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

Abstract
In the framework of commercial release for transgenic crops with novel traits, consequences of gene
flow and introgression are still one main concern. I explored the ecological and evolutionary
consequences of gene flow between oilseed rape (Brassica napus) and its wild relatives, brown
mustard (B. juncea) and wild radish (Raphanus raphanistrum), through several experiments carried
out in greenhouse, common garden and field in Beijing and Dijon.
First, I revised a comprehensive review of the literature about gene flow and its effect on
plant fitness in the Brassiceae. Second, I investigated the effects on gene flow of seed size of hybrids
between Bt-transgenic oilseed rape and mustard. Small seed size significantly reduced plant growth
and reproduction, but its influence on plant fitness varied among genetic backgrounds. Backcrosses
to oilseed rape were easier and more productive than other types of progeny of hybrids. Most of
these plants exhibited oilseed rape morphology. Together with herbicide-resistance, this trait could
help the progeny to survive in the field and disseminate the transgene to volunteers and feral
populations, which could be more troublesome than completing introgression into the genome of
the wild parent species. Third, I simulated herbivory on mustard to study the competition between
insect-resistant and susceptible plants independently to the fitness of the interspecific hybrid.
Resistant plants held a competitive advantage under herbivory pressure, and this advantage was
magnified in harsh conditions, such as low resources and high simulated herbivory pressure. The use
of insects to attack mixed populations composed of transgenic Bt-resistant and susceptible
backcrosses confirmed the same conclusion and provided evidence of no cost due to the transgene
in the absence of insect. The overall population production increased with the increasing proportion
of insect-resistant plants in the presence of insects. Fourth, wild radish populations were sampled
from four geographically distant regions, of which one region had a long history of oilseed rape
cultivation, and, therefore, higher chance to have been submitted to interspecific hybridization with
the crop. Traits divergence and polymorphism in the putative introgressed populations could be
supported as alternate hypothesis to random variation, although the differences were not marked
enough to place these populations out of the range of variation described in wild radish.
These studies pointed out different factors that could enhance the risk of transgenic flow and
introgression from transgenic crops to wild relatives, and they must be taken into account in the risk
assessment of the use of GM crops: crop traits to identify the hybrid progeny and perform
monitoring, small seed size to account for seed dispersal and dormancy, and intensity of herbivory
and competition that magnify the fitness advantage of insect-resistant transgenic plants. However,
the impact of introgression to create super-weeds was not supported.
Key words: oilseed rape (Brassica napus), wild mustard (Brassica juncea), wild radish (Raphanus
raphanistrum), competition, fitness, genetically modified (GM), gene flow, introgression,
morphological traits, population dynamics, seed production, transgene
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