An Economic Assessment of Banana Genetic Improvement and ...

IMPROVED BANANA CULTIVARS AND MANAGEMENT PRACTICES 41
1990, diagnostic studies were conducted to
identify and quantify banana production
constraints. These studies were followed
by collection of endemic germplasm with
the aim of searching for cultivars resistant
to the pests (weevils and nematode) and
diseases (especially black Sigatoka). For
the purposes of evaluating endemic materials
for yield, pest, and disease reaction,
two germplasm collections were established
at Makerere University Agricultural
Research Institute, Kabanyolo (MARIK),
and Kawanda Agricultural Research Institute
(KARI). Endemic germplasm was collected
from farmers in different parts of
the country.
Development of New Genotypes
in Uganda
Formal banana breeding did not begin in
Uganda until 1994. First, studies were conducted
to identify fertile female banana cultivars
(with potential to set seeds when pollinated)
from the assembled set of endemic
cultivars and pollen-fertile diploid clones to
serve as sources of desirable attributes. A
study of female banana fertility, conducted
from 1996 and 1998, identified 12 of the
most fertile EAHBs in the collection (Ssebuliba
2000). In addition to the low seed set
in the endemic cultivars, a seed germination
of only 1 percent was recorded when the
seeds were sown directly into soil.
Embryo culture, which is now a routine
practice in Uganda, is used to improve the
germination of hybrid seeds derived from
inter- and intraspecific crosses. The seeds
are surface sterilized and aseptically cracked
to expose the embryo. The embryo is extracted
and inoculated onto an appropriate
medium (Murashige and Skoog 1962) to
achieve germination. Seedlings are subsequently
micropropagated to obtain several
shoots for field evaluation. Embryo culture
has increased the seed germination rate to 9
percent compared to 1 percent germination
previously obtained by sowing the seeds
directly into nursery potting mixture (Ssebuliba
2000; Ssebuliba et al. 2006).
The first hybrids selected from the crossing
of the triploid landraces (3×) and the
resistant diploids (2×) were tetraploids (4×,
or four sets of chromosomes). They were
very fertile and therefore easily set seeds in
the presence of a pollen source. The presence
of seeds in bananas, especially cooking
varieties, poses a quality problem and is
not desirable. To circumvent this problem,
the tetraploids are crossed with improved
diploids to get secondary triploids (3×) that
are sterile. As of mid-2005, 11 secondary
triploids had been selected for in-depth
screening, targeting selection of clones in
which the desired traits have been maximally
accumulated.
The best-yielding highland bananas
proved sterile and hence not amenable to
conventional breeding. Genetic engineering,
through which desirable genes can
be inserted directly into plant cells and
thus plants (new genotypes) can be regenerated
without passing through seeds,
appears to be the only alternative for
improving these cultivars. The known
method through which new genes can be
engineered into bananas is based on cell
suspension, where aggregations of single
cells are first generated. Desirable trait
genes are then inserted into these cells,
after which the process of selecting transformed
cells and regenerating them into
plants for subsequent evaluations follows.
Efforts to establish the system are progressing
well, and the first plants regenerated
from single cells of an EAHB cultivar
(Nakyetengu) are planted out in the
field to test the extent to which they are
true to type (Namanya et al. 2004). Nakyetengu
is now in field evaluation, and no
somoclonal variation has been observed
to date. Three other EAHB cultivars
(Mpologoma, Nakinyika, and Nakasabira)
have been regenerated and planted
for field evaluation. Suspension culture
methods are now being optimized for a
wide range of cultivars, and arrangements
to initiate gene insertions are in their final
stages.