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developed at IITA, is exceptional in that it is
resistant to four strains of CABMV, cowpea yel-
low mosaic virus (CYMV), and cowpea mild
mottle virus (CMMV). A severely damaging dis-
ease of cowpea known as 'stunt' is caused by
a dual infection of BlCMV and CMV (Kuhn
1990). Ouedraogo and colleagues (2002a) were
able to map determinants of resistance to CPMV,
CPSMV, B1CMV and SBMV in the RIL popu-
lation 524B x IT84S-2049 using AFLP mark-
ers. Subsequently, this population was geno-
typed with the 1,536 Illumina GoldenGate Assay
allowing for the localization of SNP markers for
these resistance loci (Table 10.2).
and colleagues (2002a) were able to convert one
of these markers to a SCAR (sequence char-
acterized amplified region) that has proven to
be an effective and remarkably reliable marker
for resistance to
Striga
SG1 and SG3 conferred
by
Rsg2-1
and
Rsg4-3
. This SCAR marker, des-
ignated 61R (E-ACT/M-CAA), detects a single
polymorphic band linked to SG1 and SG3 resis-
tance in the resistant cultivars B301, IT82D-
849, and Tvu 14676, and is being tested for
use in breeding trials. Subsequently, two AFLP
markers were identified that are closely linked
to
Rsg1-1
, a gene that also confers resistance
to SG3 in Nigeria (Boukar et al. 2004). One
of those AFLP markers, designated E-ACT/M-
CAC
115
and determined to be 4.8 cM from
Rsg1-1
, was converted to a SCAR marker for
ease of use in breeding programs (Boukar et al.
2004). Because the mapping population 524B x
IT84S-2049 used in the studies undertaken by
Ouedraogo and colleagues (2002b) and Boukar
and colleagues (2004) was genotyped with the
1,536 Illumina GoldenGate array, we were able
to identify SNP markers corresponding to the
AFLP markers identified earlier (Table 10.1).
Markers for Resistance to the Parasitic
Weed Striga
The parasitic weed
Striga gesnerioides
(Wild.)
Vatke causes considerable damage to cowpea
in the Sudan savanna and Sahelian regions of
Africa. Several sources of resistance to this par-
asite have been identified and resistance has been
incorporated using conventional approaches into
advanced breeding lines by IITA (Timko et al.
2007) and by Cisse and colleagues (1995) into
cultivars for Senegal (Hall et al. 2004). Striga
exhibits strain variation such that cultivars that
are resistant in one location may be susceptible in
another (Lane et al. 1993). Genetic studies have
shown that three dominant, non-allelic genes
confer resistance to different Striga isolates but
that the mechanisms differ (Singh 1993).
Useful markers for implementation of MAS
are available only for some of the
Striga
resis-
tance genes, and these were the first candi-
dates for broad application in cowpea breed-
ing programs. Ouedraogo and colleagues (2002a,
2002b) found three AFLP markers linked to
Rsg2-1
, a gene that confers resistance to
Striga
Race 1 (SG1) present in Burkina Faso, and six
AFLP markers linked to gene
Rsg4-3
a gene that
provides resistance to
Striga
Race 3 (SG3) from
Nigeria. Two of the AFLP markers were asso-
ciated with both
Rsg2-1
and
Rsg4-3
.Ouedraogo
Markers for Resistance to
Cowpea Aphid
Cowpea aphid (
Aphis craccivora
) is one of
the most destructive pests of cowpea in Africa
and worldwide. Myers and colleagues (1996)
reported the identification of an RFLP (restric-
tion fragment length polymorphism) marker
linked to the resistance to cowpea aphid, but
MAS for aphid resistance was not implemented.
More recently we have identified SNPs associ-
ated with aphid resistance in California in an
RIL population developed from the cross 'Cali-
fornia Blackeye 27' (susceptible) x IT97K-556-
6 (resistant). IT97K-556-6 has also been shown
to be resistant to cowpea aphid in Texas and
Nigeria (B. B. Singh, personal communication,
2010), but its efficacy relative to possible aphid
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