningenUniversity,POBox430,6700AK,Wageningen,TheNetherlands
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received17April2013
Receivedinrevisedform10July2013 Accepted11July2013
Keywords: Strigahermonthica Downymildew Cenchrusamericanus Gainfromselection Resistancebreeding
a
b
s
t
r
a
c
t
Strigahermonthica(Del.)Benth.isapersistentthreattopearlmillet[Cenchrusamericanus(L.) Mor-rone,comb.nov.]production,especiallyinWestAfrica.Thisstudyaimedatevaluatingtheresponse ofadiversifiedpearlmilletgenepooltofivecyclesofrecurrentselectiontargetingStrigaresistanceand panicleyield,andtoalesserextentdownymildew[Sclerosporagraminicola(Sacc.)J.Schroet.]resistance. Two-hundredfull-sibfamilies(FS)representingtheC5selectioncyclewereevaluatedtogetherwiththe genepoolparentallandraces,experimentalvarietiesderivedfrompreviouscyclesandlocalchecksin Striga-infestedfieldsatSadoré(Niger)andCinzana(Mali).Substantialandmostlysignificantselection progresscouldbedocumented.Theaccumulatedpercentagegainfromselectionamountedto51%/1% lowerStrigainfestation(measuredbyareaunderStriganumberprogresscurve,ASNPC),46%/62%lower downymildewincidence,and49%/31%higherpanicleyieldoftheC5-FScomparedtothemeanofthe genepoolparentsatSadoré/Cinzana,respectively.Experimentalvarietiesselectedfrompreviouscycles alsorevealedlowerASNPCandmostlyhigheryieldcomparedtogenepoolparentsattheirselectionsites. SignificantgeneticvariationamongtheC5-FSandoperativeheritabilitiesof76%(Cinzana),84%(Sadoré) and34%(combinedacrosslocations)forASNPCwillenablecontinuedselectiongainforStrigaresistance. Highgenotype×environmentinteractionvariancesforalltargettraitssuggestthatdifferent experimen-talvarietiesneedtobeextractedfromthegenepoolfordifferentsites.Thegenepool-derivedvarieties willbefurthervalidatedon-farmandareexpectedtocontributetointegratedStrigacontrolinpearl milletinWestAfrica.
©2013TheAuthors.PublishedbyElsevierB.V.Allrightsreserved.
Abbreviations:ASNPC,areaunderStriganumberprogresscurve;das,daysaftersowing;DM,downymildewincidence;E,environment;FLO,timeto50%flowering;G, genotype;G×E,genotypebyenvironment;HYD,panicleyield;ICRISAT,InternationalCropsResearchInstitutefortheSemi-AridTropics;IRD,InstitutdeRecherchepourle Développement.
夽 Thisisanopen-accessarticledistributedunderthetermsoftheCreativeCommonsAttribution-NonCommercial-NoDerivativeWorksLicense,whichpermits non-commercialuse,distribution,andreproductioninanymedium,providedtheoriginalauthorandsourcearecredited.
∗ Correspondingauthor.Presentaddress:UniversityofHohenheim,InstituteofPlantBreeding,SeedScienceandPopulationGenetics(350a),70599Stuttgart,Germany. E-mail addresses: [email protected] (B.A.Kountche), [email protected] (C.T. Hash), [email protected] (H. Dodo), [email protected] (O. Laoualy),
[email protected](M.D.Sanogo),[email protected](Y.Vigouroux),[email protected](D.This),[email protected](R.Nijkamp),
[email protected](B.I.G.Haussmann).
0378-4290/$–seefrontmatter©2013TheAuthors.PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
Pearl millet [Cenchrus americanus (L.) Morrone, comb. nov.] (Chemisquy et al., 2010)is thestaple food and fodder cropof millionsof poor ruralfamilies in Africaand India (Hashet al., 2000;Senthilvel etal.,2008).Inthearidand semi-aridregions wherethiscerealiscultivated,pearlmilletplaysanimportantrole in foodsecurity.However, grainyieldsare generallylow (circa 350–600kgha−1).Grainyield is limited by abiotic stresseslike droughtandlowsoilfertility,andbioticstressessuchasdowny mildewdiseaseandtheparasiticweedStriga.
Strigahermonthica (Del.) Benth.is themostwidespread and destructivewitchweedaffectingcereals.IntheStriga-proneregions ofsub-SaharanWesternandCentralAfrica,S.hermonthicaremains themajorbioticandpersistentthreattopearlmilletproductionand productivity(Wilsonetal.,2004).About40%ofthecerealproducing areasisseverelyinfestedwithStrigainsub-SaharanAfrica(Gurney etal.,2006).Grainyieldlossesupto100%havebeenreportedin susceptiblecerealcultivarsunderhighinfestationlevels, particu-larlyunderdroughtconditions(Gurneyetal.,2006;Ejeta,2007; Amusanet al.,2008).Severaloptions have beenrecommended forStrigacontrol:handweeding,herbicides,rotationsofcashor trapcropswithcereals,improvedsoilfertility,intercroppingand biologicalcontrol(Haussmannetal.,2000).However,the diver-sityofthefarmingsystemsandoftheparasitehaverenderedthe useofa singlecontrolmethodineffective,leadingtothe devel-opmentofintegratedStrigacontrolpackagesthatcombineseveral controlmeasures(e.g.,Badu-Apraku,2010).Breedingcultivarsthat canwithstandparasiteinfectionisconsideredtobecentralto inte-gratedapproachestoStrigacontrolinpearlmillet.Employinghost resistancetoS.hermonthicaissimpleandeconomicalfor subsis-tencefarmerstoadopt(Ejeta,2007;MenkirandKling,2007;Wilson etal.,2000).
