UNIVERSITE DE BOURGOGNE THÈSE Yongbo LIU - Université de ...

moth l arvae s uffered hi gher de foliation a nd pr oduced l ess bi omass a nd s eed yield t han
transgenic oilseed rape in greenhouse and field experiments. These results demonstrate that
the f itness o f i nsect-resistant pl ants i s hi gher t han t hat of i nsect-susceptible one s unde r
moderate to high herbivore damage. High herbivore pressure might promote the invasion and
persistence o f t ransgenic ( Bt-transgene) i nsect-resistant p lants w ithin a w ild p opulation
(Vacher et al., 2004). Under these conditions, it is expected that the transgene could invade
the whole population.
Competitive i nteraction b etween i nsect-resistant a nd i nsect-susceptible pl ants a s t he
percentage of resistant plants increases
Our r esults s how t hat i nsect-resistant p lants ( simulated b y N C p lants) h ave a c ompetitive
advantage i n a m ixed p opulation, but t his a dvantage va ries w ith t he r elative pr oportion of
resistant p lants i n t he plots. A s t he pe rcentage of i nsect-resistant pl ants i ncreased, t he
performance of both susceptible (simulated by CP plants) and resistant plants decreased under
the hi gh he rbivory pr essure s imulated i n t he f irst e xperiment ( Figure 3 .6). T he e quivalent
slopes indicate that fitness of the resistant plants relative to the susceptible plants increased
geometrically with their increasing representation in the population. The difference between
NC and CP plants in seed weight, biomass, and viable seed number increased as the combined
effects of competition and defoliation increased, which suggests that these combined effects
are additive (in the sense of Weis & Hochberg, 2000; Figure 3.6). As insect-resistant plants
suffered no d amage, t hey would ha ve a ccess t o resources t hat t heir i nsect-susceptible w ild
counterparts were unable to exploit. Low resource availability exacerbates the misfortune of
susceptible pl ants. W eis & H ochberg (2000) d emonstrated t hat c ompetition m agnified t he
effect of herbivore pressure on the relative biomass advantage of resistant plants. Vacher et al.
(2004) r eported t hat hi gh-density p atches of highly da maged w ild p lants a re t he m ost
vulnerable to Bt-transgene invasion.
Allocation of resources to reproduction was not affected by simulated herbivory, but it
decreased from the plot center to the plot border due to the competitive interaction among NC
and C P pl ants f or a vailable r esources ( Figure 3.6). T his m eans t hat p roportionally m ore
resources w ere i nvested i n pr oducing pr ogeny in h arsh c onditions, a nd t hat r esource
availability may limit plant plasticity. However, Rodriguez and Brown (1998) found that slug
herbivory reduced reproductive investment in Poa annua in a greenhouse experiment. Other
variable cas es h ave b een r eported i n t he l iterature. In a pur slane ( Portulaca ol eracea)
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