Biology Reference
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limited in cultivated gene pools, including high
protein, delayed post-harvest physiological dete-
rioration, and acyanogenesis. The introgression
of this genetic diversity has been successfully
initiated through pre-breeding activities prior uti-
lization in breeding programs in South Amer-
ica and to some extent in Asia (e.g., Thailand).
However, because of the susceptibility of the
Latin American (LA) germplasm to CMD, this
germplasm cannot be used directly in Africa or
India unless it is improved for CMD resistance
(Okogbenin et al. 1998; Okogbenin et al. 2007).
A key breeding objective is continued improve-
ment of LA germplasm for CMD resistance to
give it greater utility in CMD-prone regions of
the world. Given that CMGs do not yet exist in
LA (and the accidental introduction of the CMGs
to LA is a frightening potential!), pre-emptive
breeding for CMD resistance in the absence of
the pathogen is critical to protecting the vast
wealth of genetic resources of cassava in Latin
America.
Vital breakthroughs have been made in recent
years in understanding the nature, biology and
interactions of the CMGs,
B. tabaci,
and the cas-
sava host. The information gleaned from such
efforts will help to further refocus key breeding
objectives for CMD resistance. Only determined,
well coordinated, and comprehensive breeding
objectives addressing both research and develop-
ment needs will allow the true potential of cas-
sava to be unlocked, enabling this most versatile
crop to provide food security, income, and new
commercial opportunities for a growing world
population, especially in Africa.
resistance (Jennings 1994). The earliest resis-
tance breeding programs, initiated in the 1930s in
Madagascar and at the Amani station in northeast
Tanzania, used interspecific crosses with
Mani-
hot glaziovii
Muell.-Arg. to produce progenies.
Interspecific hybrids were then backcrossed to
cassava, leading to highly resistant cultivars that
have been developed and used in East Africa.
Seeds from one of the most resistant clones,
5318/34, were sent to Nigeria, where selections
made in the 1960s were then used in the 1970s as
parents in the initial cassava improvement pro-
gram at the International Institute of Tropical
Agriculture (IITA), Ibadan, Nigeria (Hahn et al.
1980b). The program resulted in the develop-
ment of resistant clones and seeds, which were
distributed to African countries (Mahungu et al.
1994). This is the most widely deployed source
of resistance and is represented principally in
African farmer fields by clones of the Tropical
Manihot
Species (TMS) series (TMS 4(2) 142,
TMS 30337, TMS 91934, TMS 30001, TMS
60142, and TMS 30572) (Hahn et al. 1980b).
Resistance derived from
M. glaziovii
was
found to be multigenic with a recessive com-
ponent. The mechanisms of action of the multi-
genic
M. glaziovii
-derived resistances include:
prevention of initial virus infection via resis-
tance to insect vectors, reduction in the rate of
virus replication, restriction in the movement of
the virus within the plant, and tolerance to the
virus (reduction in the effects of a given virus
titer on growth and development of the plant)
(Fargette et al. 1996). Cassava breeding is com-
plicated by the biology of the crop and its het-
erozygosity, which in the absence of inbred lines
has imposed limitations to the successful use of
this multigenic source of resistance in cassava
genetic improvement.
The second major source of resistance was
identified in several Nigerian cassava landraces
that have consistently shown strong resistance to
CMD. High levels of resistance to CMD have
been described in a group of closely related
Nigerian cassava landraces (Akano et al. 2002).
These landraces, TME3 to TME7 and TME14,
Sources of CMD Resistance
A very small number of natural sources of gem-
inivirus resistance have been identified in wild
and cultivated plants. In many instances, genetic
analysis indicates these sources of resistance to
geminiviruses are controlled by multiple loci,
making their use in a breeding program very
difficult. Attempts were made in the 1930s and
1940s to breed cultivars with greater levels of
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