Marker-Assisted Breeding for Cassava Mosaic Disease Resistance - Translational Genomics for Crop Breeding: Biotic Stress - page 301

order to understand the molecular basis of CMD
resistance and to improve cassava genetic resis-
tance to the disease via MAS. Following that
initial step, and over the last decade, three CMD
genomic regions have since been identified (Fre-
gene et al. 2000; Akano et al. 2002; Okogbenin
et al. 2012).

differential resistance profile lines developed at
IITA from the M. glaziovii source supports its
multigenic character.

(b) CMD2

Classical genetic analysis and mapping of the
resistance phenotype in landraces was used to
identify the inheritance and genetic location of
CMD2 on a genetic map of cassava. The map-
ping population was an F 1 progeny from a cross
between the CMD-resistant landrace TME3 and
a susceptible improved line TMS 30555. The
progeny, comprising 158 individuals, was estab-
lished in vitro, sub-cloned, and evaluated in a
replicated field trial in two locations in Nigeria
with high CMD pressure in June 1998 (Akano
et al. 2002). The ratio of resistant to suscep-
tible plants was 1:1, fitting with the expected
segregation ratio for a single dominant locus
heterozygous in TME3. Bulk segregant analy-
sis (BSA) was used to identify simple sequence
repeat (SSR) markers linked to CMD2 (Fig-
ure 15.3). Results of single marker analysis
showed that the SSR marker explained 70%
( P

(a) CMD1

The first genetic map of cassava was developed
using an intraspecific cross between TMS30572,
an improved cultivar from IITA, Ibadan, Nige-
ria, and CM2177-2, an elite line from CIAT,
Cali, Colombia (Fregene et al. 1997). The
female parent has CMD resistance derived from
M. glaziovii . A bulk segregant analysis (BSA)
approach was used to identify a marker linked
to M. glaziovii resistance. An SSR marker,
SSRY40, on linkage group D of the genetic map
of cassava, was found to be associated with CMD
resistance and was designated CMD1 (Fregene
and Puonti-Kaerlas 2002); it explains 48% of the
phenotypic variance of CMD resistance. CMD1
appears recessive, as its effect is detected only in
backcross progeny and not in the F 1 . The reces-
sive nature of this source of resistance, however,
makes it less attractive, given cassava's outcross-
ing and heterozygous nature. Considering the
fact that resistance in M. glaziovii is based on
classical genetic analysis of crosses, which indi-
cate that the trait is polygenic with a component
that is recessive, this result implies therefore that
more loci from those sources of disease resis-
tance have yet to be mapped with markers. The

0.0001) of the phenotypic variance of CMD
resistance, confirming the hypothesis of single-
gene inheritance of this CMD resistance. Subse-
quent genetic mapping located the gene on link-
age group G of the cassava genetic map between
an RFLP (GY1) and an SSR marker, SSRY28, at
a distance of 9 cM and 8 cM respectively (Akano
et al. 2002; Beck 1980).
After the first mapping of the gene, fur-
ther activities aimed at identifying molecular

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Fig. 15.3. Polyacrylamide gel electrophoresis for CMD2 marker SSRY28 between
resistant and susceptible genotypes. For a color version of this figure, please refer to the
color plate.