Polymorphism and Genetic Diversity among Cassava Cultivars based on ITS1-5.8S-ITS2 Sequences of rDNA

Abstract

Cassava is an important crop in the southern area of the Chad and it is char- acterized by many cultivars. But these cultivars have never been evaluated at the molecular level. Therefore, the objective of this study was to analyze their genetic diversity and their phylogenetic relationships. After DNA extraction, amplification and sequencing, the nucleotide sequences of the ITS1- 5.8S-ITS2 region of the ribosomal DNA of 12 selected cultivars have been analyzed. The Neighbor-Joining method, Maximum Parsimony, Maximum Likelihood and the Bayesian approach allowed studying the ancestral links. The identified nucleotide sequences have 542 bp. The targeted genes showed 468 conserved sites and 59 polymorphic sites. The nucleotide frequency was 18.64% for Adenine, 14.01% for Thymine, 34.46% for Cytosine and 32.89% for Guanine. The (G + C) content was 67.35% compared to 32.65% for the (A+T). The substitution rate was in favor of the transversions (67.46%) against the transitions (32.54%). The analysis revealed high haplotype diversity (Hd=0.954) and low nucleotide diversity (π=0.026) with an average number of pairwise nucleo de di erences (k=14.045). On the all popula on, 9 haplotypes, including 6 individual and 3 double, were identified. Gene c di eren a on is medium (FST=0.314) with a low number of migrants (Nm=0.55) and a medium genetic distance (0.028). Phylogenetic analysis based on the Bayesian approach revealed three groups of cul vars with the existence of two strongly supported clades. The cultivars studied are characterized by demographic stability or moderate population growth.They will be incorporated in the breeding program in order to limit their genetic erosion and to select the interesting characters.