Évolution récente de la diversité génétique des riz cultivés (Oryza sativa et O. glaberrima) en Guinée

Abstract

The temporal dynamic of rice diversity was assessed in Guinea at the national level for the period 1996-2001 and at a natural regional level for the period 1979-2003. At the national level, 1,697 farms located in 79 villages of the four natural regions of the country were surveyed. The diversity descriptors used were the number of known and cultivated varieties and the Shannon diversity and evenness index. The regional analysis was based on the comparison of samples collected in six villages of Lower Guinea in 1979 and 2003. The varietal and allelic compositions at 10 microsatellite loci served as diversity descriptors. The number of varieties per farmer and per village slightly increased during the 1996-2001 period. Varietal diversity was high, especially in Forest Guinea and Lower Guinea. The rate of varietal use is however low and heterogeneous. Less than 10% of the varieties are cultivated by more than 50% of the farmers of a village. The recent dissemination of short-duration nerica varieties has not caused any reduction of pre-existing varieties as they are complementary to long-duration local varieties. For the 1979-2003 period, the varietal composition in each village evolved substantially. The average number of alleles per locus and per accession was significantly higher in 2003, and the allelic composition of homonymous accessions of the two collection dates was significantly different. However, alleles specific to each collection date had much lower frequencies compared to alleles common to the two collection dates. These results suggest that rice genetic diversity in Guinea has been maintained or even enhanced. However, at the village level, the diversity pattern remains fragile as the proportion of farmers who used all of the village varieties was low and heterogeneous. A continuous monitoring of the dynamics of rice varietal diversity in Guinea is needed.