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

apparent o nly with m ustard at l ow d ensity. S imilarly, s mall s eed s ize d ecreased t he s eed
number of GT at high density but not at low density. Plant density could cause variation of the
influences o f seed s ize on pl ant growth a nd r eproduction ( Gardner & V anderlip 1989) .
Obviously, the strong competition at high density decreased plant fitness, although the effect
varied through different plant types and seed categories. GT was the plant type more liable to
be affected by plant density and seed category, less for ntrF1 and mustard, while trF1 was not
affected.
Influences of seed size on plant fitness varied with genetic background
Seed size did not impact the growth and reproduction of trF1, and it also did not affect
the biomass and seed output in ntrF1 and wild mustard, with the exception of flowering time.
However, s eed s ize s ignificantly a ffected t he f itness of G T. T he s mall-seeded pl ants of
transgenic F 1 did not show lower fitness compared to larger-seeded p lants o f t ransgenic F1.
This i s pe rhaps due t o t he da te of t he m easurements, w hich w ere t aken a t m aturity. S ince
hybrids p roduce f ew s eeds, t heir growth c an s urpass t hat of m ustard a nd O SR, w ith hi gh
biomass and high per-seed weight, thus hindering any initial difference at the early stage of
life. This is not the case for mustard and OSR that produced copious amounts of seed and thus
ending the vegetative stage as compared with hybrids.
There were no significant differences between trF1 and ntrF1 in seed set, which indicates
costs associated with the transgene were probably negligible here. Fitness cost of transgene
expression w as obs erved t o be a ssociated with he rbivore r esistance i n s ome reports
(Bergelson & Purrington 1996; Strauss et al. 2002). Fitness costs depend on not only genetic
background i n w hich r esistance t raits oc cur, b ut a lso on the e nvironment i n w hich t he
comparisons between resistant and susceptible plants are made and on the ecological context
in which costs are measured (Bergelson & Purrington 1996). In this study, transgenic F1- and
non-transgenic F 1- plants w ere obt ained w ith t he s ame ge netic ba ckground. M ason et a l.
(2003) found no fitness cost of Bt- transgene in transgenic B. napus under low and high insect
pressure. Snow et al. (2003) also found that Bt transgene was n ot associated with a fitness
cost in sunflower under insect pressure, and transgenic plants produced more seeds since they
experienced r educed he rbivory. M oreover, s mall-seeded pl ants of t ransgenic F 1 plants ha d
high ve getative pe rformance, w hich s uggests Bt resistance p lant mig ht e xhibit s tronger
competitive cap acity and w ould co unterbalance t he d isadvantage o f s mall s eed s ize, since
45