Phenotypic, chemical and molecular characterisation of shea tree (Vitellaria paradoxa subspecies nilotica) ethno-varieties in Uganda

Abstract

The shea tree (Vitellaria paradoxa C. Gaertn.) is an African tree species that is distributed in the Sudano-Sahelian zone and grows naturally in the savannah parklands of northern and eastern Uganda. The oil from its nuts is used for cooking, cosmetics, traditional medicine and cultural ceremonies. Local communities classify shea trees into ethno-varieties on the basis of fruit and nut differences. Such folk classifications are useful as a basis for conservation and breeding programmes. This study was conducted in three farming systems (northern, Teso and West Nile) of Uganda to: 1) document folk classification and management, 2) examine morphological variation, 3) analyse fat content and fatty acid composition, and 4) assess molecular variation within and among the shea tree ethno-varieties. Folk classification was investigated using participatory rural appraisal (PRA) tools, morphological variation was assessed using morphometric measurements while chemical analysis was conducted by Near Infrared Spectroscopy (NIRS) and conventional wet chemistry (soxtec petroleum – ether and gas chromatography). Nuclear simple sequence repeats (SSRs) were used to determine molecular variation among the ethno-varieties. Thirty six uses of shea trees and their products were documented, 24 of them based on shea oil. Local management practices included weeding, pruning and on-farm retention. Traditional conservation practices are influenced by belief systems involving taboos and rituals. Fourty four ethno-varieties, based on fruit and nut morphological and organoleptic traits, were documented. Morphometric analysis showed no clear aggregation that is congruent to folk classification indicating that shea tree ethno-varieties cannot be distinguished based on morphological traits. Oleic and stearic acids were the most abundant fatty acids. Other fatty acids included palmitic, vaccenic, linoleic, linolenic and arachidic acids. There was no significant variation in fat content and fatty acid composition between ethno-varieties. Most genetic variation (86.90%) occurred within individual trees while 8.43% was found among individual trees within ethno-varieties and 4.67% was found among ethno-varieties. All shea tree ethno-varieties constitute a single out-crossing population with very low genetic differentiation. In conclusion, this study shows that the ethno-varieties as perceived by farmers in Uganda are arbitrarily defined sub-groups of a single randomly mating population.