Inheritanceof traitsassociated withresistancetoStrigaspp. hasbeenreportedforsorghum,maizeandrice(Haussmannetal., 2001a,2004;Amusanetal.,2008;Gurneyetal.,2006).Thelow stimulationofS.asiaticaseedgerminationhasbeenreportedto beunderthecontrolof a singlerecessivegene inthesorghum cultivars Framida,555, SRN 6496,and SRN 39 (Hessand Ejeta, 1992;EzeakuandGupta,2004).Onemajorgeneandseveralminor genesareinvolvedinstimulationofS.hermonthicaseed germina-tioninsorghum(Haussmannetal.,2001a),andthemajorgenehas recentlybeenmapped(Satishetal.,2011).FieldresistancetoStriga inmaizeisconsideredaquantitativelyinheritedtrait(Kim,1994; GethiandSmith,2004;Amusanetal.,2008).Theheredityof host-plantresistancetoStrigainpearlmilletisnotwelldocumented. Fewpearl milletcultivars havebeenreportedaspartially resis-tantortoleranttoStriga,andresistance(oratleastalowerlevel ofsusceptibility)incertainpearlmilletmaterialswasshowntobe dominant(Ramaiah,1987).However,theveryexistenceofStriga resistanceincultivatedpearlmillethasbeenquestionedbyothers (ChisiandEsele,1997).Lackofpreciseandvalidatedinformation aboutStrigaresistanceinpearlmilletmaybepartiallyduetothe factthatthepearlmillet/S.hermonthicapathosystemisparticularly complex.Bothhostandparasitearehighlyout-crossingspecies, whichresultsineach plantinapearlmilletlandrace,improved open-pollinatedvarietyor genepoolpopulationhavinga differ-entgenotypeandthereforecarrying potentiallydifferentalleles forStrigaresistanceorsusceptibility.Similarly,eachS.hermonthica plantinanaturalpopulationcarriespotentiallydifferentallelesfor virulence.
PartialquantitativeresistancetoS.hermonthicawasreportedin wildpearlmilletrelativesfromAfrica(Wilsonetal.,2000,2004). However,fromabreeder’spointofview,itappearspreferableto workwithinadynamicanddiversifiedgenepoolofcultivatedpearl milletsusingrecurrentselectionmethodsaswildmilletrelatives
carrymanyundesirableallelesthatneedtobeeliminated,thereby slowingdownselectionprogress.
Downymildew[causedbytheoomyceteSclerospora gramini-cola(Sacc.)J.Schroet.]isapseudo-fungaldiseaseofpearlmillet, causing up to 60% croploss (Singh, 1990; Thakur et al., 2011) and needs tobeconsidered in pearl millet improvement. Both Strigaanddownymildewpathogensarehighlyvariable,and dif-ferentecotypescanbepresentatdifferentlocations(Haussmann etal.,2001b;Thakuretal.,2006;Yoshidaetal.,2010),underlining theneedtoperformselectionatcontrastinglocationstoexploit genotype×locationinteractions and/or identifystablevarieties. Accordingtoourknowledge,thereisnoStriga-resistantpearlmillet varietyreleasedinWestAfrica.
Todate,increasedresistancetoStrigathroughrecurrent selec-tion has not been reported for pearl millet. Since 2006, the InternationalCropsResearchInstitutefortheSemi-AridTropics (ICRISAT)inNiger, inpartnershipwiththenationalagricultural researchprograminMali(Institutd’EconomieRurale,IER),has con-ductedrecurrentselectionina diversifiedpearlmilletgenepool toincreasefrequenciesofdesirableallelesforStrigaanddowny mildewresistanceandpanicleyieldusingtwotestsitesinWest Africa(SadoréinNigerandCinzanainMali).Intotal,thisgenepool wassubjectedtofivecyclesofrecurrentselection,andprovidesa uniqueopportunitytoexaminetheeffectivenessofrecurrent selec-tionforimprovingStrigaanddownymildewresistance,andyield performanceincultivatedpearlmillet.
Theobjectivesofthisstudywere(i)todescribethegenepool developmentandselectionprocess,(ii)toexaminetheresponse tofivecyclesofrecurrentselectionforpanicleyieldandresistance toS.hermonthicaanddownymildew,(iii)toestimate quantitative-geneticparametersandpredictpossiblefutureselectiongainsfrom thegenepool’sC5-Full-sibpopulation.
2. Materialsandmethods
2.1. Striga-resistantgenepooldevelopment
TheinitialstepfordevelopingthepearlmilletStriga-resistant genepoolconsistedoffieldevaluationof64pearlmilletlandraces that had been collected in Striga-infested fields jointly by IRD (theFrenchResearch InstituteforDevelopment)and ICRISATin 2003(Bezanc¸on etal., 2009).The 64 landraceswereevaluated ina four-replicatefieldtrialunderartificialStrigainfestationat the ICRISAT-Sadoré research station (13◦15 N, 2◦18 E) in the 2005rainy season.Six landraceswere identified asless Striga-sensitiveandrelativelyhigheryielding(M141,M239,M029,M197, M017and KBH).During the2005/2006off-season,both full-sib (FS)progenies(intra-andinter-population)andS1(self-pollinated) progenieswerecreatedfromthesesixselectedgenepoolparents (Table1).Theseprogeniesweresubjectedtothefirstcycleof eval-uationin the2006rainyseasonina four-replicatefield trialat ICRISAT-Sadoré. Separate,artificially infested fields (S1 and S2, sownwithintwodaysinterval)wereusedforevaluatingFSand S1progeniesinthisandthefollowingyears.Evaluationof Striga-resistantgenepoolprogeniesatCinzana,Mali(13◦17N,5◦56W) wasstartedlaterinthe2008rainyseasoninanaturallyinfested farmer’sfieldnearthestation.Thisfieldhadbeencarefullyselected forstronganduniformStrigainfestationinthepreviousyear.An overviewofthediversifiedStriga-resistantgenepooldevelopment processispresentedinFig.1.
Inthefirstandsubsequentcycles,selectionwasbasedonthe fol-lowingcriteria:above-averageStrigaresistance[i.e.below-average areaunder Striga number progress curve (ASNPC)(Haussmann etal.,2000)orStrigaemergencesupportedbytheprogeny], above-averagepanicleyield,andbelow-averagedownymildewincidence
Table1
Selectionprotocolsandphenotypicdataforeachcycleofrecurrentselection(2006–2010).
