Biology Reference
In-Depth Information
breeding in the genetic improvement of cassava
for resistance to CMD and its huge impact in
shortening the breeding cycle, achieved through
a fast-track breeding scheme hastening the
release of novel cassava cultivars for increased
production and commercialization of cassava.
Marker-assisted breeding encompasses differ-
ent breeding strategies, such as marker-assisted
selection (MAS) - the selection of specific
alleles at a few loci; marker-assisted breeding
backcross (MABC) - the transfer of a limited
number of loci from one genetic background
to another; marker assisted recurrent selection
(MARS) - the identification and selection of sev-
eral genomic regions involved in the expression
of complex traits in order to assemble the best
performing genotype within a single or across
related populations (Ribaut et al. 2010); and
genome-wide selection (GWS) - selection based
on markers without significance testing and with-
out identifying a prior subset of markers associ-
ated with the traits (i.e., without quantitative trait
locus (QTL) mapping) (Bernardo 2007).
In this chapter we report on the significant
advances made in the last decade in the integra-
tion of molecular markers for CMD-resistance
breeding. The challenges of combining CMD
resistance with other key traits in marker-assisted
breeding in a heterozygous genetic background
are also highlighted. Also covered in this chap-
ter are the future prospects and opportunities for
cassava breeding based on access to advanced
molecular technologies, high throughput plat-
forms, and innovative paradigms.
eties has reached 90%, generating gains worth
US$12 billion in the last two decades (CGIAR
2011). New technologies in the area of tissue cul-
ture and molecular biology have further strength-
ened this, resulting in more positive contribu-
tions (Schopke et al. 1996; Fregene et al. 1997,
Fregene et al. 2000; De Vries and Toenniessen
2001; Taylor et al. 2001; Taylor et al. 2011).
Cassava is allogamous and most cultivars are
therefore highly heterozygous. Cassava genetic
improvement starts with the production of new
recombinant genotypes derived from selected
elite clones. Hybridization of select parents is
necessary to create source populations with high
genetic variability. With large separate male and
female flowers, which do not open at the same
time, cassava flowers are very easy to hand-
pollinate. One successful pollination yields a
maximum of three seeds - a very low yield
compared to cereal crops - but this is seldom
obtained, and in most situations one seed per pol-
linated flower is a good average. Seeds mature
2.5-3 months after pollination. The success rate
of hand-crossing seems to vary widely. Fukuda
(1980) reports that 80% of pollinated flowers set
fruit. The breeding scheme often involves elab-
orate evaluation stages, which require a longer
time for output delivery (Figure 15.1).
Genetic improvement of clonally propagated
non-inbred crops such as cassava is helped by
the fact that a superior genotype can be fixed at
any stage in the breeding scheme. Nonadditive
gene actions, including dominance and epistasis,
are important components of the genetic vari-
ance, which can be manipulated by the breeder
(Jaramillo et al. 2005; Calle et al. 2005; Perez
et al. 2005). The multiplication rate of cassava
planting materials is low, as five to ten cuttings
can be obtained from one plant (Blair et al.
2007). A typical selection scheme cycle in cas-
sava begins with the crossing of elite clones
and finishes when the few clones surviving the
selection process reach the stage of regional tri-
als across several locations (Figure 15.1). As
the number of plants representing each geno-
type increases, the weight of selection criteria
Cassava Breeding and Genetics
Plant breeding has one of the highest rates
of return among investments in agricultural
research. Cassava has benefitted from breeding
(Kawano 2003), resulting in the development of
new cultivars in Africa, Asia, Latin America, and
the Caribbean, bringing US$440 million in the
late 1990s, with immense benefit to rural house-
holds. Recent estimates indicate that in Thai-
land and Vietnam, adoption of improved vari-
Search WWH ::
Custom Search