R E V I EW AR T I C L E
Cowpea (Vigna unguiculata): Genetics, genomics and breeding
Ousmane Boukar1 | Nouhoun Belko1 | Siva Chamarthi1 | Abou Togola1 |
Joseph Batieno2 | Emmanuel Owusu3 | Mohammed Haruna3 | Sory Diallo4 |
Muhammed Lawan Umar5 | Olusoji Olufajo5 | Christian Fatokun6
1International Institute of Tropical
Agriculture, Kano, Nigeria
2Institut National d’Environnement et de
Recherches Agricoles, Ouagadougou,
Burkina Faso
3Savanna Agricultural Research Institute,
Tamale, Ghana
4Institut d’Economie Rurale, Bamako, Mali
5Institute of Agricultural Research, Zaria,
Nigeria
6International Institute of Tropical
Agriculture, Ibadan, Nigeria
Correspondence
Ousmane Boukar, International Institute of
Tropical Agriculture, Kano, Nigeria.
Email: o.boukar@cgiar.org
Communicated by: C. Ojiewo
Abstract
Cowpea, Vigna unguiculata (L.), is an important grain legume grown in the tropics
where it constitutes a valuable source of protein in the diets of millions of people.
Some abiotic and biotic stresses adversely affect its productivity. A review of the
genetics, genomics and breeding of cowpea is presented in this article. Cowpea
breeding programmes have studied intensively qualitative and quantitative genetics
of the crop to better enhance its improvement. A number of initiatives including
Tropical Legumes projects have contributed to the development of cowpea genomic
resources. Recent progress in the development of consensus genetic map containing
37,372 SNPs mapped to 3,280 bins will strengthen cowpea trait discovery pipeline.
Several informative markers associated with quantitative trait loci (QTL) related to
desirable attributes of cowpea were generated. Cowpea genetic improvement activi-
ties aim at the development of drought tolerant, phosphorus use efficient, bacterial
blight and virus resistant lines through exploiting available genetic resources as well
as deployment of modern breeding tools that will enhance genetic gain when grown
by sub-Saharan Africa farmers.
K E YWORD S
breeding, cowpea, genetics, genomics, productivity, Vigna unguiculata
1 | INTRODUCTION
Cowpea (Vigna unguiculata (L.) Walpers) also known as black eye pea
is a herbaceous annual crop mostly grown in the dry agro-ecologies of
the tropics in Latin America, Africa and south Asia. It is a legume and
belongs to the family Fabacea, tribe Phaseoleae, genus Vigna and sec-
tion Catiang (Marechal, Mascherpa, & Stainer, 1978). Cultivated cow-
pea, which is in subspecies unguiculata, is divided into five cultivar
groups namely Unguiculata, Sesquipedalis (yard-long-bean), Textilis,
Biflora and Melanophthalmus. The commonly cultivated cowpea
belongs to cultivar group Unguiculata while members of cultivar group
Textilis, characterized by long peduncles are grown in some parts of
Nigeria for production of fibre. Cowpea is a diploid with 2n = 22 and
a genome size of about 620 million base pairs. According to Padulosi
and Ng (1997), the area where maximum diversity of land races and
cultivated cowpeas is present is West and Central Africa.
The bulk of cowpea production and consumption is in sub-Saharan
Africa (SSA) particularly West and Central Africa. Nigeria produces the
most quantity of cowpea grains annually at approximately 2.14 million
metric tonnes (FAOStat, 2017) and consumes more than 3.0 million
metric tonnes. The other major producers are Niger Republic and
Burkina Faso with an average of 1.59 and 0.57 million metric tonnes,
respectively (Table 1). Another major cowpea producing country is
Brazil with an annual production of more than 491,000 tonnes on an
area of about 1.5 million hectares (Singh et al., 2002).