Selection cycle(year)
Selection method
Location No.ofprogeny Proportionof
selectedunits(%) Traits†
Evaluated Recombined ASNPC HYD DM FLO
h2 TM h2 TM h2 TM h2 TM C0(2006) Full-sib Sadoré 169 53 31 45.0 1308.5 53.9 126.7 61.5 37.5 44.7 85.7 S1 116 25 22 30.6 325.1 30.6 160.6 51.1 18.3 25.8 107.5 C1(2007) Full-sib Sadoré 164 33 20 29.1 354.9 47.8 190.0 43.8 7.5 48.2 72.5 S1 91 20 22 37.9 10.5 49.1 102.5 71.0 7.0 44.2 81.7 C2(2008) Full-sib Cinzana 70 – – 59.1 46.3 32.3 390.2 51.1 14.9 35.3 71.5 Sadoré 196 59 30 64.7 1263.0 37.0 291.9 61.3 19.9 47.9 74.8 S1 Sadoré 91 12 13 19.7 904.2 55.9 193.1 60.4 20.4 55.6 80.0 C3(2009) Full-sib Cinzana 183 65 25 57.6 2074.9 16.4 280.5 32.8 9.4 27.5 67.8 Sadoré 183 55.9 4265.9 34.1 483.5 60.5 7.2 72.4 42.6 C4(2010) Full-sib Cinzana 103 56 54 55.5 196.9 22.7 201.5 66.9 7.0 34.3 76.4 Sadoré 162 61.8 39.6 27.9 140.8 52.7 11.6 23.9 85.9 S1 Sadoré 55 16 20 29.9 244.2 54.5 207.8 28.9 22.5 55.2 67.4
† ASNPC:areaunderStriganumberprogresscurve;HYD:panicleyield(gm−2);DM:downymildewincidence(%);FLO:timeto50%flowering(daysaftersowing).Thetrial
means(TM)andbroad-senseheritability(h2)onaplotbasisarereportedforeachtrait.
(DM) (number of infested hills recorded during plant tillering andupdatedafterflowering,dividedbythetotalnumberofhills afteremergence,multipliedby100).Inlatercyclesalsoan over-allagronomicsuitabilityscorewastakenintoaccount,andonly entrieswithabove-averagescoreswereselected.Thepercentage ofselectedentriesrangedfrom13to54%,anddependedonthe numberofentriestested,thenumberofsuperiorentriesidentified forthedesirabletraitcombinations,andthedesiretohaveatleast 50progeniescontributingtothenextgeneration,inordertoavoid inbreedingdepressioninsubsequentgenerationsofthegenepool. In the following selection cycles, S1 and FS selection were alternated,withnewFSprogeniesbeingcreatedfromselectedS1
progeniesandbothnewFSprogeniesandS1progeniesbeing cre-atedfromtheselectedFSprogenies.Duringeachrecombination step,crossingplanswereestablishedtoassurethateachselected progenywascrossedtoseveralotherselectedprogeniesandalso thateachselectedprogenycontributed aboutthesamenumber of progenytothe nextgeneration.Thiswasdoneto maximize theeffectivepopulationsizeinthefollowinggenerationsofthe genepool,andtherebytoavoidinbreedingdepression.
Thegenepoolwaskeptopen,sothatpotentiallyinteresting lan-draces(reportedbyfarmerstobeStrigaresistant)wereincluded astestentriesduringtheevaluationcycles(allfour-replicatefield trials),andifabove-averageStrigaresistanceandacceptableyield
Ev a lu a ti on Test of 64 landraces for
selecting the
genepool parents C2Evaluation using 196 FS & 91 S1 at Sadoré & 70 FS at
Cinzana (Mali)
& checks† C3Evaluation at Sadoré & Cinzana using 183 FS & checks† C4Evaluation using 162 FS & 55 S1at Sadoré & 103 FS at Cinzana & checks† Full-sib (FS) & selfed progeny (S1) creation using 6 landraces representing the genepool parents
25 selected S1 used for new FS creation & 53 selected FS
used for both new FS/S1 creation Creation of two experimental varieties C5Evaluation &
Test of selection
progress at Sadoré
& Cinzana using
200 FS & checks†
Introduction of M017 & PE5461 as new germplasm
Introduction of PE00702, PE05606
& Serkloc as new
germplasm Creatioexperimental n of three
varieties
Rainy season 2005 Rainy season 2006 Rainy season 2007 Rainy season 2008 Rainy season 2009 Rainy season 2010 Rainy season 2011
Off-season 2005/2006 Off-season 2006/2007 Off-season 2007/2008 Off-season 2008/2009 Off-season 2009/2010 Off-season 2010/2011 Off-season 2011/2012
Re com b inat ion & Creat ion of new te st uni ts 20 selected S1 used for new FS creation & 33 selected FS
used for both new FS/S1
creation
12 selected S1 used for new FS creation & 59 selected FS
used for both new FS/S1
creation
65 selected FS used for both
new FS/S1 creation
16 selected S1 & 56 selected FS used for
new FS creation
28 selected FS used for new FS creation C0Evaluation at Sadoré(Niger) using 169 FS & 116 S1 & checks† M017 & PE5461 C1Evaluation at Sadoré using 164 FS & 91 S1 & checks†
PE00702, Serkloc & PE05606
Creation of two experimental
varieties
Legend:
FS = Full-sib families; S1= Self-pollinated progenies; = New germplasm included in field evaluations; † Checks included genepool parents and other controls.
Fig.1. SchematicillustrationofthegenepooldevelopmentprocessfollowinganopenrecurrentselectionschemewithcyclicFSandS1evaluation,selectionandrecombination combinedwithintrogressionofnewgermplasmandextractionofnewexperimentalvarieties.
performancewasproven,thelandracewasincludedintheFSandS1 creationforthenextselectioncycle.Atotaloffivenewgermplasm wereincludedattworecombinationcycles(inC1 andC2,Fig.1). Basedonselectionsmadefrom2008and2009rainyseasondata, thefirstexperimentalvarietieswereextractedfromthegenepool (afterthe3rdand4thselectioncycle).Fortheexperimental vari-etiesderivedin2008,15bestprogeniesatSadoréwithlongpanicles (EV08-Sad-LP),and15bestprogeniesatSadoréwithshorter pan-icles(EV08-Sad-SP)wererecombinedduringthe2008/2009dry season.Fortheexperimentalvarietiesderivedin2009,the15best progeniesfromeachsite,Sadoré(EV09-Sad)andCinzana (EV09-Cin),andfromacombinedanalysisacrosstwosites(EV09-Comb) wererecombined.Withtherecombinationof15bestprogenies,a higherselectionintensitywasappliedfortheexperimental vari-etiescomparedtotheoneappliedtothegenepool.
