生命科学概论论文.docx

生命科学概论论文.docx

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生命科学概论论文

Anintroductiontolifescience

Courseassignments

BtandMonarch

Despitewidespreadadoptionofgeneticallymodifiedcropsinmanycountries,heatedcontroversiesabouttheiradvantagesanddisadvantagescontinue.Especiallyfordevelopingcountries,thereareconcernsthatgeneticallymodifiedcropsfailtobenefitsmallholderfarmersandcontributetosocialandeconomichard-ship.Manyeconomicstudiescontradictthisview,butmostofthemlookatshort-termimpactsonly,sothatuncertaintyaboutlonger-termeffectsprevails.So,BtornotBt,thatisaquestion.

Plantresistancetoinsectpestshasevolvednaturallyovermanymillionsofyearsandinvolves（i）bothconstitutiveandinduciblephytochemicalandmorphologicalmechanismsinplants,（ii）counteradaptationstoplantdefensesbytheherbivores,and（iii）bioticinteractionsofthemultitrophiclevelcommunitiesofinsectpathogens,parasites,andpredators（1–4）.Theoutcomesofsuchcomplexbioticinteractionsaresometimesdeterminedbylocalmosaicsofabioticenvironmentalconditionsorregionalclimatechangesthatdirectlyinfluencethecomponentparticipantsandtheirbehavioral,physiological,andgeneticadaptations（5–7）.Theintentionalselectionandbreedingofinsectandyorpathogen-resistantplantgenotypessuchascorn（Zeamays）hasresultedinslow,butsteady,progressagainststalk-boringlarvaesuchastheEuropeancornborerandothersuchspecies,andhostplantresistanceingeneralhassignificantlyreducedtheneedforbroad-spectruminsecticidesacrossagro-ecosystemsandforests（8,9）.Ithasrecentlybeenpossibletogeneticallyengineerplants,suchasZ.mays,toavoidproblemswithinsecticidalspraydriftandmoreefficientlyexpresstoxinsinparticularplanttissuesagainstlepidopterouspests.Suchefficient,effective,andrelativelysafetoolsforpestmanagementhaveeludedclassicalplantbreedersandintegratedpestmanagementpractitionersformanydecades（10）.However,eventhoughsafeformostothertaxonomicgroupsofinsectsotherthanLepidoptera,thenontargetimpactsofBtrequirecarefulandthoroughevaluationcombinedwithextensivepublicizing.Ifnotdone,therealrisks,whichmaybeminimal,canbesignificantlyamplifiedinthepublicmindsbyperceivedrisksanddistrustofbiotechcorporationsandtheregulatoryfederalagenciesandsimilargoal-affiliatedagriculturalresearchinstitutionsanduniversities（11,12）.AdditionalrisksforcroppestmanagementsuchastherapiddevelopmentofinsectresistancetotheBtinsecticidesalsoneedseriousattentionandconsiderationinlong-termagroecosystemdesignaswell（13,14）

Recentpublicconcernsoverthetrans-genic（geneticallymodified）plantsandnontargetimpactssuchasthosefromBt-toxinexpressingcornpollenonthemonarchbutterflypopulationshaveescalated,despitegoodpestmanagementintentionsandgoodscience（15–23）.Asinglelaboratorystudyonmonarchbutterflieshaspromptedwidespreadconcernthatcornpollen,engineeredtoexpressBacillusthuringiensis（Bt）endotoxin,mighttravelbeyondcornfieldsandcausemortalityinnontargetlepidopterans.Amongthelepidopteransathighpotentialriskfromthistechnologyistheblackswallowtailbutterfly,Papiliopolyxenes,whosehostplantsinthemidwesternU.S.arelocatedprincipallyinnarrowstripsbetweenroadsandcropfields.AfieldstudywasperformedtoassesswhethermortalityofearlyinstarblackswallowtailswasassociatedeitherwithproximitytoafieldofBtcornorbylevelsofBtpollendepositiononhostplants.PottedhostplantswereinfestedwithfirstinstarblackswallowtailsandplacedatintervalsfromtheedgeofafieldofBtcorn（Pioneer34R07containingMonsantoevent810）atthebeginningofanthesis.TheyconfirmedbyELISAthatpollenfromtheseplantscontainedCry1Abendotoxin（2.12560.289ngyg）.Althoughmanyofthelarvaediedduringthe7daysthattheexperimentswererun,therewasnorelationshipbetweenmortalityandproximitytothefieldorpollendepositiononhostplants.Moreover,pollenfromthesesameplantsfailedtocausemortalityinthelaboratoryatthehighestpollendosetested（10,000grainsycm2）,alevelthatfarexceededthehighestpollendensityobservedinthefield（200grainsycm2）.theyconcludethatBtpollenofthevarietytestedisunlikelytoaffectwildpopulationsofblackswallowtails.Thus,theirresultssuggestthatatleastsomepotentialnontargeteffectsoftheuseoftransgenicplantsmaybemanageable.

