Assessment of genetic variability in a traditional cassava (Manihot esculenta Crantz) farming system, using AFLP markers

Despite the urgent need to conserve domesticated plant genetic resources, a nd developing 'on farm' strategies of conservation, the impact of tradition al farming practices and of their interaction with ecological factors on th e structure and evolutionary dynamics of the genetic variability of crop po pulations has been little documented. We assessed the genetic variability o f 31 varieties of cassava (M. esculenta Crantz) traditionally grown by Maku shi Amerindians from Guyana, using AFLP markers. We used a sample of 38 var ieties from an es situ core collection as a reference. Accessions of wild c assava were also included. While clonality of the varieties was expected du e to the vegetative propagation of cassava, 21 varieties presented intravar ietal polymorphism. Among the varieties from a single site in Guyana, genet ic diversity was the same as that in the accessions from the core collectio n. We suggest that incorporation of volunteer seedlings, produced by sexual reproduction, into the stock of varieties grown by the Makushi plays a maj or role in explaining both intravarietal polymorphism and the high level of genetic diversity. No correspondence was found between the structure of mo lecular diversity and variation observed for agronomic traits that are targ ets for selection by cultivators. As found in previous studies, all wild fo rms of cassava clustered together and separately from the cultivated variet ies in a Neighbour-Joining dendrogram 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. Our results show that local varieties are an important source of genetic divers ity, and highlight the importance of the interaction between human and ecol ogical factors in the dynamics of this diversity.