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the da maged nt rBC2 pl ants pos sibly i nducing defense m echanisms a nd m oved t owards
healthy and attractive trBC2 plants. Alternately, it c ould be a further evidence o f the lower
innate fitness of the transgenic plants, leaving more and more resources to their neighbor.
Population production with varied proportion of transgenic plants
Population production, biomass and seed output, increased with the percentage of transgenic
BC2 in populations under insect pressure, perhaps because of the generalized protection effect
as proposed above. It was not the case when the populations were not subjected to herbivory,
again w ith a m aximum of s eed pr oduction at a 50% pr oportion of t ransgenic pl ants. Th e
increased population production in the presence of insects is important to predict the increase
of the colonizing ability of the B. juncea populations hosting more and more insect-resistant
plants. F ew s tudies f ocused on the d ynamics of resistant pl ants i n s usceptible popul ations,
although some studies showed that herbivory strongly affected population dynamics of plants
(e.g. P almisano a nd Fox 1997) . R amachandran e t a l. (2000) s howed t hat i nsects d ecreased
total biomass but did not affect seed yield in mixed and pure stands of Bt-transgenic and/ or
non-transgenic oi lseed r ape. T hey found t hat no s ignificant di fferences oc curred i n t otal
biomass and seed production when there was no i nsect, and that biomass and seed yield was
significantly different among mixed plantings for insect-infested plots with a maximum at the
proportion of 50% i n t wo f ield e xperiments, but t hat i t w as not di fferent i n a nother f ield
experiment in the presence of insects. In our e xperiments simulating insect-resistant plants
through clipping leaves, the increase of healthy plants in a population of damaged plants did
not increased the population production but resulted in a maximum seed output when healthy
plants dom inated t he popul ation ( 75% of he althy plants) (Liu e t a l.2010b). E nhanced s eed
production c ould m ake a popul ation m ore a ble t o s pread t o ne ighboring ha bitats, e venly
displacing other species suffering of insect damages. Thus, the occurrence of insect attack is
the major determinant of the fate and dispersal of the transgene: while pests are over-frequent
in ar able l ands co vered b y the s ame cr op o ver h ectares, t he o ccurrence o f s uch i nsect
infestations is not so permanent in wild habitats, which could seriously hamper the spread of
the Bt transgene in and among wild populations, as observed in the case of virus-resistance
genes i n s quash ( Fuchs e t a l 2004a ; 2004b) . Laughlin et a l. ( 2009) showed t hat vi rusresistance
transgenes and conventional crop genes can increase fecundity of wild plants under
virus pressure, and viruses play a role in the population dynamics of wild squash (Cucurbita
pepo).
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