Being drought tolerant, cowpea is grown predominantly in the
dry savannahs to the Sahel in the fringes of the Sahara Desert with
annual rainfall of about 300 mm or even less. Cowpea is cultivated
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This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
© 2018 The Authors. Plant Breeding Published by Blackwell Verlag GmbH
Received: 12 December 2017 | Accepted: 24 February 2018
DOI: 10.1111/pbr.12589
Plant Breeding. 2018;1–10. wileyonlinelibrary.com/journal/pbr | 1
mainly for its grains, which are rich in protein with most improved
varieties containing between 20 and 25 per cent protein on dry
weight basis. Fresh leaves are also used as pot herb especially in
East Africa. Samireddypalle et al. (2017) reported that cowpea haulm
(dried leaves, stems and pod walls) could be a good source of
income for farmers in the dry savannah areas where the farmers also
keep livestock. In an earlier study, Duke (1990) found that cowpea
fodder could contain up to 18.6 g protein per 100 g dry weight.
Depending on the region, seed coat colour and texture could be
very important to consumers. For example, in northern parts of
Nigeria where cowpea is generally produced because of favourable
climatic conditions white-coloured grains are preferred by con-
sumers, whereas in the southern parts of the country, preference is
for brown-seeded types. Consumers also have preference for vari-
eties with short cooking time, as less fuel is needed when cooking
such varieties. This article reviews key achievements in the genetics,
genomics and breeding of cowpea which contribute to the enhance-
ment of the productivity of this crop.
2 | PRODUCTION CONSTRAINTS
2.1 | Insect pests
The productivity of cowpea in typical SSA farmers’ fields is abysmally
low at less than 600 kg/ha when compared with a potential grain
yield of over 2,000 kg/ha. A number of abiotic and biotic factors are
responsible for these low yields. The adverse effects of these yield
limiting constraints can be reduced to their barest minimum through
genetic improvement. Cowpea seedlings can be attacked and even
killed by aphids (Aphis craccivora) if not controlled with insecticides or
planting of resistant variety. Aphids are more devastating when
drought occurs shortly after seedling emergence in the field. The sin-
gle dominant gene that conferred resistance to aphid and assigned
the symbol Rac-1 by Bata, Singh, Singh, and Ladeinde (1987) has
become ineffective against aphid. This is a typical example of a major
resistance gene breaking down in cowpea. Ombakho, Tyagi, and
Pathak (1987) and Pathak (1988) reported another dominant gene,
derived from induced mutation and confers resistance to aphid, was
given the gene symbol Rac-2. The flower bud thrips (Megalurothrips
sjostedti) is yet another major insect pest of cowpea with devastating
effects mainly on flower buds thereby inhibiting anthesis and pod for-
mation. As no pods develop on plants attacked by this insect, they
tend to remain vegetative for a longer period of time. The legume
pod borer, Maruca vitrata, the most cosmopolitan of cowpea pests, is
capable of causing extensive grain yield reduction (20%–80%) if not
controlled (Singh, Jackai, Dos Santos, & Adalla, 1990). A number of
pod sucking bugs (Clavigralla tomentosicollis, Riptortus dentipes and
Nezara viridula among others) attack pods and suck sap from the
seeds while still developing within the pods. Such seeds become seri-
ously reduced in size and malformed thus making them unviable and
unattractive to farmers and consumers. When cowpea seeds are
placed in storage, weevils (Callosobruchus maculatus), which often
accompany seeds from the field, cause extensive damage by the
insect’s larvae, that feed and develop inside with adults boring holes
through which they emerge. Low levels of resistance to a few of
these insect pests have been identified among some cowpea germ-
plasm lines. Host plant resistance through incorporating desired genes
in improved varieties would be the preferred approach to protect
cowpea plants in the field and seeds in storage against these pests.
Effective insecticides are not readily available and could be unafford-
able to majority of the farmers.
