Biology Reference
In-Depth Information
response to the disease. Resistant cultivars are
much less readily infected than are susceptible
cultivars. While in resistant cultivars the local-
ized distribution of the virus and overall con-
centrations tend to be low, they are nevertheless
a potent source of inoculum from which spread
can occur (Fargette et al. 1988).
The earliest resistance-breeding programs,
initiated almost simultaneously in the 1930s in
Madagascar and at the Amani station in north-
east Tanzania, used both intraspecific and inter-
specific crosses with a cultivar of cassava wild
species to produce progeny with increased levels
of CMD resistance. Most success was achieved
with the interspecific
Manihot glaziovii
Muell.
Arg. crosses, and the Amani group then used
a series of backcrosses to restore root quality
whilst retaining resistance. High levels of resis-
tance were obtained, and the program was ter-
minated in the late 1950s, but seeds from one of
the most resistant clones, 5318/34, was used to
reinitiate the work at the International Institute of
Tropical Agriculture, IITA, Nigeria, beginning in
1970. One of the clones derived from this seed,
designated 58308, had a good combination of
CMD resistance and root quality and formed the
basis for much of the resistance breeding work
that followed at IITA (Hahn et al. 1980b). Some
of the most important clones from the Tropical
Manihot
Species (TMS) series that resulted from
this work have been widely distributed across
the continent, with sizeable production of the
varieties in some countries, including Nigeria,
Uganda, and Ghana (FAO 2005).
increase the precision of selection and reduce the
breeding cycle required for the development of
improved cultivars for the dynamic demands of
trade.
The pace of development of molecular mark-
ers has been phenomenal in recent years. Mark-
ers are very useful and available to breeders for
gene discovery and crop improvement via MAS.
With conventional approaches, genetic improve-
ment can be expected to increase over time, but it
could be slow and up to a defined limit. However,
with molecular markers, results can be achieved
much sooner, and obstacles to success can be
overcome by the introduction of novel genes
and through the combination of more positive
alleles.
The increased potential for breeders to use
genetic rather than phenotypic selection in their
programs means that many of the previous
approaches may be effectively redesigned. The
ways in which the new breeding tools will
directly alter returns from the program are: (1)
enhanced productivity, through novel genes and
traits, including the pyramiding of existing genes
more rapidly; (2) improved cultivars, resulting
from more thorough testing before commercial
release; and (3) accelerated breeding, by short-
ening the breeding cycle, and genes that promote
the development of new products or provide new
options for producers. Desirable breeding tech-
nologies are those that allow a significant change
in the number of tests that can be carried out in
a given stage of the program, permit the eval-
uation of traits(s) that was/were not previously
possible, or provide information on lines at an
earlier stage of the program. In some cases, this
can simply mean an increase in the number of
lines that can be assessed in the early stages
because of the improved selection efficiency for
the late stages of the program (Bennan and Mar-
tin 2007). Such tools will be highly beneficial to
CMD-resistance breeding, which under normal
conventional breeding schemes, requires active
intensive screening and selection for stable and
durable CMD resistance at the early stages, after
which, at about the third year of the scheme, the
Benefits of Molecular Markers for
Genetic Improvement
The pressure to meet the food and indus-
trial requirements of the world's population has
necessitated the need to explore and develop new
tools to complement conventional breeding prac-
tices for efficient development and rapid delivery
of superior cultivars to stakeholders. Molecular
markers are tools that could easily be amenable to
and integrated into breeding programs to rapidly
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