Morphological and Molecular Characterization of Cultivated Guinea Yam Accessions and their Wild Relatives (Dioscorea cayenensis Lam. complex) from South and Southwest Ethiopia.
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Date
2008-03
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Addis Ababa University
Abstract
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.
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Keywords
Guinea yams, morphometry, AFLP, microsatellite, taxonomic status, Genetic diversity