Virus-Resistant Transgenic Horticultural Crops: Safety Issues and Lessons from Risk Assessment Studies - Transgenic Horticultural Crops: Challenges and Opportunities - page 273

Field.trials.relating.to.gene.low.from.virus-resistant.transgenic.summer.squash.to.
a.free-living.relative.( C. pepo .ssp.. ovifera .var.. texana ).and.the.persistence.of.trans-
genes.among.hybrids.of.transgenic.and.free-living. C. pepo .have. been.carried.out..
Gene.low.occurred.with.sympatric.populations.(populations.of.related.species.exist-
ing.in.the.same.geographic.area).under.conditions.of.low.disease.pressure. 84 .Hybrid.
plants.containing.transgenes.were.likely.to.produce.more.fruit,.seed,.and.be.more.
vigorous. than. free-living. C. pepo . and. non-transgenic. hybrids. under. conditions. of.
high. disease. pressure. 85 . This. was. not. true. under. conditions. of. low. virus. pressure,.
where.free-living.plants.outperformed.the.transgenic.hybrids. 85 .These.results.clearly.
indicated.the.advantage.that.the.transgenes.might.provide.under.conditions.of.high.
disease.pressure,.although.it.is.not.clear.whether.this.poses.a.signiicant.risk.in.terms.
of.population.dynamics..Surveys.of.free-living. C. pepo .for.viruses.in.areas.where.
transgenic.summer.squash.had.not.yet.been.released.showed.an.extremely.low.inci-
dence.of.viruses,.including.CMV,.ZYMV,.and.WMV. 86 .These.results.suggested.that.
viruses.have.a.limited.effect.on.the.dynamics.of.free-living. C. pepo .populations. 86 .
Also,. the. studies. on. gene. low. and. its. consequence. in. squash. do. not. necessarily.
indicate.any.risk.of.engineered.virus.resistance.beyond.that.of.conventionally.bred.
resistance.genes,.as.far.as.free-living.populations.are.concerned. 2
An. additional. study. by. Fuchs. et. al. 4 . investigated. the. likelihood. of. transencap-
sidation. in. transgenic. squash,. tomato,. and. melon,. so. as. to. allow. the. transmission.
of. an. aphid. non-transmissible. strain. of. CMV. by. aphids. through. interaction. with.
the.coat.protein.transgene.derived.from.an.aphid.transmissible.strain.of.CMV..The.
results.of.this.study.failed.to.demonstrate.that.transencapsidation.could.occur.over.
two. consecutive. growing. seasons. 4 . However,. transencapsidation. of. an. aphid. non-
transmissible.strain.of.ZYMV.likely.occurred.in.transgenic.squash.expressing.the.
coat.protein.gene.of.an.aphid.transmissible.strain.of.WMV.at.a.very.low.rate.and.
without.triggering.an.epidemic. 87
Similarly,.virus-resistant.transgenic.summer.squash.had.no.effect.on.the.genetic.
diversity. of. CMV. strains,. suggesting. that. these. plants. did. not. facilitate. the. emer-
gence.of.recombinant.viruses. 88
p apaya r eSIStant to P APAYA r ingsPot V irus
Background
An. early. success. story. in. the. development. and. commercialization. of. virus-resistant.
transgenic. fruit. crop. plants. is. the. case. of. papaya. resistant. to. PRSV. 6 . This. virus.
causes. one. of. the. most. devastating. viral. diseases. of. papaya.. There. is. no. practical.
resistance.known.to.PRSV.in. Carica .germplasm;.therefore,.control.of.this.virus.has.
relied. on. exclusion,. movement. to. new. growing. regions. where. PRSV. is. not. found.
(often. involving. the. destruction. of. native. rainforest. habitats),. and. the. use. of. mild.
isolates.of.PRSV. in.attempts.to.control.the.disease.via.cross-protection. 6 .PRSV.is.
an.aphid-borne.potyvirus.and.can.readily.spread.over.long.distances.by.its.vectors..
Papaya.is.grown.in.tropical.and.semitropical.regions.and.Hawaii.is.by.far.the.largest.
producer.of.papaya.in.the.United.States..The.production.center.for.Hawaiian.papaya.
is.in.the.Puna.district.of.the.island.of.Hawaii..Production.from.this.district.comprises.