LaboratorytestswereconductedbyRichardLtoestablishtherelativetoxicityofBacillusthuringiensis（Bt）toxinsandpollenfromBtcorntomonarchlarvae.ToxinstestedincludedCry1Ab,Cry1Ac,Cry9C,andCry1F.Threemethodswereused:

（i）purifiedtoxinsincorporatedintoartificialdiet,（ii）pollencollectedfromBtcornhybridsapplieddirectlytomilkweedleafdiscs,and（iii）Btpollencontaminatedwithcorntasselmaterialapplieddirectlytomilkweedleafdiscs.BioassaysofpurifiedBttoxinsindicatethatCry9CandCry1Fproteinsarerelativelynon-toxictomonarchfirstinstars,whereasfirstinstarsaresensitivetoCry1AbandCry1Acproteins.Olderinstarswere12to23timeslesssusceptibletoCry1Abtoxincomparedwithfirstinstars.Pollenbio-assayssuggestthatpollencontaminants,anartifactofpollenpro-cessing,candramaticallyinfluencelarvalsurvivalandweightgainsandproducespuriousresults.TheonlytransgeniccornpollenthatconsistentlyaffectedmonarchlarvaewasfromCry1Abevent176hybrids,currently<2%cornplantedandforwhichre-registrationhasnotbeenapplied.ResultsfromtheothertypesofBtcornsuggestthatpollenfromtheCry1Ab（eventsBt11andMon810）andCry1F,andexperimentalCry9Chybrids,willhavenoacuteeffectsonmonarchbutterflylarvaeinfieldsettings.

ToassessthelikelihoodthatmonarchlarvaewillbeexposedtoBacillusthuringiensis（Bt）pollen,KarenS.etalstudiedmilkweedandmonarchdensitiesinhabitatswhichcomprisemuchofthelandavailabletobreedingmonarchs,e.g.,cornfields,cornfieldedges,otheragriculturalfields,andnonagriculturalareas,infourregionsofthemonarchbreedingrange.Theyfoundthatmonarchsusemilkweedincornfieldsthroughouttheirbreedingseason,andthatperplantdensitiesareashighorhigherinagriculturalhabitatsasinnonagriculturalhabitats.Asaresultoftheprevalenceofagriculturalland,mostofthemonarchsproducedintheupperMidwestarelikelytooriginateincornfieldsorotheragriculturalhabitats.Therewasagreatertemporaloverlapbetweensusceptiblemonarchsandcornanthesisinthenorthernthanthesouthernpartofthesummerbreedingrange,becauseofearlierpollenshedinthesouth.Theimportanceofagriculturalhabitatstomonarchproductionsuggeststhat,regardlessoftheimpactofgeneticallymodifiedcrops,agriculturalpracticessuchasweedcontrolandfoliarinsecticideusecouldhavelargeimpactsonmonarchpopulations.

AcollaborativeresearcheffortbyscientistsinseveralstatesandinCanadahasproducedinformationtodevelopaformalriskassessmentoftheimpactofBtcornonmonarchbutterfly（Danausplexippus）populations.InformationwassoughtontheacutetoxiceffectsofBtcornpollenandthedegreetowhichmonarchlarvaewouldbeexposedtotoxicamountsofBtpollenonitshostplant,thecommonmilkweed,Asclepiassyriaca,foundinandaroundcornfields.ExpressionofCryproteins,theactivetoxicantfoundinBtcorntissues,differedamonghybrids,andespeciallysointheconcentrationsfoundinpollenofdifferentevents.Inmostcommercialhybrids,Btexpressioninpollenislow,andlaboratoryandfieldstudiesshownoacutetoxiceffectsatanypollendensitythatwouldbeencounteredinthefield.Otherfactorsmitigatingexpo-

sureoflarvaeincludethevariableandlimitedoverlapbetweenpollenshedandlarvalactivityperiods,thefactthatonlyaportionofthemonarchpopulationutilizesmilkweedstandsinandnearcornfields,andthecurrentadoptionrateofBtcornat19%ofNorthAmericancorn-growingareas.Their2-yearstudysuggeststhattheimpactofBtcornpollenfromcurrentcommercialhybridsonmonarchbutterflypopulationsisnegligible

ConceptualModel.Riskassessmentrequiresknowledgeoffouressentialcomponents:

（i）hazardidentification,（ii）natureofdoseresponsetoatoxin,（iii）probabilityofexposuretoaneffectivedose,and（iv）characterizationofrisk（24）.ComponentsofariskassessmentapproachasappliedtothecaseofBtcornandmonarchbutterflyaredepictedinFig.1.Btproteinsexpressedincornplanttissuescanbringaboutspecificreactionsinthegutoflepidopteranlarvae（25）,includingnontargetlarvaethatconsumeBtcornpollen.Themagnitudeofthereactionwilldependonthetypeofproteinproducedbyvarioustransgeniceventso-fhybridBtcorn,theamountofproteinexpressedinpollengrainsfromdifferentevents,theamountofpollenconsumedbylarvaeofdifferentdevelopmental

stages,andthesusceptibilityoflarvaetotheBtprotein.ThatahazardmayexistwassuggestedbyLoseyetal.（26）.Characterizationoftoxiceffectsisnecessarytoestablishthefirstcomponentofrisk.LaboratoryandfieldassaysoflethalandsublethaltoxicityresultingfromexposuretodosesofBtpollenarerequiredtoestablishtoxicitythresholdsforcomparisonagainstthedoseencounteredwithintheenvironment.ThesetoxicitythresholdswillvarybasedonexpressionlevelsforindividualBtcorneventsinconjunctionwithenvironmentalfactorsdeterminingecologicalexposure.

OnlythreepapersconcerningtheimpactofBtcornpollenonnontargetLepidopterahavebeenpublished（26,33,34）before,andtheyarelimitedintheirapplicatio