Biology Reference
In-Depth Information
biotype differences is unknown in other parts of
Africa.
ber of breeding lines and cultivars by IITA, using
conventional breeding approaches (Singh 1993),
and by ISRA (Institut Senegalais de Recherches
Agricoles) (Cisse et al. 1995). The resistance
provides effective protection from weevil dam-
age for about two months, but levels of damage
in resistant cultivars approach that found in sus-
ceptible cultivars after six months of infestation.
This resistance is useful in developing countries
but when used alone provides insufficient pro-
tection to ensure that the grain retains its market
value and seed quality characteristics. It would
be helpful to identify SNP markers for resistance
to cowpea weevil and to look for options that
provide higher levels of protection.
Markers for Resistance to Other Insects
Insect resistance is a good candidate for MAS
in cowpea since assessments of host plant resis-
tance to insects are often difficult to conduct in
the field or greenhouse. However, other than for
aphid, insect-resistance factors in cowpea dis-
covered thus far do not provide immunity to
the pest and often have low heritability under
field conditions. Field screenings that rely on
natural insect infestations are subject to nat-
ural fluctuations in pest pressure. When such
variability is combined with incomplete resis-
tance, field screens can lead to misclassifica-
tion and selection of lines lacking the strongest
resistance. For example, this has been the case
with screening cowpea breeding lines and acces-
sions for resistance to Lygus bug (
Lygus hes-
perus
) and pod sucking bugs (such as
Nezara
viridula, Clavigralla tomentosicollis, Riptortus
dentipes
). In addition, colonies of insects may be
difficult to rear without specialized facilities and
trained entomologists to monitor insect growth
and screening tests. Such resources have not been
available to cowpea breeding programs histori-
cally. Resistance to the pod bug
C. tomentosicol-
lis
has been identified in the wild cowpea (ssp.
dekindtiana
) germplasm line TVNu 151 (Koona
et al. 2002). MAS could be used to introgress
resistance factors from such wild cowpea into
cultivated forms using a rapid backcrossing
approach, based on simultaneous selection for
the resistance genes (markers) and against mark-
ers associated with unwanted wild germplasm
characteristics such as small seed size and pod
shattering.
The cowpea storage weevil (or cowpea
Bruchid) (
Callosobruchus maculatus
Fabricius)
is a devastating pest of stored cowpea in Africa,
Asia, and the Americas. Resistance to the cow-
pea storage weevil has been identified (Singh
and Jackai 1985) and incorporated into a num-
Genetically-Modified Cowpea for
Control of Maruca PodBorer and
Cowpea Weevil
Cowpea was one of the last major grain legume
species for which genetic transformation was
achieved (Popelka et al. 2006). This technology
now presents an additional option for address-
ing constraints that have not yielded to other
approaches.
Maruca pod borer (
Maruca testulalis
) is one
of three major insect pests affecting cowpea pro-
duction in Africa and parts of Asia (Singh et al.
1996). Major efforts to screen both wild and cul-
tivated cowpea for resistance to this pest have not
revealed meaningful levels of resistance. Effec-
tive transgenes have been identified for control
of cowpea weevil and the Maruca pod borer, and
these genes have been used to transform cowpea
(Higgins et al. 2012). 'Bt cowpea' is currently
undergoing yield and efficacy testing in confined
yield trials in Burkina Faso and Nigeria. Since
the distribution of wild cowpea species overlaps
with areas of cowpea cultivation in Africa and
since many of these wild cowpeas can form fer-
tile hybrids with cultivated cowpeas, the escape
of the 'Bt gene' into wild cowpea is a proba-
ble outcome of release of
Bt
-cowpea. An expert
panel formed to address this 'geneflow' issue
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