Cassava mosaic disease (CMD) is the most-important disease of cassava ( Manihot esculenta) in Africa, and is a potential threat to Latin American (LA) cassava production. Although this viral disease is still unknown in LA, its vector - the whitefly - has recently been found. The disease is best controlled through host-plant resistance, which was first found in third backcross derivatives of an interspecific cross between cassava and Manihot glaziovii, and is thought to be polygenic. Recently, high levels of resistance were also found in several Nigerian cassava landraces. Classical genetic analysis and molecular genetic-mapping of the landraces showed that a major dominant gene confers this resistance. Bulk segregant analysis (BSA) was used to quickly identify a simple sequence repeat (SSR) marker linked to the CMD-resistance gene. The marker, SSRY28, is located on linkage group R of the male-parent-derived molecular genetic map. The gene, designated as CMD2, is flanked by the SSR and RFLP marker GY1 at 9 and 8 cM, respectively. To our knowledge, this is the first report of qualitative virus resistance in cassava, and of molecular markers that tag CMD resistance in cassava. We discuss the use of markers linked to CMD2 for marker-assisted breeding of CMD resistance in Latin America and for increasing the cost-effectiveness of resistance breeding in Africa.
Cassava mosaic disease (CMD) is a viral disease of the important tropical staple crop cassava (Manihot esculenta) and preferred management involves use of host-plant resistance. The best available resistance is controlled by a single dominant gene. Serial analysis of gene expression (SAGE) was used to analyze the gene expression pattern in a bulk of 40 each of CMD resistant and susceptible genotypes drawn from a gene mapping progeny. Messenger RNA used for the SAGE analysis came from plants that were exposed to heavy disease pressure over a period of 2 years in the field. A total of 12,786 tags were studied, divided into 5733 and 7053 tags from the resistant and susceptible genotypes, respectively. Tag annotation was by PCR amplification using the tag sequence as sense primer and 4000 cassava ESTs generated from the bulk of CMD resistant genotypes. Annotation of more than 30 differentially expressed tags revealed several genes expressed during systemic acquired resistance (SAR) in plants and other genes involved in cell-to-cell and cytoplasm-to-nucleus virus trafficking. Differential expression of the most abundantly expressed tag, corresponding to a beta-tubulin gene, was confirmed by Northern Analysis. RFLP analysis of the tags in the parents and bulks of the CMD mapping progeny revealed only one tag, a WRKY transcription factor, associated with the region bearing the dominant CMD gene.
Molecular‐marker‐aided evaluation of germplasm plays an important role in defining the genetic diversity of plant genotypes for genetic and population improvement studies. A collection of African cassava landraces and elite cultivars was analysed for genetic diversity using 20 amplified fragment length polymorphic (AFLP) DNA primer combinations and 50 simple sequence repeat (SSR) markers. Within‐population diversity estimates obtained with both markers were correlated, showing little variation in their fixation index. The amount of within‐population variation was higher for landraces as illustrated by both markers, allowing discrimination among accessions along their geographical origins, with some overlap indicating the pattern of germplasm movement between countries. Elite cultivars were grouped in most cases in agreement with their pedigree and showed a narrow genetic variation. Both SSR and AFLP markers showed some similarity in results for the landraces, although SSR provided better genetic differentiation estimates. Genetic differentiation (Fst) in the landrace population was 0.746 for SSR and 0.656 for AFLP. The molecular variance among cultivars in both populations accounted for up to 83% of the overall variation, while 17% was found within populations. Gene diversity (He) estimated within each population varied with an average value of 0.607 for the landraces and 0.594 for the elite lines. Analyses of SSR data using ordination techniques identified additional cluster groups not detected by AFLP and also captured maximum variation within and between both populations. Our results indicate the importance of SSR and AFLP as efficient markers for the analysis of genetic diversity and population structure in cassava. Genetic differentiation analysis of the evaluated populations provides high prospects for identifying diverse parental combinations for the development of segregating populations for genetic studies and the introgression of desirable genes from diverse sources into the existing genetic base.
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