Perceptual selection and the unconscious selection of ‘volunteer’ seedlings in clonally propagated crops: an example with African cassava (Manihot esculenta Crantz) using ethnobotany and population genetics
The role of sexual reproduction in magnifying genetic diversity in clonally propagated crops is now well documented. But whereas many studies have emphasized the importance of perceptual distinctiveness and its role in arousing farmers’ curiosity towards plants with unusual morphological traits, few have considered its corollary, perceptual indistinctiveness. In this study, we investigated which factors influence farmers’ behaviours towards self-sown ‘volunteer’ seedlings of cassava (Manihot esculenta Crantz) in a cross-cultural survey in Gabon, Central Africa. Interviews revealed that farmers were seven times more likely to spare seedlings that resembled a named landrace compared to those that did not. Similarly, farmers were more likely to relax weeding pressures on cassava volunteers if they believed that volunteers were resurgences from old cuttings. The model fit significantly improved when cultural factors were accounted for, suggesting strong culture-dependent differences among communities in their perception of and attitudes towards cassava volunteers. An analysis of the structure of genetic diversity of cassava landrace populations at the community level showed that most landraces included singleton genotypes, even in villages where farmers imposed the strongest weeding pressures, suggesting that there is always some background incorporation of cassava seedlings. We show that by channelling the selective incorporation of cassava seedlings that are morphologically indistinguishable from familiar landraces, perceptual selection favors the recruitment, sometimes deliberate but more often unconscious, of new genotypes from plants germinating from seeds and contributes to balancing the disadvantages of strictly clonal propagation, while maintaining landraces within consensual combinations of perceptually distinct traits.