ABSTRACT
Cassava mosaic disease caused by begomoviruses is the most widespread disease of cassava in Africa. Simple sequence repeat (SSR) genetic analysis of an F1 population of cassava was carried out to identify new sources of CMD resistance. Genomic DNA extracted from parents and progeny was amplified using polymerase chain reaction (PCR). PCR amplifications were run on polyacrylamide and agarose gels. SSR marker technology was used to identify markers linked to CMD resistance via bulk segregant analysis (BSA). One hundred and forty molecular markers from the International Center for Tropical Agriculture (CIAT) were used to screen the parents, contrasting bulks and the individuals that make up the contrasting bulks.
The bulk segregant analysis produced forty polymorphic markers. Screening of the contrasting individuals with the polymorphic markers revealed four candidate markers (SSRY 238, SSRY 51, SSRY 76 and SSRY 20) linked to CMD resistance. The correlation coefficient values between genotypic and phenotypic data classes for candidate markers were generally low. The t-test value between both genotypic classes (absence of band versus presence of band) were not significant (P>0.05) in each of the four markers. The results from this study suggest that there are at least two new CMD resistance genes in the mapping population.
INTRODUCTION
Cassava (Manihot esculenta Crantz) is an important starchy crop grown in the tropical and sub-tropical regions of the world. Cassava originated from Latin America and was introduced to Africa between the 15th and 17th century by the Portuguese traders. The crop ranks sixth among the important staple food crops (Mann, 1997), with more than 800 million people depending on it for their calorie needs (FAO, 2001).
Many diseases and pests like cassava bacteria blight, cassava anthracnose disease, cassava mosaic disease (CMD), cassava mealy bugs, and green spider mites attack cassava. Among these, cassava mosaic disease (CMD) constitutes a major constraint and is the most important economically. It is also the most widespread disease of cassava in Africa. Cassava root yield losses arising from attack of CMD could be as high as 95%, depending on the location and innoculum pressure conditions (Jenings, 1994; Thresh et. al., 1994b; Fauguet and Fargette, 1987). Legg and Thresh (2003) in a country-level survey carried out in all major cassava producing countries estimated that continental losses in 2003 ranged from 19 to 27 million metric tonnes, amounting to about 1.9-2.7 billion US dollars.
Conventional breeding efforts have been made to combat the problem of CMD. This method is considerably slow and expensive. It may take up to 10-15 years to transfer a trait from a donor species into a receiving cultivar destined for improvement. The time needed to transfer the desired gene into a crop cultivar depends on the source of the gene and the evolutionary distance of that source to the recipient crop. If the gene source is a landrace or a related species to the crop, breeding for disease resistance may take up to 5-10years. Less related wild species may be rich reservoirs of resistance genes, but to transfer such genes into the crop cultivar may take longer years (Prem, 2006). Pre- and post-fertilization barriers may impede sexual hybridization between the donor and the recipient crops, thereby compounding the problem of alien gene transfer (Prem 2006). Furthermore, conventional breeding efficiency is affected by genotype x environment interaction and resistance could break down due to sudden emergence of new biotypes of the virus. There is therefore the need for a more efficient breeding strategy that will enhance identification of resistance genes that will aid CMD resistance in cassava. But considering the time and resources required for developing resistant cassava varieties, it is important to develop varieties carrying as many different genes for resistance as possible. Identifying and concentrating different resistance genes will provide stable resistance against a broad spectrum of the virus.
Molecular markers are useful tools in gene tagging studies. Molecular markers are segments of an organism’s DNA that show genetic variability between individuals in the same species or population. They are not affected by the environment. They enhance the efficiency of conventional plant breeding by carrying out selection not directly on the trait of interest but on the molecular markers linked to the trait of interest. Simple sequence repeat (SSR) marker technology is one of the widely used molecular markers in genetic studies. SSR markers are small tandem repeats of DNA (usually 2-5 base pair (bp) in length). Once molecular markers closely linked to CMD resistance are identified, marker – assisted selection (MAS) can be done in segregating populations. For MAS to be effective in a plant breeding programme, the following conditions must be fulfilled: (a) marker (s) should co- segregate or be closely linked with the desired traits; (b) availability of an efficient means of screening large populations (c) the screening technique should have high reproducibility across laboratories, (d) and should be user-friendly (Mohan et al., 1997).
Cassava breeding scheme is typically a long process and very expensive. Considering the long time required to develop and release new varieties, there has been demand to seek effective strategies to make the breeding process much more efficient. Molecular markers are currently being used to understand the genetics of several traits in crop improvement programmes and to rapidly increase efficiency of breeding activities. Efficiency of cassava breeding can be enhanced through the use of molecular markers. Markers enable the efficiency of selection by enhancing the precise identification of genotypes without the confounding effect of the environment, thus increasing heritability. Breeding for disease resistance using conventional methods is not only time-consuming, but less-effective. This is attributable to the dynamic nature of the pathogens that have the potentials of altering virulence as new strains evolve. MAS has the advantage of precision in identifying, tagging and isolating DNA segments that have the capacity of precisely addressing many pathogenic problems. These considerations have prompted the present study with the following objectives:
(i) to identify new sources of CMD resistance gene; and
(ii) to elucidate the genetic control of CMD using the SSR marker technology.
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SIMPLE SEQUENCE REPEAT (SSR) GENETIC ANALYSIS OF CASSAVA MOSAIC DISEASE RESISTANCE IN SELECTED F1 POPULATIONS OF CASSAVA
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