ABSTRACT
Shea (Vitellaria paradoxa C. F. Gaertn.) is a major economic oil tree species in the savanna regions of Africa. Genetic diversity, which is the variation that exists within or between species, forms the bases of all crop breeding and improvement programmes. Despite the multipurpose uses, there is a dearth of genetic diversity research on shea necessary for its domestication and genetic enhancement in Nigeria. The aim of this research was to characterize the genetic diversity of the shea tree in three states of the Guinea Savanna of Nigeria using Simple Sequence Repeat (SSR) markers. Plant materials (leaves and fruits) were collected from a total of 60 shea genotypes from six locations: Gure and Kosuboso in Kwara State, Emishurun and Gbakogi-kasara in Niger State, and Abaji and Sati-agirigi in Benue State. In each location, ten trees separated by a minimum distance of 25 meters were randomly sampled to avoid the selection of closely related individuals. Morphological traits relating to trees, leaves, fruits, and nuts were measured. Proximate and biochemical analyses were performed on shea nuts and butter, respectively. DNA was extracted from young shea leaves. The quality of extracted DNA was checked using agarose gel electrophoresis while the quantity was determined by a nanodrop spectrophotometer. Polymerase Chain Reaction (PCR) was conducted for DNA amplification using ten SSR primer sets. The banding patterns of amplification products were visualized by electrophoresis. Genetic distance and polymorphic information content (PIC) were calculated using PowerMarker version 3.25. Dendrogram for molecular data was developed using NTSYS, version 2.02. There were significant variations in all tree traits (p < 0.05) except trunk height. Significant variations (p < 0.05) were observed in the leaf traits measured among genotypes from the different locations. The highest mean values for the geometric diameter (4.71 cm) and area (70.03 cm2 ) of fruits were from genotypes in Abaji. There were highly significant and positive correlations between most fruit and nut traits. The highest mean values for crude fibre (11.40%), lipid (46.47%) and protein (10.43%), were obtained from genotypes in Emishurun while those for moisture (11.88%), ash (8.12%) and soluble carbohydrate (48.40%), were obtained from genotypes in Sati-agirigi, Kosuboso and Abaji, respectively. The percentage free fatty acid value of genotypes in Benue (8.23) significantly differed (p < 0.05) from values in Niger (9.29) and Kwara states (9.20). The concentration of extracted DNA ranged from 70.8-379.09, 74.45-315.69, and 64.29-316.86 ng/µl for shea genotypes xvii from Niger, Kwara and Benue States, respectively. Over 80% of DNA extracted fell within purity range of 1.7-2.0. Four SSR primers were responsive to PCR amplifications and two were polymorphic. The average number of observed alleles per locus, genetic distance and PIC were 11.5, 0.857, and 0.842, respectively. The dendrogram based on molecular data grouped the genotypes into four. The results showed genetic diversity for the morphological traits and high polymorphism among the 60 shea genotypes from the SSR analysis. A breeding programme can be initiated for genetic improvement of shea by combining distinct genotypes from the different states.
