Traditional management of cassava morphological and genetic diversity by the Makushi Amerindians (Guyana, South America): Perspectives for on-farm conservation of crop genetic resources

In this paper we present original data on morphological and genetic diversi ty of cassava managed by the Makushi Amerindians from Guyana. Although they propagate cassava exclusively vegetatively by means of stem cuttings, many Amerindian farmers also use and multiply volunteer plants grown from seeds produced by sexual reproduction. Morphological characters were recorded fo r 29 varieties cultivated by the Makushi and two populations of plants orig inating from volunteer cassava seedlings. Genetic characterisation with AFL P markers was available for 21 of the examined varieties. The morphological and agronomic characters were highly variable among varieties. Every varie ty could be differentiated from any other one, except for one pair of varie ties. However, high intra-varietal variability existed, which might lead to confusions between phenotypically similar varieties by the Makushi. Seedli ngs were on average different from the pool of the varieties studied, but 6 7% were found to resemble closely enough one of the varieties to be liable to be assigned to it. Confusion between very similar varieties, as well as assignment of seedlings to a variety, should generate genetic variability w ithin varieties, which was detected with AFLP markers. As in other sites in Amazonia, there was only a weak correlation between inter-varietal distanc es assessed with molecular and with morphological markers, suggesting that diversification of morphological characters has taken place repeatedly and independently across the Amazonian range of the crop. Diversifying selectio n, exchanges of varieties between farmers, and incorporation of sexually pr oduced volunteer plants are key mechanisms responsible for the high diversi ty observed. Strategies of conservation of genetic resources should take th ese dynamic processes into account.