Farmers' selection of quality traits in cassava (Manihot esculenta Crantz) landraces from Vanuatu

Cassava (Manihot esculenta Crantz) was introduced into Vanuatu shortly after 1850. Farmers have since selected and clonally propagated volunteer seedlings and new morphological variants found in cultivated plots. As cassava is free of serious pests and diseases in Vanuatu, it is unknown what are the farmers' sought traits leading to diversification. The aim of the present study is to investigate the diversification process and to identify the cultural factors that contribute to shape cassava diversity in Vanuatu. We characterized morphologically, using twelve descriptors, and chemically (dry matter, starch, sugars, proteins, cellulose, minerals), the national germplasm collection composed of 145 landraces collected from eleven different islands. Farmers' traditional knowledge on landraces was documented in ten villages, each located on a different island. A wide morphological variation was found among the accessions with few duplicates, the number of distinct morphotypes is 141. However, the sampling strategy most likely underestimated the total number of existing landraces. Cluster analysis revealed no geographical structuring among morphological groups, confirming the wide movement of germplasm between islands across the archipelago. Dry matter (DM) and starch are the least variable compounds with CV % of respectively 12.3 and 3.4 %; these two traits being highly correlated. Sugars (CV 42 %), cellulose (30.3 %), proteins (34.4 %) and total minerals (17.4 %) are more variable. The human selection pressure is focusing on good quality landraces based on high DM and starch contents, with different farmers in different islands focusing independently on the same traits. Implications for germplasm conservation and use strategies are discussed. (Résumé d'auteur