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ABSTRACT: Cassava (Manihot esculenta) is a tropical crop that is grown in Africa, Latin America and Southeast Asia. Cassava was introduced from Latin America
into West and East Africa at two independent events. In Uganda a serious threat to cassava's survival is the cassava mosaic
disease (CMD). Uganda has had two notable CMD epidemics since the introduction of cassava in the 1850s causing severe losses.
SSR markers were used to study the effect of CMD on the genetic diversity in five agroecologies in Uganda with high and low
incidence of CMD. Surprisingly, high gene diversity was detected. Most of the diversity was found within populations, while
the diversity was very small among agroecological zones and the high and low CMD incidence areas. The high genetic diversity
suggests a mechanism by which diversity is maintained by the active involvement of the Ugandan farmer in continuously testing
and adopting new genotypes that will serve their diverse needs. However, in spite of the high genetic diversity we found a
loss of rare alleles in areas with high CMD incidence. To study the effect of the introgression history on the gene pool the
genetic differentiation between East and West Africa was also studied. Genetic similarities were found between the varieties
in Uganda and Tanzania in East Africa and Ghana in West Africa. Thus, there is no evidence for a differentiation of the cassava
gene pool into a western and an eastern genetic lineage. However, a possible difference in the genetic constitution of the
introduced cassava into East and West Africa may have been diminished by germplasm movement.
Euphytica 04/2012; 146(1):45-54. · 1.55 Impact Factor
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ABSTRACT: Cassava (Manihot esculenta Crantz) is a starchy root crop grown in the tropics mainly by small-scale farmers even though agro-industrial processing is rapidly increasing. For this processing market improved varieties with high dry matter root content (DMC) is required. Potentially toxic cyanogenic glucosides are synthesized in the leaves and translocated to the roots. Selection for varieties with low cyanogenic glucoside potential (CNP) and high DMC is among the principal objectives in cassava breeding programs. However, these traits are highly influenced by the environmental conditions and the genetic control of these traits is not well understood. An S(1) population derived from a cross between two bred cassava varieties (MCOL 1684 and Rayong 1) that differ in CNP and DMC was used to study the heritability and genetic basis of these traits. A broad-sense heritability of 0.43 and 0.42 was found for CNP and DMC, respectively. The moderate heritabilities for DMC and CNP indicate that the phenotypic variation of these traits is explained by a genetic component. We found two quantitative trait loci (QTL) on two different linkage groups controlling CNP and six QTL on four different linkage groups controlling DMC. One QTL for CNP and one QTL for DMC mapped near each other, suggesting pleiotrophy and/or linkage of QTL. The two QTL for CNP showed additive effects while the six QTL for DMC showed additive effect, dominance or overdominance. This study is a first step towards developing molecular marker tools for efficient breeding of CNP and DMC in cassava.
Hereditas 10/2007; 144(4):129-36. · 0.79 Impact Factor
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ABSTRACT: Cassava is a tropical crop and grown for its tuberous starchy roots. In Africa it is mainly cultivated by small-scale farmers who observe, select and name their cassava varieties based on morphology, food, social and economic interest. Here we have used an interdisciplinary approach involving farmer interviews, genetic markers and morphological descriptors to study the composition of cassava varieties on small-scale farms in 11 villages located in three districts in Uganda, the genetic structure within and between these varieties and their morphology. The composition of local, newly introduced and improved varieties differed widely between villages and districts. The Ugandan farmers in our study seemed to adopt improved varieties to a greater extent when there was a nearby market, prevalence of disease epidemics and good extension service. We found considerable genetic variation both within and between cassava varieties though the variation was larger between varieties. However, most local and improved varieties showed predominating genotypes at many loci. Accessions of commonly grown varieties meeting farmers' preferences could therefore be selected and implemented in future breeding programmes involving development, dissemination and adoption. The like-named varieties in different villages were genetically similar, demonstrating farmers' ability to differentiate and maintain the same variety over large areas. However, some varieties with different names in different villages showed both genetic and morphological similarity, suggesting that farmers may rename plants when they are introduced into their fields. The large differences found in variety and genetic composition between villages and districts in Uganda may be a result of the diverse needs and growing conditions characteristic for traditional farming system. This suggests that efforts to conserve and increase the genetic diversity in farmers' fields will require policies tailored to each area.
Genetica 08/2007; 130(3):301-18. · 2.15 Impact Factor
