Biology Reference
In-Depth Information
Marker-Assisted Breeding for
CMD2 Resistance in Cassava
in cassava, and screening for new sources of
resistance through negative selection for iden-
tified known sources of resistance in African
germplasm. Gene pyramiding through selection
of parents with different sources of CMD resis-
tance (to improve stability and durability of resis-
tance in cassava) and fixing of CMD genes
in a clonally propagated crop such as cassava,
through selection for homozygosity at loci of
interest, are other added advantages of MAS.
The CMD2 markers have been applied in MAB
basically in the following major areas of cassava
breeding, as outlined in the sections that follow.
Cassava genetic improvement can be made more
efficient through the use of easily assayable
molecular markers, which allow for the precise
identification of genotype without the confound-
ing effect of the environment, thereby increasing
heritability. The selection of progenies based on
genetic marker data substantially increases the
rate of genetic gain, especially if the number of
cycles can be reduced (Meuwissen et al. 2001).
An ideal target for MAS is the breeding for
disease resistance, since one or only a few genes
are often involved. The discovery of CMD resis-
tance in TME3, a landrace from Nigeria, and
the major nature of the gene also means that
a genetic marker for marker-assisted selection
(MAS) can be easily identified. MAS could thus
become an invaluable tool for breeding CMD
resistance in Africa, where the disease is most
prevalent, as well as in Latin America, where pre-
emptive breeding for CMD disease resistance has
assumed importance, with the aim of mitigating
possible accidental introduction of the disease
to the region. The occurrence in the Americas
of B. tabaci biotype B ( Bemisia argentifolia ),
which has a wide range of hosts, including cas-
sava, makes the threat of the virus more fright-
ening for cassava production in the Neotropics
(Polston and Anderson 1997). Using the CMD2
gene, MAS has been applied for CMD resis-
tance breeding in Latin America and Africa.
The CMD2 markers are gradually being used
for breeding and genetic analysis of CMD resis-
tance in Asia (Biu 2010). MAS in cassava has
been applied in breeding for CMD resistance in
the absence of the gene, in the Americas, by using
CMD2 markers to introgress CMD resistance
into valuable LA susceptible germplasm. The
LA germplasm were selected with CMD markers
and then introduced into CMD-infected environ-
ments in Africa and Asia. The other benefits of
MAS include improved potential to reduce large
population sizes early in the breeding scheme,
fast-tracking the long breeding scheme typical
(a) Breeding in the Absence of the
Pathogen in the Americas
CMD is a threat to South America, where the dis-
ease has not been reported but where the vector
of CMD viruses has recently become widespread
on other hosts, although not on cassava (Polston
and Anderson 1997), increasing the possibility
that these vectors could adapt to cassava and
that the CMGs could cross over to cassava in
the Neotropics. Latin American cassava is highly
susceptible to CMD (Okogbenin et al. 1998). The
absence of the pathogen in the Neotropics has
imposed limited breeding capacity to develop LA
cultivars with CMD resistance. In the absence
over a period of decades of the proper environ-
ment that would support CMD-resistance breed-
ing in LA, it is not unexpected that elite LA lines
would be highly susceptible to the disease.
Even in instances where CMD resistance
breeding was initiated, the environment was not
appropriate to test the materials. In the 1990s,
the use of the polygenic source for improving
CMD resistance in cassava of LA genetic back-
ground was not successful because of its reces-
sive nature. Several hundreds of thousands of
seeds from CIAT developed from crosses involv-
ing donor lines for the M. glaziovii source of
resistance succumbed to the disease when tested
in Africa. The large quantities tested were partly
a result of the inability to preselect for the
Search WWH ::




Custom Search