Biology Reference
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worth US$1.49 billion in Nigeria, US$675 mil-
lion in Ghana, and US$52 million in Uganda,
if developed through MAS. If developed solely
by conventional breeding they would be worth
about US$676 million in Nigeria, US$304 mil-
lion in Ghana, and US$18 million in Uganda.
The difference is mostly due to the faster timing
of release for the cultivars developed with MAS
and the higher probability of success. Several
sensitivity analyses were conducted and benefits
for MAS range from US$1.7 billion to US$4.3
billion for all three traits, depending on assump-
tions. In all cases, the research investment is
highly profitable from a societal standpoint (Rudi
et al. 2010).
The benefits from improved cultivars through
breeding have been widely demonstrated, partic-
ularly in international agricultural research cen-
ters (Heisey et al. 2002; Everson and Gollin
2003) and also in national programs (Brennan
et al. 2004). In any plant-breeding program,
breeders inevitably have more potential selec-
tion traits than can be feasibly targeted with their
current resources. As a result, the pressure on
all breeding programs to be more efficient and to
utilize the latest technologies has increased enor-
mously. Many breeding programs must demon-
strate commercial success if they are to con-
tinue. Given the circumstances, there are pow-
erful incentives for all breeders to introduce new
breeding technologies and to do so to maximum
effect (Brennan and Martin 2007).
related to taste and palatability of the tuberous
roots, growth habit, and overall suitability for use
in the cropping system.
Improving cassava to meet the dietary needs
of consumers for various nutritional attributes
aimed at balancing diets and improving the
health status of millions is a top-priority breed-
ing target. The expansion of the cassava industry
has meant that the crop has to be improved for
commercial traits if it is to enhance the value
chain and improve income generation for farm-
ers and processors. Effective MAB strategies
that enhance the combination of CMD resistance
with high yield and value-added traits in a fast-
track scheme will be crucial to the successful
development of next-generation cassava culti-
vars that will drive the economies of developing
countries. Exploring robust marker-based breed-
ing strategies such as MARs and, in the near
future, genome-wide selection (GWS) (Bernardo
and Yu 2007) is critical to this step. Once the draft
sequence of the cassava genome becomes avail-
able, rapid improvement in the use of genomic
resources as in genomic selection is likely to be
the future direction for improvement of complex
traits.
For unknown reasons cassava has evolved
with CMG resistance in wild species, and these
and African cassava germplasm have been
identified as sources of high CMD resistance,
the latter probably under the heavy pressure of
CMGs in West Africa. The discovery of highly
resistant CMD cultivars provides hope for the
detection of new genes or QTLs associated
with CMD resistance. These will be explored
to identify more genomic regions involved
in CMD disease resistance. The synergistic
relationships among CMGs that lead to higher
virulence would probably require such multiple
sources of resistance stacked into one cultivar
to build a strong resistance that would be more
difficult to break by newly emerging CMGs.
The identification by the IITA breeding team of
additional sources of resistance to CMD opens
up
Future MAB Targets for CMD
Resistance
Abiotic stresses and especially CMD account
for major fraction-of-yield losses on farmers'
fields. Breeding for CMD resistance is there-
fore critical to boosting productivity in Africa
and Asia. Farmers and consumers desire virus-
resistant cultivars that meet their criteria. How-
ever, yield increase alone is not sufficient for
addressing the myriad of problems contended
with by cassava farmers, most of whom are poor
resource holders. Their criteria include factors
possibilities
for
pyramiding
these
genes,
thereby assuring durability.
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