Only one improvedpopulation/genepoolresulted from each selectioncycle.ThereasonsforusingS1recurrentselectionin com-binationwithFSrecurrentselectionwerethefollowing:selection amongS1 progeniesgiveshigherchancestoselectfordesirable recessive resistancealleles and has theoretically theadvantage oflargergeneticvarianceamongthetestedentries.Thevariable numbersofS1 andFSprogeniesincludedineachselectioncycle (Table1)wereduetovariablesuccessinFS/S1creationinthe off-seasonnurseries–duetoirrigationproblemsandconcomitantheat stress,grasshopperdamageorpoorseedsetting.A summaryof eachselectioncycleincludingevaluatedunits,recombinationunits, heritabilityandtrialmeanisgiveninTable1.
2.2. Evaluationofselectionprogressafterfiveselectioncycles 2.2.1. Geneticmaterials
Thetestofselectionprogressafterfiveselectioncycleswas inte-gratedintotherecurrentselectionprogenyevaluationprocess.A setof200C5-FSprogenieswereproducedinthe2010/2011dry sea-sonbyrecombining56FSprogeniesand16S1progeniesselected fromtheC4evaluationcycle,and3experimentalvarieties (EV09-Sad,EV09-CinandEV09-Comb).Remnantseedsofeachselected progenyweresowninaone-rowplotofsevenhillsandcrosses wereperformed accordingtoa defined crossing planto assure thateachprogenywascrossedtoatleastthreedifferentselected progeniesandthateachprogenycontributedthesamenumberof progenies(about three)tothenextgeneration.For thepresent study,the200C5 derivedFSfamilies togetherwith20 controls [including genepoolparental varieties,5 experimentalvarieties (seeabove)andasetof8agronomicallyeliteopen-pollinated vari-eties(OPVs)]wereevaluatedforStrigaresistanceduringthe2011 rainyseason.Theseelitevarietieshaveacceptablelevelsof resis-tancetodownymildewandwideadaptabilitybutaregenerally susceptibletoStriga.
2.2.2. Fieldexperimentprocedures
Theexperiment wasconducted attwo sites: inNiger under artificialStriga infestationatICRISAT-Sadoré(13◦15 N,2◦18 E) andinMaliundernaturallyhighlyStriga-infestedfieldconditions nearIER-Cinzana(13◦17N,5◦56W).Theseenvironmentsare con-trastingintermsofecology(SahelianvsSudanian),andsoiltype. Randomizedincompleteblockdesignswithfourreplicationswere usedineachtestenvironment.Eachfull-sibfamilywasgrownina single-rowplotof3mlengthwithinter-rowspacingof1.60mof distancetoprovidemorelightforStrigaemergence,enoughspace forStrigacountingandtoreducebordereffects(Haussmannetal., 2000).Withinplot-rows,distancebetweenhillswas0.50m, pro-vidingsevenhillsperrow.Initiallyabout15seedsweresownper hill.Thehillswerethenthinnedtotwoplantsperhillthreeweeks afteremergence.Fertilizationofthetrialswasperformedduring soilpreparationwhen50kgha−1of15–15–15NPKfertilizerwas
applied.WeedsotherthanStrigawereeliminatedbyhandona regularbasistokeepthefieldcleanfor Strigaobservations.The StrigaseedsusedforartificialinfestationatICRISAT-Sadoréwere collectedfrompearlmilletfieldtrialsthereandfarmers’fieldsnear Sadoré attheend ofpreviousseasons and hadbeenstored for atleastoneyeartobreakseeddormancy.TheStrigaseedswere mixedwithfinelysievedsandthatservedascarrierandensured uniforminfestation(Kim,1991).About50,000viableStrigaseeds wereappliedperm2toensureasufficientlyhighStrigapressure (Haussmannetal.,2000).
2.2.3. Measuredvariables
Datawerecollectedatbothsitesonaplotbasisfortraitsrelated topearlmilletphenologicaland agronomicalcharacters,downy mildew(DM)incidence,andStrigaresistance.Thefloweringtime (FLO)indaysaftersowing(das)wasrecordedwhen50% ofthe plantshadexertedstigmas.Duringharvest,thenumberof produc-tivepanicleswascountedandweighedtodeterminethepanicle yield (HYD) (gm−2).The number of hills per plotwith downy mildewwasrecordedattillering,verified/adjusted after flower-ingandusedforcalculatingthedownymildew(DM)incidence(in percentage):numberofinfestedhills,dividedbythetotalnumber ofhillsafteremergence,multipliedby100.Inaddition,thenumber ofemergedStrigaplantsforeachplotwasrecordedat67,101and 121das.SuccessiveStrigacountswerethenusedtocalculatethe “AreaunderStrigaNumberProgressCurve”(ASNPC)(Haussmann etal.,2000): ASNPC= n−1
i=0 Yi+Y(i+1) 2 (t(i+1)−ti)wherenisthenumberofStrigaassessmentdates,YitheStrigacount attheithassessmentdate,tithedaysaftersowingattheith assess-mentdate,t0is0,andY0is0.LowASNPCmeansvaluesindicate resistance,andhighvaluessusceptibilitytoStriga.
2.2.4. Statisticalanalysis
Analysisofvariance(ANOVA)wasperformedforallmeasured traitsfollowingtheincompleteblockdesign.Datafromthetwo locations(SadoréandCinzana)wereanalyzedseparatelyandthen theadjustedmeansofeachenvironmentwereusedforcombined analysis.ThestatisticalsoftwarePLABSTATv3A(Utz,2005)was usedfordataanalysis.Thediscretedownymildewdatawere sub-jectedtoalog-transformationtomeetANOVAassumptions.The binomialdistributionwiththeLogitfunctionwasusedfor analyz-ingthetransformeddownymildewdata.Thegeneralizedlinear model(GLM)commandimplementedintherestrictedmaximum likelihood (REML) package from theGenStat 14thEd. software (Payneetal.,2011)wereusedfortheanalysis.Back-transformed meanswereestimated.Allsourcesof variation,entries, replica-tionsandincompleteblockswithinreplications,wereconsideredas randomeffects.Forfurtheranalysis,theC5full-sibfamilies (C5-Full-sib),genepoolparent(Parent),agronomiccontrols(Control)and experimentalvarieties(EVs)wereseparatedintodifferentgroups. t-testswereperformedforpairedgroupmeanscomparisonsby dividingthemeansdifferencewiththeirstandarderrorof differ-ence.Theselectiondifferential(S)isthedifferencebetweenthe meanofselectedunitsandtheoverallmeanofthebase popula-tion.Estimatesofbroad-senseheritability(h2)onaplotbasisin areplicated trialwerecalculatedforalltraitsfollowing the for-muladescribed byHaussmannetal.(2000).Predictedgain was calculatedbymultiplyingheritabilitybytheselectiondifferential
(Falconer,1981).Thepercentagegainpercycle(G%)wascalculated followingtheformula:
G%=
( ¯Xn− ¯X0)/ ¯X0
×100n ,
whereG%istheaveragepercentagegainpercycle, ¯Xnisthemean atlatestselectioncycle, ¯X0isthegenepoolparentsmean,andnis thenumberofselectioncycles.
