Molecular Marker Analysis and Validation of Resistance to Cassava Mosaic Disease in Selected Elite Genotypes in Nigeria.

Abstract

Cassava mosaic disease (CMD) is the most important constraint to cassava production in Africa.  The discovery of the CMD2 dominant gene is facilitating both marker-assisted selection and the genetic analysis of CMD resistance in cassava breeding.  Molecular markers for CMD2 gene were used to introgress CMD resistance into Latin American (LA) cassava elite genotypes and field evaluation study was conducted in four years to validate disease resistance stability for this gene. Two selected and recently released CMD resistant Nigerian cassava varieties (TMS97/2205 and TMS 98/0505) were also analyzed with markers for CMD resistance. Field phenotypic evaluation of TMS97/2205 revealed this variety as highly resistant to CMD with little or no disease symptoms in three ecologies in Nigeria.  Due to the high resistance and near immunity of TMS97/2205, further genetic analysis of this variety using a segregating F₁ population derived from cross between TMS 97/2205 and NR8083  was initiated via bulk segregant analysis (BSA) involving 500 SSR markers to identify possible new QTLs for CMD resistance.  Field screening identified Latin American genotypes with CMD resistance.  The LA genotypes representing 64 families were found to show good and stable resistance to the disease.  Results also indicate that CMD resistance in the two African varieties was also mediated by the CMD2 gene.  BSA identified a marker (NS 198) associated to a QTL for CMD resistance.  The phenotypic variance explained by this QTL is 16% indicating that the high CMD resistance observed in TMS97/2205 is due to the combined effects of this QTL and the CMD2 gene.  The CMD2 gene disease response curve in the growth season among the genotypes (LA genotypes, two African varieties and F₁ population) evaluated indicate that the disease resistance expression of the CMD2 gene is not uniform and thus affected by genetic background effects.  Some of the genotypes showed very high disease resistance (low infection level) while others showed moderate infection levels at CMD pressure peak in the season but soon quickly recovered to express high disease resistance.The discovery of new QTL (CMD3) for CMD resistance in TMS97/2205 offer new opportunities for pyramiding CMD genes in cassava for enhanced durability of CMD resistance in this crop.