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Diversité génétique du riz (Oryza sativa L.) dans la région de Vakinankaratra, Madagascar. Structuration, distribution éco-géographique & gestion in situ

Abstract : In situ conservation of crop genetic resources is widely recommended but, as yet, no methods have been developed to rank the genetic entities to preserve and the social organisations to involve. Increased knowledge of the dynamic of agrobiodiversity is essential for the integration of conservation and valorization in the strategies of crop productivity improvement in traditional agrosystems. The highlands of Madagascar have been identified as a key site for rice, Oryza sativa, genetic diversity. To define conservation strategies, we performed multidisciplinary analysis of rice genetic diversity and factors shaping its distribution in the Vakinankaratra region of Madagascar. Individual and collective surveys, collect and characterization of samples of cultivated rice varieties in 1050 farms located in 32 villages were realized. Averages numbers of rice varieties used per village (10.9) and per farm (2.2) are comparables with other traditional agrosystems, nevertheless great regional variability is observed. The determinants of this variability are altitude, production system of villages, type of rice cultivation system and farm economic richness. An important disparity in the frequency of the use of varieties was observed with large proportion of "minor" varieties used by less than 10% of farms. Local seed management is characterized by the lack of selection for varietal purity. The adoption of new practices of seed and variety management, associated with the recent development of upland rice in this region, shows the capacity of the system to innovations. Along with the indica and japonica rice subspecies of O. sativa, we confirmed the presence of an atypical rice group with a preferential habitat of 1250-1750 m. Spatial distribution of genetic diversity was uneven. The most determining factor of this unevenness was the altitude authorising or not the presence of different rice cropping systems and the associated types of varieties. Village and individual farmer's wealth also had a determining role on the amount of rice diversity they hosted. While molecular variance between villages in a given interval of altitude represented 16% of the total variance, within-village variance represented more than 75% of the total, and within-farm variance 70% of within-village variance. This hierarchical distribution of molecular variance suggests that a small number of samples per scale (altitude interval, village and farm) could allow to capture most of the genetic diversity observed. However, within-variety diversity was also important making ex situ conservation strategies impractical and costly. Implications of the within-variety diversity are discussed in terms of adaptive advantages, evolutionary processes, and need for in situ conservation.
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Tendro Radanielina. Diversité génétique du riz (Oryza sativa L.) dans la région de Vakinankaratra, Madagascar. Structuration, distribution éco-géographique & gestion in situ. Amélioration des plantes. ENSIA (AgroParisTech), 2010. Français. ⟨tel-00818536⟩

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