Biology Reference
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genotypes being tested for CMD resistance via
MAS thereby reducing breeding cost.
type (AR40-6) was tolerant of CBSD (Mkamilo
pers. com.). LA germplasm selected with
CMD2
markers that were shipped into East Africa were
first evaluated on the field for CMD resistance.
A selection based on CMD resistance, harvest
index, and total biomass was made and the best
genotypes (more than 80) were selected from the
503 LA genotypes received by Tanzania. The
CMD-resistant LA genotypes were then intro-
gressed into Tanzanian germplasm with CBSD
tolerance. They were planted in controlled cross-
ing blocks together with 54 local genotypes (with
resistance to CBSD) from the Eastern and South-
ern zones of the country. More than 40,000
crosses were made between the improved geno-
types and the local cultivars, producing more
than 60,000 seeds. The resultant progenies devel-
oped from the cross between LA genotypes and
Tanzania lines were screened with
CMD2
mark-
ers (the second round of MAS selection for CMD
resistance in the scheme) to reduce population
size, and then field tested to select those com-
bining CMD and CBSD resistance. The geno-
types having both CMD and CBSD resistance
were selected and then integrated into the nor-
mal breeding scheme for the evaluation of agro-
nomic traits. The materials were then planted
and evaluated through the breeding scheme. The
genotypes were tested extensively in the breed-
ing scheme, leading to the identification of ten
best genotypes with vigor, high yield, good dry
matter, and combining both CMD and CBSD
resistance.
Yield obtained for the best 10 genotypes were
between 50 and 87 tons per ha (Table 15.2).
These are very high yields - roughly four to
eight times higher than the normal yield aver-
age of local cultivars - suggesting the possibil-
ity of heterosis from cross combination between
LA germplasm and Tanzania local germplasm.
Similar activities are underway in other African
countries that have received LA germplasm from
CIAT. While it is possible to release in West
Africa the MAS-developed CMD-resistant LA
genotypes produced by CIAT, it is not possible
to do so easily in East Africa because of the need
(c) Exploring Heterosis
Genetic diversity studies using molecular mark-
ers have shown genetic differentiation in cassava
between the primary center of diversity in Latin
America and the secondary center of diversity in
Africa (Fregene et al. 2000; Fregene et al. 2003;
Kizito et al. 2005). This genetic divergence is
believed to represent heterotic groups that may
be explored in breeding to enhance cassava pro-
duction.
A marker-assisted selection (MAS) project
was initiated in Tanzania in 2003 with fund-
ing support from the Rockefeller Foundation to
improve gene pools for resistance to disease and
pests (CMD and CGM) and for high yield and dry
matter content (Blair et al. 2007; Fregene et al.
2006). This initiative was later supported with
funding by the CGIAR GCP and Alliance for
Green Revolution in Africa (AGRA) with tech-
nical assistance from IITA. The project sought
to explore heterosis, in order to improve yield
and dry matter content, by carrying out exten-
sive crosses between the LA germplasm (intro-
gressed with CMD resistance via MAS) and
Tanzanian germplasm. While the CR and AR
genotypes from LA have CMD resistance that
was developed via MAS, the AR has CGM resis-
tance introgressed from wild relative
M. flabeli-
folia,
offering sources of resistance to the cassava
green mite in addition to the resistance
CMD2
provides.
The marker-assisted breeding scheme used
in connection with MAS developed in LA
germplasm and implemented in East African
NARS is different from that used in West Africa
resulting from the prevalence of cassava brown
streak disease (CBSD) in East Africa. CBSD,
which is a highly devastating viral disease,
has yet to be reported in West Africa. LA
germplasm is susceptible to the disease (CIAT
2005; Mkamilo pers. com.). A total of 503 LA
genotypes were shipped and only one geno-
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