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interacting f actors s uch as genetic ba ckground, intensity of h erbivory, resource l evel and
relative abundance of resistant plants (Verkaar 1987).
Studies on t he d ynamics of m ixed popul ations i nvolving r esistant a nd s usceptible
plants a nd t heir pos sible c hange from a pur e susceptible popul ation t o a pur e resistant
population are scant (Ramachandran et al. 2000). In addition, it is not clear if results from a
given i nterspecific cr oss can b e v alidated i n t he cas e o f o ther w ild s pecies w ith d ifferent
growth habit. Brassica juncea (2n=36, AABB), the wild brown mustard, is closely related to
B. napus (2n=38, A ACC), the oilseed r ape, and both are pol yploids. They easily intercross
and produce viable progeny (Frello et al. 1995; Song et al. 2010; Liu et al. 2010a). In this
study, w e a im at d etecting if th e f itness advantage of fered b y t he B t transgene t o w ild
populations of B. juncea varies according to its frequency within the population. Backcrossed
Bt and non-Bt hybrid progeny were employed to detect the vegetative and reproductive output
of m ixed popul ation w ith va rious pr oportions o f ne arly i sogenic r esistant a nd s usceptible
plants, and under no insect and insect pressure.
Material and methods
Plants
Seeds of wild brown mustard (Brassica juncea var. gracilis, 2n =36, AABB) originated from
a lo cal field c ollection p rovided b y Nanjing Agricultural U niversity (NAU). T ransgenic
oilseed r ape ( B. napus cv. W estar, 2n =38, A ACC) w as obt ained after t ransformation b y
pSAM 12 plasmid containing genetically linked GFP and Bt Cry1Ac cassettes (GT) regulated
by i ndependent C aMV 35S pr omoters ( Halfill et a l. 2001) . T he t hird g eneration of t he
progenies of the transgenic event 1 w as used in this study. The non-transgenic oilseed rape
parent (Westar), a spring type, was also employed. Transgenic and non-transgenic F1 hybrids
were formed from wild mustard as female fertilized by the transgenic and the Westar parent,
respectively. Backcrossed generations BC2 were obtained through pollinating mustard as the
female by pollen of hybrids.
Field experiment
Medium seeds (1.2mm