CBGG

Cassava (Manihot esculenta Crantz) is a major tropical crop. Regarded as an orphan crop a few decades ago, it now receives considerable attention from governments, industry and agencies investing in agricultural research. As a result, the cassava community generates vast amounts of information and develops useful technologies and products. This positions cassava as a key industrial commodity and a reliable food security staple. Significant genetic gains have been achieved through the early 2000s. Gains are particularly noticeable under better agronomic conditions, as was the case for the green revolution of cereals. However, further gains obtained in the past two decades have not been as impressive. Cassava breeding cycle is long, and its multiplication rate slow. Therefore, it takes no less than 8 years to develop a new variety. Cassava breeding is based on the use of heterozygous progenitors which has important drawbacks. One of them is the impossibility to implement conventional back-crossing. Cassava researchers have recently introgressed a single recessive trait (amylose-free starch) into elite varieties. This was unprecedented in cassava and revealed important problems incorporating single genes into elite varieties. In the absence of backcross, the process required essentially the development of new varieties, which exposed strong effects of undesirable genetic linkages. Breeding approaches to overcome the problems of introgressing single-gene traits have been developed and will be implemented to introduce resistance to cassava mosaic disease into SE Asia breeding populations. These methods will also be useful to exploit recently identified immunity to cassava brown streak disease, a serious problem currently restricted to Africa. Trait introgression is an important process in crop breeding and will be more important in the future as gene discovery and editing identify useful genes. This article consolidates and integrates information from cassava and other crops and proposes guidelines to maximize the returns on investments on this important commodity.

back-cross; genetic load; genetic linkages; inbreeding; reverse genetics; trait introgression