| dc.description.abstract | Postharvest physiological deterioration (PPD) is one of the major constraints causing major economic loss across the cassava value chain (Manihot esculenta). Cassava roots start undergoing physiological deterioration within 72 hours after harvest, constraining the crop’s usability for commercial and industrial purposes. To remedy this challenge, this study was designed i) to determine the genetic variation of PPD expression in Ugandan germplasm and ii) to conduct a genome-wide association study to identify genomic regions associated with PPD in cassava, which could be critical to increasing breeding efficiency. Three hundred and seventy-six (376) cassava accessions constituting part of a pre-breeding population were sourced from the International Center for Tropical Agriculture (CIAT), West Africa and East Africa. These were evaluated across two locations, Namulonge and Serere and at 12 months after planting (MAP), PPD was assessed in mature roots after 3 and 7 days of storage using a visual score method. Results showed that accessions UGIS190308, UGIC18234, UGIC190083, UG2101643, UGIN190170 were ranked top five for low PPD based on Best Linear Unbiased Predictors (BLUPs) and they had a combined mean PPD score of 11.36%. These top-ranked accessions could be evaluated further in several agroecologies to confirm stability for PPD tolerance and other economically important agronomic traits or recycled as progenitors for population improvement. The study also found that accessions with high provitamin A (yellow roots) generally had low PPD scores compared to white and cream-fleshed accessions across two locations. However, a few white-fleshed accessions had low PPD scores comparable to yellow-fleshed clones across locations. A moderate heritability of 0.57 was estimated for PPD in the population, implying there is substantial genetic control of PPD which could positively improve response to selection.
Analysis of variance revealed that a large portion of the phenotypic variance associated with PPD was accounted for by genotypic effects, root color, location and size of the roots assessed. This confirmed that there was substantial genetic variability among the accessions for PPD and the environment was also a big influencer of the expression of the trait. Assessment of genetic diversity was done using a total of 38,738 single nucleotide polymorphism (SNP) markers. The SNP markers were highly informative, with polymorphic information content ranging from 0.26 to 0.38 across all the accessions and observed heterozygosity ranging from 0.31 to 0.49. Cluster analysis based on the ward. D2 method grouped cassava accessions into two main clusters based on SNP data with admixtures and the largest group comprised largely of CIAT accessions. These groups could be harnessed to maximize heterosis for PPD resistance through crossing of distinct accessions selected from the two relatively genetically differentiated groups.
The genetic architecture of PPD was determined using a genome-wide association study (GWAS) with PPD BLUPs and SNP data from the 331 accessions. The study found genomic regions on chromosomes 1 and 4 with five SNPs significantly associated with PPD in cassava. This study provides one of the pioneer insights into the practical application of GWAS for dissecting the genetic basis of PPD trait in cassava. Identified SNPs should be further annotated to reveal candidate genes linked to PPD expression or deployed in genomic breeding strategies to accelerate genetic gains for resistance. Overall, this work has shown that there is potential for improvement of PPD resistance in cassava in Uganda through a combined conventional and genomic harnessing of existing genetic variability for selection. | en_US |
