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When one of four leaves (1/4) was clipped, CP and NC plants produced similar silique
numbers, biomass and seed numbers (Fig. 3.2). Significant differences were observed when
plants suffered severe leaf damage, when one of two (1/2), two successive leaves of four (2/4)
and three of four (3/4) leaves were clipped. At the 3/4 clipping treatment, the average silique
number of NC was almost 5-fold higher (78±10) than CP (16±2) (df=1, F=9.42, P=0.022), the
biomass w as a lmost 3 -fold hi gher ( 9.72±0.39 v ersus 3.38± 0.22) (F=50.96, P =0.0004), a nd
seed num ber a lmost 6 -fold hi gher ( 504±84 v ersus 86± 12) ( F=6.12, P =0.048). T herefore,
silique number, biomass and seed number of CP plants decreased as the proportion of clipped
leaves increased.
DISCUSSION
Simulated herbivory pressure enhances the fitness of insect-resistant populations
Experiments w here s imulated he rbivory was employed t o m easure t he d ifferences b etween
transgenic a nd w ild r elatives ha ve a lready b een c arried out ( Sutherland e t a l. 2006;
Letourneau and Hagen 2009). Here, NC plants stand for a transgenic insect-resistant hybrid
population a nd C P f or a popul ation of a n i nsect-susceptible w ild r elative. A s ex pected,
clipping l eaves h ad a n egative i mpact o n t he p erformance o f Brassica j uncea: N C p lants
produced more siliques, higher biomass and more seeds than CP. Our results demonstrate that
simulated herbivory pressure can enhance the fitness of an insect-resistant population.
Most s tudies ha ve s hown a s tatistically s ignificant i ncrease i n s eed out put a nd f inal
biomass w hen G M Bt p lants w ere e xposed t o i nsect he rbivores or t o s imulated he rbivory,
compared to non-GM plants. Ramachandran et al. (2000) reported such an effect in seed yield
and biomass in transgenic oilseed rape infested by diamondback moth larvae in greenhouse
and field experiments. Sutherland et al. (2006) found no s ignificantly different fitness effect
when o nly cotyledons were d amaged m echanically o f B. napus and B. r apa and t heir F1
hybrids, w hile a s light e ffect upon f inal bi omass w as r ecorded. In a c omparison be tween
transgenic B t B. r apa and B. r apa × B .napus introgressed h ybrids i n greenhouse an d f ield
experiments, Moon et al. (2007) demonstrated no significant difference in seed output and dry
weight with low herbivore pressure. Letourneau and Hagen (2009) showed that seed output
and s ilique num ber of B.rapa were s ignificantly greater w hen p lants were p rotected f rom
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