marker-assisted selection in wheat - ictsd

Chapter 7 – Marker-assisted selection in common beans and cassava 99Figure 4Schematic representation of steps employed in breeding for resistance to the CMDin Latin America cassava gene poolsThe entire process from sexual seeds to tissue plants for shipment or transfer to thescreen house takes approximately three months.of resistance, overlapping outbreaks ofCGM, cassava bacterial blight (CBB), andCMD are common (Legg and Ogwal, 1998)and the need for modest-sized breedingpopulations make MAS for CMD resistancea powerful tool to accelerate cassavaimprovement even in Africa. A MAS andparticipatory plant breeding (PPB) projectwas initiated in 2003 with funding fromthe Rockefeller Foundation to improvethe resistance of local cassava varieties inthe United Republic of Tanzania to CMDand CGM and also to provide proof ofconcept for the use of MAS to acceleratecassava improvement. The United Republicof Tanzania is the fourth largest producerof cassava in Africa with average yieldsof about 8 tonnes/ha (FAO, 2001). This isbelow the continent’s average of 10 tonnes/ha, and well below the average yield of14 tonnes/ha of Africa’s (and the world’s)largest producer, Nigeria.The low yield in the United Republicof Tanzania is caused by many factors,including the susceptibility of commonlygrown varieties to major diseases and pestssuch as CMD and the cassava brown streakdisease (CBSD). The project crosses farmerpreferredgermplasm, by agro-ecology, toimproved introductions that are resistantto CMD and to CGM. Markers associatedwith resistance to CMD are used to reduce