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could not be differentiated with a genetic simi-
larity greater than 95% using 76 AFLP (ampli-
fied fragment length polymorphism) or 36 SSR
(simple sequence repeat) markers (Fregene et al.
2000; Fregene et al. 2003). The Nigerian lan-
draces are highly resistant in the field to ACMV
(Akano et al. 2002), and resistant to EACMV-UG
(CIAT 2003), ICMV (CIAT 2005), and EACMV
(CIAT 2006). Resistant plants showed little or
no symptoms during the entire life cycle of the
plant in Nigeria, Uganda, and India. However, in
Tanzania, resistant plants developed symptoms;
these symptoms were followed by a complete
recovery (CIAT 2006). These field observations
have been confirmed by laboratory infection of
TME3, TME7, and TME14, validating these lan-
draces as completely resistant to ACMV and par-
tially resistant to EACMV, which can induce
fairly strong systemic symptoms followed by
complete recovery 45 days after infection (Fau-
quet pers. com.).
Although, the source of CMD resistance in
the Nigerian landraces is currently not known, it
has been speculated that, given the large num-
ber of closely related landraces, they are 'lost'
sibling lines from a CMD breeding program run
by British breeders in Nigeria in the 1950s and
1960s (Beck 1980). However, the possibility that
the resistant landraces arose via a spontaneous
mutation in the CMD resistance gene(s) in a sin-
gle line, followed by selection and dissemina-
tion by farmers, cannot be disregarded. In con-
trast to the recessive multigenic inheritance of
resistance from the
M. glaziovii
source (Hahn
et al. 1980b) genetic studies demonstrated that
this CMD resistance was single locus and dom-
inant (Akano et al. 2002). Since the 1990s, the
IITA and African National Agricultural Research
Systems (NARS) have been exploiting this resis-
tance derived from Nigerian landraces (Akano
et al. 2002). Under the CMD resistance project of
the IITA, resistance profiles of cassava cultivars
were tested in multilocal trials in Nigeria, reveal-
ing the top highly CMD-resistant lines, which are
being considered as possible additional sources
of CMD resistance. This set of cultivars is being
evaluated with molecular markers to understand
the genetic basis of resistance and to identify the
loci associated with CMD resistance.
Genetic Mapping of
CMD
Resistance Genes
Molecular markers are valuable tools for under-
standing the genes and genomes underlying nat-
ural variation. Although the genetics of cassava
is the least understood of the major staples in
the world, increasing research investment in the
last two decades has resulted in the development
of ample genomic resources, which are readily
available to enhance the genetic improvement
of the crop. Since the development of the first
genetic map of cassava (Fregene et al. 1997)
other new maps have since followed (Mba et al.
2001; Okogbenin et al. 2006; Kunkeaw et al.
2010; Sraphet et al. 2011). The first genetic
map (Fregene et al. 1997) was constructed
based mainly on RFLPs (restriction fragment
length polymorphisms), RAPDs (random ampli-
fied polymorphic DNA), isozyme, candidate
genes, and AFLPs (Amplified Fragment Length
Polymorphism). The need for change to a more
polymorphic, easily accessible, and user-friendly
marker system led to the development of SSR
markers and construction of SSR-based genetic
maps, between 2001 and 2005 at CIAT (Mba
et al. 2001; Okogbenin et al. 2006; Zarate
2004). However, since 2008, efforts in genomic
resources development in cassava have rapidly
shifted to SNPs (single nucleotide polymor-
phisms), which are more abundant in genomes
compared to SSRs. SNPs are also well suited to
high-throughput genotyping platforms, making
them the markers of choice for application and
successful implementation of efficient marker-
assisted breeding.
Considering the importance of CMD, one of
the primary objectives of the first genetic map-
ping studies conducted in cassava at CIAT was
to use molecular markers to tag genomic regions
controlling CMD resistance, first begun under a
Rockefeller Foundation (RF) initiated project, in
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