Browsing by Author "Abebe, Wendawek"

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    Morphological and Molecular Characterization of Cultivated Guinea Yam Accessions and their Wild Relatives (Dioscorea cayenensis Lam. complex) from South and Southwest Ethiopia.
    (Addis Ababa University, 2008-03) Abebe, Wendawek; Demissew, Sebsebe (Professor); Nordal, Inger (Professor); Wilkin, Paul (PhD)
    Yams (Dioscorea species L.) are among the most important of the tuber crops mainly cultivated in the tropics. They are also an important source of diosgenein, a starting material for the industrial production of sex hormones and steroidal drugs with pharmaceutical properties. Despite their cultural and economic importance, there are taxonomic confusions in the groups called Guinea yams that belong to the D. cayenensis complex. Identification of living or dried specimens using the currently used classification scheme can be extremely difficult. Establishing the taxonomic identity and understanding the systematic and genetic relationships among the accessions of Guinea yams and their wild relatives is vital to the conservation and management of the crop. Therefore, the major objectives of this study were to evaluate the existing taxonomy and to determine the amount and distribution of genetic variation within Dioscorea cayenensis complex in Ethiopia. Collections of plant material were conducted between the months of July and September 2005 and 2006 from different localities in South and Southwestern parts of Ethiopia. Morphometric analyses were carried out based on a similarity matrix constructed using 26 morphological characters on 40 accessions of Dioscorea cayenensis complex . The results of the cluster and ordination analyses revealed that the wild and cultivated Guinea yams are closely related. None of the UPGMA clusters entirely contained those accessions considered as discrete taxa according to the existing classification system. The three primer combinations used in the Amplified Fragment Length Polymorphism (AFLP) analyses generated 158 scorable bands, with an overall polymorphism of 78%. Ordination and cluster analyses of AFLP data failed to produce any clear species boundary between the species within D. cayenensis complex. The average genetic similarity between the accessions ranged from 60 % to 100 %. The first, second and third principal coordinates axes cumulatively account 77.5 % of the total variation. AFLP analyses also revealed a higher genetic divergence among cultivated Guinea yams accessions of the Sheko cultivars. Estimates of population parameters using microsatellite or simple sequence repeat (SSR) markers were made by studying 7 loci. The total number of alleles amplified for the 7 loci were found to be 60, with an average of 8.6 alleles per locus. Analyses of the data indicated that Guinea yams and their wild relatives in the study area displayed a tremendous genetic diversity. The wild forms exhibited greater allelic diversity than the cultigens. Contrary to what is expected in vegetatively propagated crops, none of the seven loci studied showed a significant excess of heterozygotes. The levels of heterozgosity found in the study group were, in most cases lower than expected. Analyses of the taxonomic status using microsattellite data also revealed comparable results with both morphometry and AFLP. The accessions tended to group based on their geographical origin rather than their supposed taxonomic identity. In the present studies, the phenograms and scatter plots based on morphological, AFLP and microsatellite markers failed to produce a clear partitioning of the study individuals studied into discrete taxa according to the existing classification system. Therefore, we believe that at least the wild or managed populations and cultivated Guinea yams of South and Southwest Ethiopia form a single taxonomic entity. It also appears that the Sheko population displayed the greatest genetic diversity. From a conservation perspective, it is important that both the range of cultivars and the diversity within them is protected both in-situ in the Sheko region, and perhaps also in ex-situ in selected areas in gardens. Future studies must be undertaken at the population scale and in a broad range of ecosystems, so as to take the diversity of each of the yams currently regarded as distinct species into account. Key words: Guinea yams, morphometry, AFLP, microsatellite, taxonomic status, Genetic diversity.