3. Results
3.1. Assessmentofthegenepooldevelopment:S1vsFSselection Due to the genetic load of the pearl millet landraces used (which were never self-pollinated before), the comparatively poorerseedlingestablishmentoftheS1 materialsturnedoutto bea disadvantageintheS1 selection process,leadingtohigher experimentalerrors(lowerheritabilities)intheS1intheC0,C2and C4trials,especiallyforASNPCandDM(Table1).IntheC0 evalua-tion,S1progeniesfloweredthreeweekslaterthantheFS.Thiswas duetopoorerS1 plantestablishmentafteraseveredroughtthat occurredafterplanting.Relativelyhighpanicleyield(484gm−2) wasrecordedinC3intheFS,whichwasduetothefavorable2009 rainyseasonconditions.SincebothS1andFSprogenieswere eval-uatedinseparatefieldtrials,andsinceboth S1 andFSselection schemeswerecombined,arealcomparisonofmeansandselection efficienciesisnotpossibleinthisstudy.
3.2. Entrymeans,geneticdifferentiationandcorrelationsamong traits
Meanperformance of the200C5-FSfor panicle yield (HYD) amountedto151gm−2atSadoré,whichwashigherthanthemean panicleyieldrecordedatCinzana(130gm−2)andacrossthetwo sites(141gm−2)(Table2).MeanvaluesfortheareaunderStriga numberprogresscurve(ASNPC)wereloweratCinzana(246.9)than atSadoré(1068.1)(Table2).Highlysignificantdifferences(P<0.01) weredetectedamongthegenotypesforfloweringtime(FLO), pan-icleyield(HYD),ASNPCanddownymildewincidence(DM)from thesingle-siteanalysesofvariance(Table2).Excellentplot-based heritabilitiesof84and76%wereestimatedforASNPCatSadoré andCinzana,respectively.
Resultsfromthecombinedanalysesofvarianceshowedhighly significantdifferences (P<0.01) among genotypes (G), environ-ments(E)(datanotshown)andgenotype-by-environment(G×E) interactionsforallthemeasuredtraits.G×Einteractionvariances (VG×E)werelargerthangeneticvariances(VG)fortheStrigaand downymildewresistance-relatedtraits. Incontrast,thegenetic variancecomponentofpearlmilletfloweringtimewashigherand theoneforpanicleyieldsimilartotherespectiveG×Evariance component(Table2).
Forthepresenttrial,estimatesofbroad-senseheritability(h2) fromthecombinedanalysiswerehighforfloweringtime(h2=71%) andpanicleyield(h2=50%).Moderateheritabilityvalues(34%and 38%)wereobservedforStrigaanddownymildewresistancetraits (Table2),duetothelargerG×Evariancecomponentsforthese traits.
Highpanicleyieldwassignificantlybutonlyweaklyassociated withresistancetoStriga(coefficientofcorrelationr=−0.38**and −0.15*)anddownymildew(r=−0.34**and−0.22**)forSadoréand Cinzana,respectively.Combinedacrossthetwosites,highpanicle yieldwasalsoweaklyandsignificantlyassociatedwithlater flow-ering.NeitherStrigaresistancenordownymildewresistancewere relatedtofloweringtime.
3.3. Testofselectionprogress
UnderSadoréconditions,themeanofthefull-sibfamilies(FS) createdfromtheC5selectioncyclewassignificantlylower(P<0.01) thanthemeanofthegenepoolparents(Parent)forbothStriga resis-tance(ASNPC:−51%)anddownymildewincidence(DM:−46%) (Table3).Inotherwords,theadvancedC5-FSpopulationpresents significantlybetterresistancetoStrigaanddownymildewthanthe genepoolparentalvarietiesfromwhichitwasderived.Panicleyield wassignificantlyhigher(+49%)intheFSprogeniescomparedto thegenepoolparentalpopulation.Similarly,significantselection gainswereobservedatCinzanaforpanicleyield(+31%)anddowny mildewincidence(−62%),butnotforStrigaresistance,wherethe C5-FSmeanforareaunderStriganumberprogresscurvewasonly 1%lowerthantheParentgroup,andtheobserveddifferencewas statisticallynon-significant.Atbothsites,nosignificantdifference wasdetectedforfloweringtime betweenFSandParent groups (Table3),indicatingthattheselectionfortheothertraitsdidnot resultinfloweringtimechangesinthegenepool.
TheaveragerealizedpercentagegainfromselectionforStriga resistance(ASNPC)was−10%cycle−1atSadoréand−0.3%cycle−1 atCinzana,downymildewsusceptibilitydecreasedby9%cycle−1 atSadoréand21%cycle−1atCinzana.Theselectiongainforpanicle yieldwasequalatbothSadoréandCinzana,where10%cycle−1was quantified(Table3).
Under Sadoré conditions, most experimental varieties (EVs) derivedfromthegenepoolin2008(3rdselectioncycle)or2009 (4thselectioncycle)presentbetterresistancetoStrigaanddowny mildewthanthegenepoolparentalpopulation,exceptEV09-Cin (forASNPC),EV08-Sad-SPandEV09-Comb(forDM)(Table3).All EVswerehigheryieldingcomparedtothegenepoolparents,except forEV08-Sad-SP.AtCinzana,thederivedvarietiesEV08-Sad-SP, EV09-CinandEV09-CombshowedlowermeanforASNPC,while allEVsweremoreresistanttodownymildewandhigheryielding comparedtotheParentgroup,exceptforEV09-Cin(forHYD).At bothsites,onlynon-significantchangeswereobservedbetween EVsandgenepoolparentalmeansforfloweringtime.Underboth sites,theControlgroupsupportedthehighestStrigadamageand
Table2
SinglesiteandcombinedanalysisofvarianceresultsfromtheC5-Full-sibpopulationevaluatedatSadoréandCinzanaduringthe2011rainyseason.
