Résumé
Understanding the effects of actions undertaken by human societies on crop
evolution processes is a major challenge for the conservation of genetic
resources. This study investigated the mechanisms whereby social
boundaries associated with patterns of ethnolinguistic diversity have
influenced the on-farm distribution of sorghum diversity. Social
boundaries limit the diffusion of planting material, practices and
knowledge, thus shaping crop diversity in situ. To assess the effect of
social boundaries, this study was conducted in the contact zone between
the Chuka, Mbeere and Tharaka ethnolinguistic groups in eastern Kenya.
Sorghum varieties were inventoried and samples collected in 130
households. In all, 297 individual plants derived from seeds collected
under sixteen variety names were characterized using a set of 18 SSR
molecular markers and 15 morphological descriptors. The genetic structure
was investigated using both a Bayesian assignment method and
distance-based clustering. Principal Coordinates Analysis was used to
describe the structure of the morphological diversity of the panicles. The
distribution of the varieties and the main genetic clusters across
ethnolinguistic groups was described using a non-parametric MANOVA and
pairwise Fisher tests. The spatial distribution of landrace names and the
overall genetic spatial patterns were significantly correlated with
ethnolinguistic partition. However, the genetic structure inferred from
molecular makers did not discriminate the short-cycle landraces despite
their morphological distinctness. The cases of two improved varieties
highlighted possible fates of improved materials. The most recent one was
often given the name of local landraces. The second one, that was
introduced a dozen years ago, displays traces of admixture with local
landraces with differential intensity among ethnic groups. The patterns of
congruence or discordance between the nomenclature of farmers’ varieties
and the structure of both genetic and morphological diversity highlight
the effects of the social organization of communities on the diffusion of
seed, practices, and variety nomenclature.