MOLECULAR CHARACTERIZATION OF IMPROVED CASSAVA CULTIVARS AND COMMONLY GROWN NIGERIAN LANDRACES USING SIMPLE SEQUENCE REPEAT MARKERS

Abstract

Cassava is a starchy staple food of tropical Africa whose yield is affected by several biotic stresses. Improved cassava cultivars that are resistant to biotic stresses were developed to boost cassava production. Also, some Nigerian landraces of cassava that are resistant to some of these stresses have been discovered. Simple sequence repeats (SSR) are molecular markers with high discriminatory power and technical and analytical simplicity. SSR markers have been applied successfully to crops such as rice, cowpea, sorghum, and sunflower but scarcely used on cassava. Based on this, the study evaluated the genetic diversity between improved cassava cultivars and commonly grown. Nigerian landraces using simple sequence repeat (SSR) markers and also determined the SSR markers that could readily be used for genotype identification of cultivated cassava in Nigeria. For the evaluation of genetic diversity, 31 improved cultivars and 5 Nigerian landraces of cassava were assessed at genomic deoxyribonucleic acid (DNA) level with SSR markers. Polymerase chain reaction (PCR) amplification of the genomic DNA of the cultivars were carried out with 16 polymorphic SSR primers. A total of 38 distinct and scorable DNA bands generated were used for data analysis by Numerical Taxonomy and Multivariate Analysis System (NTSYS). Principal component analysis, which revealed the major underlying sources of variation, was also carried out using Statistical Analysis System (SAS). For genotype identification study, 16 SSR markers were assessed using 36 genotypes of cassava. Data from each primer were analyzed by NTSYS and primers that generated between 6 and 9 cluster groups at 0.70 similarity coefficients were selected. Combinations of data from selected primers were also analyzed by NTSRS to select minimum number of SSR markers for genotype identification of cultivated cassava in Nigeria. The results of genetic diversity study identified 12 distinct DNA cluster groups at 0.70 similarity coefficient, the similarity indices ranged from 0.42 to 0.81. The closest genetic relationship between improved cultivars and Nigerian landraces was observed at 0.82 similarity coefficient; while the most distant relationship was at 0.55 similarity coefficient. Ten principal components that contributed 70.59% of the variation observed among the cassava cultivars were revealed. The first and tenth principal components contributed 11.70 and 4.03% of the variance of genetic distance, respectively. The results of the determination of SSR markers for genotype identification study revealed five polymorphic SSR markers that could readily be used for genotype identification of cultivated cassava, because they were able to distinguish the 36 cassava genotypes at 0.95 similarity coefficient. Furthermore, the results of this study revealed that SSR primers that amplified DNA from improved cassava successful so in Nigerian landraces. SSR markers detected polymorphisms among improved cultivars and Nigerian landraces of cassava and are therefore ideal molecular tools genetic and genotype identification of cultivated cassava in Nigeria that could be exploited in cassava breeding programs.