Trait Sadoré Cinzana Combinedacrosstwosites
Mean VG h2(%) Mean VG h2(%) Mean VG VG×E h2(%)
ASNPC 1068.1 231.7** 83.5 246.9 11.1** 76.2 656.4 27.5** 100.4** 34.1
HYD(gm−2) 151.2 971.6** 36.1 130.3 310.3** 21.5 140.7 334.4** 317.5** 49.8
DM(%) 20.1 213.4** 42.9 6.7 90.8** 53.7 13.4 51.1** 119.1** 38.3
FLO(das) 71.1 9.5** 48.6 51.6 6.0** 38.6 61.3 5.9** 2.5** 70.6
TheanalysisofvarianceisperformedonareaunderStriganumberprogresscurve(ASNPC),panicleyield(HYD,gm−2),downymildew(DM)incidence(%)andtimeto50%
flowering(FLO)indaysaftersowing(das).
TheanalysiswasperformedusingPLABSTAT.Foralltraits,genetic(VG)andG×Einteraction(VG×E)variancecomponentsarehighlysignificant.Basicdatadescription(trial meansandheritability)aregivenforeachselectedtrait.
Table3
Realizedresponsetoselection:estimatedmeansofgenepoolparents,C5-Full-sib(FS)population,newexperimentalvarieties(EVs)extractedfromthegenepoolandthe
controlgroup,evaluatedduringthe2011rainyseasonatSadoréandCinzana.
Trait† Location Meanperformance‡
Genepool parents
C5-Full-sibpopulation Realized
gain(%)
Gainper cycle(G%)
EV08-Sad-LP EV08-Sad-SP EV09-Sad EV09-Cin EV09-Comb Control
ASNPC Sadoré 1969.4 965.2** −51.0 −10.2 611.3 687.3 262.1 1019.0 601.3 3447.3 Cinzana 224.5 222.5ns −0.9 −0.3 301.6 202.6 242.6 86.2 186.7 993.1 HYD(gm−2) Sadoré 105.0 156.0** 48.6 9.7 152.4 105.5 125.9 131.5 162.2 80.1 Cinzana 101.8 133.3** 30.9 10.3 118.2 108.2 122.7 93.8 111.6 89.8 DM(%) Sadoré 36.2 19.5** −46.1 −9.2 8.4 33.5 19.6 15.5 30.8 21.1 Cinzana 15.6 5.9** −62.2 −20.7 0.0 7.8 3.7 3.2 7.5 19.0
†ASNPC=areaunderStriganumberprogresscurve;HYD=panicleyield(gm-2);DM=downymildewincidence(%);EV=Strigaresistantexperimentalvariety;08/09=year
ofextractionfromthegenepool;Sad/Cin/Comb=selectionbasedonperformanceatSadoré,Cinzanaandcombinedacrossthetwolocations.
‡**andns:Differencetogenepoolparentssignificantbyt-testat1%andnon-significancefortheC
5-Full-sibpopulationvsgenepoolparentsmeanscomparison.
lowestpanicleyield,whileitsperformancewasbetteratSadoré fordownymildewresistancecomparedtothegenepoolparental means(Table3).
3.4. Predictedresponsefromfutureselection
Considering the200C5-FS asthebasepopulationfor future selection,thesignificantgeneticvariationandmoderatetohigh heritabilitiesforthetargettraitsatbothsitesindividuallyandin thecombinedanalysisshouldenablefurtherpositiveresponseto selection.
Predictedgainscycle−1 forStrigaresistance(recordedonthe areaunderStriganumber progress curve)were −572,−81and −122forSadoré,Cinzanaandacrossthetwosites,respectively, andrangedfrom−3%to−6%fordownymildewincidence(Table4). Forbothtraits,theexpectedgainscycle−1atSadoréwere remark-ablyhighercomparedtothetwootherconditions.Predictedgains cycle−1forpanicleyieldweresimilarbetweenSadoréandacross thetwosites(12gm−2cycle−1).Comparativelysmallerbut posi-tivegeneticgainsforresistancetoStrigaanddownymildewandfor increasesinpanicleyieldmaythereforebeexpectedunderCinzana conditions(Table4).
Resultsfromthecombinedanalysesofvariancerevealedahigh amountofG×Einteractionvariance especiallyforresistanceto Strigaanddownymildew,butalsoforpanicleyieldandlower her-itabilitiesforASNPCandDMcomparedtothesingle-siteanalysis (Table2).Thesesuggestthatselectionforspecificadaptationto eachofthetwotestsitesmaybemorepromisingthanselection forwideadaptationacrossthesetwosites.Thismeansthat dif-ferentFSfamiliesneedtobeselectedandrecombinedforthetwo
contrastingsitesSadoréandCinzanatomaximizegainsfrom selec-tionandtoderivethebestvarietiesforeachlocation.
4. Discussion
4.1. Recurrentselection:aneffectivetoolforenhancingStriga resistanceinpearlmillet
Recurrentselectioninbroad-basedpopulationscanbeusedto enhancequantitativeresistancetoatargetpestordisease.Such quantitativeresistance,whichisduetomanygenelociwithsmaller effects,isusuallymoredurablethanmonogenic,qualitative resis-tance(Berneretal.,1995;MenkirandKling,2007).Accordingtoour knowledge,thisisthefirstreportdescribingsignificantprogress fromrecurrentselectionforquantitativeStrigaresistanceinpearl millet.Simultaneously,Strigaresistance,downymildewresistance andpanicleyieldwereimprovedwhilemaintainingfloweringtime whichdeterminesgeneraladaptationtothetargetsites.The rig-orousevaluationunderfieldconditionsusingfourreplicatesand ASNPC as combined measure for Striga resistance (resultingin lowerexperimentalerrorandhigherbroad-senseheritabilities), and thelargenumber of progenyevaluated in each generation seemtobeeffectiveforimprovingthegenepool.Self-pollinated progenyevaluationisexpectedtoleadtobettergenetic differen-tiation,buttheS1 evaluation partiallysufferedfrompoorplant establishment(seedlinginbreedingdepression),resultinginlower heritabilitiesinsomeseasons.TheFSselection,ontheotherhand, haslessprobabilitytoidentifyandmoveforwarddesirable reces-sivealleles compared toS1 selection, but hastheadvantageof thetestentriesbeingintheirnatural,highlyheterozygousstate.
