Unmanaged sexual reproduction and the dynamics of genetic diversity of a vegetatively propagated crop plant, cassava (Manihot esculenta Crantz), in atraditional farming system

Occurrence of intervarietal or interspecific natural crosses has been repor ted for many crop plants in traditional farming systems, underlining the po tential importance of this source of genetic exchange for the dynamics of g enetic diversity of crop plants. In this study, we use microsatellite loci to investigate the role of volunteer seedlings (plants originating from unm anaged sexual reproduction) in the dynamics of genetic diversity of cassava (Manihot esculenta Crantz), a vegetatively propagated crop, in a tradition al farming system in Guyana. A previous field study showed that farmers inc orporate such plants into the germplasm for vegetative propagation, and tha t many of them are likely to be assigned by farmers to recognized varieties . Under strict vegetative propagation clonality of varieties is expected. T he high proportion of polyclonal varieties observed suggests that incorpora tion of seedlings into the germplasm for propagation is a frequent event. T he molecular variability assessed with microsatellite markers shows that th ere is high differentiation among heterozygous varieties, whereas populatio ns of seedlings do not depart from the proportions expected under Hardy-Wei nberg assumptions. Assignment of seedlings to a recognized variety on the b asis of morphological similarity greatly increases genetic diversity within the variety. We argue that recombination and gene flow play a major role i n the dynamics of genetic diversity of cassava in traditional farming syste ms. Documenting unmanaged sexual reproduction and its genetic consequences is a prerequisite for defining strategies of in situ conservation of crop p lant genetic resources.