Sequencing wild and cultivated cassava and related species reveals extensive interspecific hybridization and genetic diversity

Cassava (Manihot esculenta) provides calories and nutrition for more than half a billion people. It was domesticated by native Amazonian peoples through cultivation of the wild progenitor M. esculenta ssp. flabellifolia and is now grown in tropical regions worldwide. Here we provide a high-quality genome assembly for cassava with improved contiguity, linkage, and completeness; almost 97% of genes are anchored to chromosomes. We find that paleotetraploidy in cassava is shared with the related rubber tree Hevea, providing a resource for comparative studies. We also sequence a global collection of 58 Manihot accessions, including cultivated and wild cassava accessions and related species such as Ceará or India rubber (M. glaziovii), and genotype 268 African cassava varieties. We find widespread interspecific admixture, and detect the genetic signature of past cassava breeding programs. As a clonally propagated crop, cassava is especially vulnerable to pathogens and abiotic stresses. This genomic resource will inform future genome-enabled breeding efforts to improve this staple crop

Mapping of QTL associated with resistance to Cassava Brown Streak and Mosaic Diseases in outcrossing cassava cultivars locally cultivated in Tanzania

Cassava brown streak (CBSD) and mosaic (CMD) diseases are the major cassava production constraints in the East African region. Efforts to control these diseases were initiated in northeastern Tanganyika in the 1930’s using conventional breeding methods. Despite these efforts, CBSD is spreading, threatening central and western Africa. Identification of molecular markers associated with observed plant resistance and tolerance would facilitate marker assisted breeding leading to efficient breeding, knowledge-based deployment of resistance genes and pre-emptive breeding in West Africa. QTL analysis using a bi-parental F1 mapping population was undertaken to identify QTL for CMD resistance in the variety ‘Albert’ and CBSD resistance in the variety ‘Namikonga’. A one-step genetic linkage map composed of 986 SNP markers and 18 linkage groups spanning 1826.3 cM was generated and used to create a framework map of 242 loci. Phenotyping data was obtained from two disease hotspots in Tanzania from 226 F1 progeny. Significant QTL were identified on chromosomes VIII, XV and I for CMD, CBSD foliar and root symptoms respectively. The maximum LOD score of 19.32 explained variation 34.9% of variation for CMD resistance. CBSD foliar and root symptoms revealed different QTL with moderate LOD scores of up to 5.09 and 7.33, explaining 10.7 and 16.2% respectively of variation. Several other minor significant QTL for all traits were also identified. Genes within QTL regions are being characterized