Table4
Predictedfuturegainfromselection.
Traits† Location C5-Full-sib
population mean
SelectedFS familiesmean
Selectiondifferential Heritability Predictedgain
ASNPC Sadoré 965.2 284.1 −681.1 0.84 −572.1 Cinzana 222.5 115.4 −107.1 0.76 −81.4 Across-sites 593.9 235.1 −358.8 0.34 −122.0 HYD(gm−2) Sadoré 156.0 188.2 32.2 0.36 11.6 Cinzana 133.3 148.5 15.2 0.22 3.3 Across-sites 144.7 168.1 23.4 0.50 11.7 DM(%) Sadoré 19.5 5.2 −14.3 0.43 −6.1 Cinzana 5.9 1.3 −4.6 0.54 −2.5 Across-sites 12.7 4.4 −8.3 0.38 −3.2
FLO(das) Sadoré 71.3 71.0 −0.3 0.49 −0.1
Cinzana 51.6 51.0 −0.6 0.39 −0.2
Across-sites 61.4 61.2 −0.2 0.71 −0.1
Sincebothselectionschemeswerecombinedwhichsavedtime(no additionalcycleneededforrecombinationofS1s),amoredetailed comparisonofselectionefficienciesinS1vsFSselectionisnot pos-siblein thisstudy.Based onthepresent findings,therecurrent selectionobjective–improvingtheStriga-resistantgenepoolmean whilemaintainingthegeneticvariability–islargelyattainedby thefifthselectioncycle,especiallyatSadoré.Differentrecurrent selectionmethodshavebeenreportedbyRattundeandWitcombe (1993)toimprovesubstantiallygrainyieldandresistancetodowny mildewinfivepearlmilletcomposites.Theseresultsareinlinewith ourfindingswhichequallyrevealedlargegainsoverfivecyclesof selection.Also,recurrentselectionhasbeenusedsuccessfullyin pearlmillettoimprovedownymildewresistance,grainyieldand otheragronomictraits(e.g.,WeltzienandKing,1995;Goviletal., 1982;BidingerandRaju,2000;Baskaranetal.,2009).InNigeria, recurrentselectionofmaizeforgrainyieldandStrigaresistance hadbeenreportedtobesuccessful(Badu-Apraku,2010;Menkir andKling,2007).
Thelowermeasuredresponsefromselectionfor Striga resis-tanceatCinzanamaybeattributedtothefollowingfactors: -predominanceofselectionatSadoré,i.e.,lowernumberandlater
startofthegenepoolevaluationsatCinzana; -selectionofthegenepoolparentsmainlyfromNiger;
-significantG×EinteractionforASNPC acrossSadoré and Cin-zana,combinedwithpredominanceofselectionatSadorémight haveresultedinspecificadaptationofthegenepooltotheStriga populationatSadoré;
-fluctuations inselection pressure, resulting fromyear-to-year variation in the level of parasiteinfestation, especially under theconditionsof naturalStriga infestationaswasthecase at Cinzana(artificial Strigainfestationmight thereforebe gener-allypreferable,butrequiresstrictisolationtopreventanyStriga seedtomigrate;butsuchisolationwasnotpossibleatCinzana; obviously,artificialinfestationcannotberecommendedif farm-ers’fieldsareusedduringevaluation–herecarefulfieldselection inthepreviousyeariskeytosuccess);
-thelowStrigainfestationlevelatCinzanaduringtheevaluation ofselectionprogress,whichmighthavebeensub-optimalfor dif-ferentiationamongthetestedentries–butthisislessprobable sincetheestimatedplot-basedheritabilitywassufficientlyhigh atCinzanaduringthetestyear.
4.2. FutureusesoftheStriga-resistantpearlmilletgenepool Inthepresentstudy,markedpredictedresponseswere esti-matedfor alltarget traits,suggesting that enhanced Striga and downymildewresistance,aswellasincreasedpanicleyieldare expectedalsointhenextcycleofselection.Thefactthatuseful geneticvariationstillexistsinthepopulationmaybeduetothe introgressionoffurtherresistantgermplasminoneselectioncycle andmaintenance of alargeenougheffective populationsize in thegenepoolineach selectioncycle.Thebroad-basedgenepool can therefore serve as base population for furtherselection in collaborativebreedingprogramsandforextractionofnew exper-imentalvarietieswithspecificadaptationtodifferenttargetsites. Experimentalvarietieshave beenand are beingextractedfrom thegenepool for Sadoré and Cinzana, and are being also vali-dated inon-farm trials.The genepoolmayalsobesharedwith otherpearlmilletbreedersinWestAfrica,especiallyBurkinaFaso, Nigeria,andSenegal,sothattheresistanceisvalidatedandnew experimentalvarietiesareextractedfornewtargetsitesinthose countries.Suchcultivarswithquantitativeresistancemayplayan importantroleinintegratedStrigacontrolinpearlmilletinWest Africa.
