Such situations offer opportunities for studying the effects of gene flow, disruptive selection and genetic drift in crop populations, which often are enmeshed in crop/wild/weed complexes ( De Wet & Harlan, 1975). Moreover, related and interfertile wild forms sometimes occur sympatrically. It is common to find numerous varieties in the same field. Traditional agrosystems are of particular interest because crop diversity within them is generally high.
Defining plans for ‘on farm’ or in situ conservation of plant genetic resources requires assessment of the pattern of both molecular and phenotypic diversity and of processes that drive evolution of crop populations ( Miller et al., 1995). A wide range of theoretical and empirical knowledge about wild plant and animal populations, has led to conservation strategies, but equivalent information is missing for populations of domesticated crops. Our results show that local varieties are an important source of genetic diversity, and highlight the importance of the interaction between human and ecological factors in the dynamics of this diversity.ĭefining the scientific bases from which efficient strategies for conserving biological diversity can be developed, still remains a challenge for the scientific community. These results are consistent with the hypothesis of a limited domestication event in a restricted area, followed by rapid diffusion of cultivated phenotypes and convergent evolution. As found in previous studies, all wild forms of cassava clustered together and separately from the cultivated varieties in a Neighbour-Joining dendrogram. No correspondence was found between the structure of molecular diversity and variation observed for agronomic traits that are targets for selection by cultivators. We suggest that incorporation of volunteer seedlings, produced by sexual reproduction, into the stock of varieties grown by the Makushi plays a major role in explaining both intravarietal polymorphism and the high level of genetic diversity. Among the varieties from a single site in Guyana, genetic diversity was the same as that in the accessions from the core collection. While clonality of the varieties was expected due to the vegetative propagation of cassava, 21 varieties presented intravarietal polymorphism.
Accessions of wild cassava were also included. We used a sample of 38 varieties from an ex situ core collection as a reference. esculenta Crantz) traditionally grown by Makushi Amerindians from Guyana, using AFLP markers. We assessed the genetic variability of 31 varieties of cassava ( M. Despite the urgent need to conserve domesticated plant genetic resources, and developing ‘on farm’ strategies of conservation, the impact of traditional farming practices and of their interaction with ecological factors on the structure and evolutionary dynamics of the genetic variability of crop populations has been little documented.
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