The process of introgression between a transgenic crop modified for better agronomic characters and a wild relative could lead potentially to increased weediness and adaptation to the environment of the wild species. However, the formation of hybrid and hybrid progeny could be associated with functional imbalance and low fitness, which reduces the risk of gene escape and establishment of the wild species in the field. Our work compares the fitness components of parents and different types of backcross in the sixth generation of hybrids between transgenic oilseed rape (Brassica napus, AACC, 2n = 38) resistant to the herbicide glufosinate and wild radish (Raphanus raphanistrum, RrRr, 2n = 18). The backcross with oilseed rape cytoplasm (OBC) has a fitness value 100 times lower than that of the backcross with wild radish cytoplasm (RBC). The herbicide-resistant RBC has similar growth to the susceptible RBC, but final male and female fitness values are two times lower. In turn, susceptible RBC exhibit similar fitness to the control wild radishes. The relative fitnesses of the different types are the same whether or not they grow under competitive conditions. The consequence on fitness of the chromosome location of the transgene conferring resistance and the relevance of these results to the impact of gene flow on the environment are discussed.

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The effect of two-component rice cultivar mixtures on the control of rice blast disease was studied in three different experiments under rainfed upland conditions in the Madagascar Highlands. The mixtures involved a susceptible cultivar (either susceptible or very susceptible) and a resistant cultivar in different mixture arrangements (random or row mixtures) and with different proportions of the susceptible cultivar (50, 20 and 16AE7%), which were compared to the susceptible cultivar grown in a pure stand. The effect of these mixtures on the incidence and severity of leaf and panicle blast was measured weekly, and on yield and yield components at harvest time. The mixture effect was more efficient in reducing disease with a proportion of 16AE7% susceptible component than with a proportion of 50%. Blast epidemic was significantly reduced in all three experiments. However, under high blast pressure, there was no reduction in the disease by the end of the epidemic and yields of the susceptible cultivar were almost zero whatever the mixture. In two other experiments performed under lower blast pressure, disease incidence and severity were significantly lower in mixtures, and yields of the susceptible cultivars grown in mixtures were higher than those of their respective pure stands. Cultivar mixtures are a promising strategy that could contribute to a more sustainable cultivation of rice under upland conditions in the context of subsistence agriculture in Madagascar, where all cropping operations are manual.

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