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interspecific h ybridization, ba ckcrossing a nd nu merous ge nerations w here a h ybrid f itness
penalty could limit the spread of the resistance.
Intensity of s imulated herbivory a mplifies t he f itness ad vantage of an i nsect-resistant
population
The C P a nd N C pl ants di d not s how s ignificantly di fferent m ean va lues w hen one o f f our
leaves w ere cl ipped. As t he pr oportion o f c lipped l eaves i ncreased, lower growth a nd
reproduction were obs erved f or C P pl ants. T he absence of a few l eaves di d not a ffect t he
performance o f B. j uncea, w hile m any c lipped l eaves obvi ously l imited i t. T hree of f our
leaves clipped might be too serious. At the 3/4 clipping treatment, the average values of NC
plants were about 5-fold higher than CP. Because clipping leaves limited the growth of CP
plants, N C c ould ut ilize m ore of t he r esources in t he pot . CP pl ants s howed s ignificantly
different silique numbers and biomass among the treatments of four different proportions of
clipped leaves.
Consequently, the difference between insect-resistant hybrids and related wild plants
was amplified as the intensity of herbivore pressure increased because of the appropriation of
their neighbors’ resources. Most studies suggest that moderate to high herbivore or simulated
damage pr oduces a n i ncrease i n pl ant f itness ( Vacher et a l. 2004; S utherland e t a l. 2006;
Moon et al. 2007). The high herbivore pressure might promote the invasion and persistence of
transgenic ( Bt transgenes) i nsect-resistant h ybrids i n m ixed s tands w ith a w ild popul ation.
That m ight be on e o f t he r easons w hy even v ery low l evels of i ntrogression a nd s election
could lead to a high probability of fixing a transgene in a population (Meirmans et al. 2008).
Within-population competition magnifies the fitness advantage of insect-resistant plants
Among treatments of different NC percentages, CP plants showed significant differences for
the measured traits, while no difference was recorded for NC. Competition pressure from NC
plants significantly affected the development of CP plants, while CP plants did not affect NC
plants in the first experiment. The more NC plants there were in the pot, the less fertile, heavy
and r eproductive t he C P p lants w ere. Because i nsect-resistant h ybrids w ould s uffer l ess
damage, t hey would h ave acc ess t o resources t hat t heir i nsect-susceptible wild counterparts
would have otherwise exploited. As a result, resistant plants not only escape herbivore attack,
but t hey also c apitalize on t heir ne ighbors’ m isfortune. T hus, t he r elative f itness of i nsect-
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