High outcrossing rates in fields with mixed sorghum landraces: how are landraces maintained?

The effect of mating system on genetic diversity is a major theme in plant evolutionary genetics, because gene flow plays a large role in structuring the genetic variability within and among populations. Understanding crop mating systems and their consequences for gene flow can aid in managing agricultural systems and conserving genetic resources. We evaluated the extent of pollen flow, its links with farming practices and its impact on the dynamics of diversity of sorghum in fields of Duupa farmers in Cameroon. Duupa farmers grow numerous landraces mixed in a field, a practice that favours extensive pollen flow. We estimated parameters of the mating system of five landraces representative of the genetic diversity cultivated in the study site, using a direct method based on progeny array. The multilocus outcrossing rate calculated from all progenies was 18% and ranged from 0 to 73% among progenies. Outcrossing rates varied greatly among landraces, from 5 to 40%. Our results also showed that individual maternal plants were usually pollinated by more than eight pollen donors, except for one landrace (three pollen donors). Although the biological traits of sorghum (inflorescence morphology, floral traits, phenology) and the spatial planting practices of Duupa farmers led to extensive pollen flow among landraces, selection exerted by farmers appears to be a key parameter affecting the fate of new genetic combinations from outcrossing events. Because both natural and human-mediated factors shape evolution in crop populations, understanding evolutionary processes and designing in situ conservation measures requires that biologists and anthropologists work together.