4.3. Needforselectionforspecificadaptation
Thepresence of significantG×E interactionfor Striga resis-tance and other traits in our study may be attributed to the differencesinenvironmentalfactorssuchassoiltype,amountof rainfall(SahelianvsSudanian),andputativedifferencesfor viru-lencebetweentheparasitepopulationsatSadoréandCinzana.The totalobservedG×Einteractionconsistsofinteractionsofthepearl milletgenepoolwithenvironmentalfactorsofthetestlocations, interactions betweenpearl millet genepool and S. hermonthica populations at the two test sites and the threefoldinteraction betweenpearlmilletgenepool,S.hermonthicapopulationand envi-ronmentalfactors,butthesedifferenttypesofinteractioneffects cannotbeseparatedinfieldtrialsduetoquarantineregulations. Onealternativewouldbetomovebeyondthetwosites(Cinzana andSadoré)andrathertestatnumerousenvironmentsinseveral countriesinordertoperformanAMMIanalysis,apowerfultool forunderstandingcomplexG×Einteractions.However,thefact thattheG×Einteractionvarianceswereconsiderablyhigherthan thegeneticvariancesforresistancetoStrigaanddownymildew suggeststhatbestFSfamiliesdifferbetweenthetwoevaluation environments,andthatthebestFSidentifiedateachsiteshouldbe recombinedseparatelytocreateexperimentalvarietieswith spe-cificadaptationtoeachsiteanditsbroaderenvironmentalcontext. AlsoBadu-Apraku (2010) foundsignificant G×E interaction forgrainyieldandStrigaresistanceparametersinanextra-early maturingmaizepopulationinthreetestlocationsinNigeria,and Haussmannetal.(2001b)reportedhighlysignificantG×E inter-actionforgrainyieldandareaunderStrigaseverityprogresscurve (ASVPC,ameasureofStrigaemergenceandvigorthroughoutthe season)inAfricansorghumtrials.Bothstudiesthereforealso sup-porttheneedtoselectforspecificadaptationinStrigaresistance breeding,especiallyifthetargetsitesdiffersubstantiallyfromeach otherandrepresentputativedifferentStrigaecotypes.
4.4. FutureprospectsforStrigaresistancebreedingand managementinpearlmillet
Intra-speciesgeneticdiversityhasbeenreportedinS. hermon-thica(Yoshidaetal.,2010), indicatingthattheparasiteishighly variable,andthereforerespondstoselectionpressurerapidly.The complexnatureofthepathosystemandhighadaptabilityofStriga toachanginghostpopulationlikelyrequiresthatStriga-resistant varietiesbemanagedand combinedwithintegrated Striga con-trolmeasures,tohindertheparasitetoovercometheresistance. Genetically heterogeneous open-pollinated varieties where dif-ferentplantscarrydifferentresistanceallelesmaycontributeto stabilityofresistanceovertime,becausetheyresultinnon-uniform selectionpressureontheparasite.Maintenancebreedingofsuch Striga-resistant open-pollinated varieties may need to be done underStrigainfestedconditions,inordertomaintainandimprove theirresistance.Thisisespeciallytrueincasethereareany“costsof resistance”thatleadtolowerperformanceofresistantplantsinthe absenceofStriga.Toourknowledge,anycostsofStrigaresistance –ifexistentatall–arenotyetwellunderstood.
Severalmechanismshave beenimplicatedinloweringStriga parasitism,includinglowgerminationstimulantproduction(Hess etal.,1992;EjetaandButler,1993;Haussmannetal.,2001a;Jamil etal.,2011),reducedcapacitytoelicithaustorialinductionofStriga (Gurneyetal.,2003;Richetal.,2004),mechanicalbarriers(Maiti et al.,1984; Gurney et al.,2006; Amusan etal., 2008; Yoshida andShirasu,2009),andhypersensitiveresponse(Mohamedetal., 2003;Cissokoetal.,2011).Ejetaetal.(2000)hadalsosummarized otherpotentialpost-germinationmechanismsofresistancethat impedeattachmentandemergenceofStrigaincrops.Whetherthe observedgainsinStrigaresistancewithselectioninthepearlmillet
Striga-resistantgenepoolareduetotheseorothermechanismsis yettobedetermined.AbetterunderstandingofStrigaresistance mechanismsinpearlmilletanddevelopmentofappropriate, cost-efficientscreeningproceduresforcomponenttraitscouldrender futurerecurrentselectionmoreefficient.
Thedevelopmentofamarker-assistedpopulationimprovement schemeforenhancingquantitativeStrigaresistanceinpearlmillet couldhelprenderselectionevenmoreeffective.Duetodecreasing genotypingcosts(Elshireetal.,2011),largernumbersofentries couldbe screened for markers linked to resistancealleles, fol-lowed byfield phenotyping of a selectedsubset ofthe entries, andthiswouldresultinanincreasedeffectiveselectionintensity comparedtophenotypicselectionalone.Theso-called“non-Striga years”(yearswithoveralllimitedStrigainfestation)thatoftenlead topoordifferentiationamongthetestedentrieswouldnolonger hinderselectionprogressasmuchastheydoatpresent,because selection wouldbe doneusingthemarkers. In years whenthe fieldphenotypingissuccessfulindifferentiatingthetestedentries, theresultscanbeusedtore-calibratethemarker-basedselection index.Effortsarepresentlyunderwayinourgrouptodevelopsuch amarker-assistedFSpopulationimprovementschemeforStriga resistanceinpearlmillet.
5. Conclusions
Fivecyclesof phenotypicrecurrentselectionin a diversified pearl millet genepool resulted in significant improvement for Strigaanddownymildewresistance,andincreasedpanicleyield. Significantgeneticvarianceandsufficientlylargebroad-sense her-itabilitiesintheC5populationindicategoodprospectsforfurther selection within and extraction of new Striga-resistant experi-mental varieties out of this genepool. Large G×E interactions observedinthepresentStrigaresistancetrialsunderlinetheneed formulti-locationtestingtodevelopnewvarietaloptionswith spe-cificadaptationtocontrastingtargetenvironments.Becauseofthe Strigavariability,resistantvarietiesneedtobecarefullymanaged andcombinedwithotherintegratedStrigacontrolmeasures,in ordertohindertheparasitetoovercomethedeployedresistance.
Acknowledgements
ThisworkhasbeenundertakenaspartoftheCGIARResearch Program onDrylandCereals. The financial support of the McK-nightFoundationisgratefullyacknowledged(ICRISATgrantsno. 06-14and10-134).ThisresearchwasconductedatICRISAT-Sadoré andIER-Cinzana;theauthorsexpresstheirappreciationtoBoubé Guida,HamadouBoubé,AdamaTraoréandallotherfieldassistants atSadoréandCinzanafortheirgreathelpwiththeexperiments. OurgratefulthanksareextendedtoDr.RogerSternandZobinou Mawusi(ReadingUniversity,UK)fortheirvaluablestatistical assis-tance,andtotheresourcepersonsofthee-writeshoporganizedby theCollaborativeCropResearchProgramoftheMcKnight Founda-tion.Finally,thispublicationwasmadepossiblethroughsupport providedbytheIRD-DSFtothefirstauthor.
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