2.2 | Diseases
Cowpea is also affected by several fungal, bacterial and viral diseases
causing extensive yield reductions. Among fungal diseases are pre-
emergence and postemergence damping off caused by Pythium ulti-
mum, Fusarium wilt caused by Fusarium oxysporium, macrophomina
blight caused by Macrophomina phaseolina, web blight and root rot
caused by Thanatephorus cucumeris (Rhizoctonia solani), stem rot by
Phytophthora vignae, scab by Sphaceloma sp., cercospora leaf spot by
Pseudocercospora cruenta and Cercospora apii. The major bacterial
disease of cowpea is bacterial blight caused by (Xanthomonas axono-
podis pv phaseoli). Under serious infection, bacterial blight causes
stem canker, which could eventually kill the affected plant. Bacterial
pustule caused by Xanthomonas sp. occurs sporadically and is less
damaging to cowpea compared to bacterial blight. Cowpea farmers
in sub-Saharan African are looking forward with great delight to the
TABLE 1 Top 20 cowpea producing countries in the world (2014)
Rank Country
Production
(tons) Area (ha)
Yield
(Kg/ha)
1 Nigeria 2,137,900 3,701,500 578
2 Niger 1,593,166 5,325,168 299
3 Burkina Faso 573,048 1,205,162 475
4 United Republic
of Tanzania
190,500 197,323 965
5 Cameroon 174,251 209,019 834
6 Mali 149,248 353,382 422
7 Kenya 138,673 281,877 492
8 Myanmar 115,200 132,000 873
9 Mozambique 103,837 377,900 275
10 Sudan 80,000 260,000 308
11 D R C 70,042 159,945 438
12 Senegal 64,088 153,142 418
13 Malawi 35,903 81,753 439
14 Haiti 29,895 41,525 720
15 United States
of America
21,591 12,060 1,790
16 Peru 17,588 12,779 1,376
17 Serbia 16,189 4,777 3,389
18 Sri Lanka 15,281 11,519 1,327
19 China, mainland 13,500 13,000 1,038
20 Uganda 10,100 25,000 404
Source: FAOSTAT, 2017.
2 | BOUKAR ET AL.
development and deployment of improved varieties characterized by
resistance to these various biotic constraints.
2.3 | Abiotic constraints
Although cowpea is known to be drought tolerant when compared to
other crops, the productivity of cowpea could be hampered by erratic
rainfall in the beginning and towards the end of the rainy season—a
common phenomenon, in the semi-arid tropics where cowpea is mostly
grown. With the current effects of climate change, the pattern of rainfall
in the subregion, which either comes late or stops earlier than usual,
requires that efforts be made to enhance the level of drought tolerance
in currently existing improved crop varieties being grown by farmers
(Fatokun, Boukar, & Muranaka, 2012). Cowpea yield can also be
affected considerably by heat in sensitive varieties. When the night
temperature reaches about 35°C, cowpea flowers abort due to poor
pollen development, which can result in poor seed and pod set (Hall,
1993). Low soil fertility due to low organic matter and low phosphorus
in the savannah soils is also a major constraint for cowpea production.
3 | GENETICS
Comprehensive reviews of cowpea genetics were published by Fery
(1980, 1985), Fery and Singh (1997) and Singh (2002). These reviews
covered relevant literature on cytologic, qualitative and quantitative
genetics. We will cover here some more recent literature. Padi
(2003) studied the inheritance of leaf node pigmentation, flower
(petal) colour, immature pod colour, seed coat colour, seed eye col-
our and seed eye colour pattern and reported that presence of pig-
ment was dominant over the absence of pigment and the black seed
eye was dominant over brown eye. He further reported partial domi-
nance of the very small eye pattern to the Holstein eye type.
Mustapha and Singh (2008) also reported that pod pigmentation
is digenic while pigmentation in the pod tip followed two patterns of
inheritance (monogenic and digenic), which agreed with Harland’s
(1920) findings. Cowpea seed coat colour is an important market
trait of the crop in West Africa. Using six different biparental
crosses, Egbadzor et al. (2014) could not classify segregating materi-
als based on seed coat colour as chi-square goodness-of-fit test
could not be conducted on them. They suspected that many genes
might be involved in cowpea seed coat colour inheritance. Fery
(1985) already mentioned that the complete mechanism of seed coat
pigmentation was complex and not yet understood. Lachyan, Desai,
and Dalvi (2016) studied also the inheritance of some traits in cow-
pea. The results showed monogenic inheritance for all four qualita-
tive traits including growth habit, flower colour, seed coat colour
and seed coat colour pattern. Joint segregation was observed
between seed coat colour and seed coat colour pattern. Lopes,
Gomes, and Filho (2003) reported that the number of genes that
control 100-seed weight in cowpea is five and the high values for
narrow sense heritability indicates that selection for seed size can be
made in early generations. Pandey and Dhanasekar (2004) reported
the presence of connate foliaceous stipules of primary leaves and
their inheritance in cowpea genotype EC394736. They found the
rudimentary stipules (RS) to be dominant over foliaceous stipules
(FS). The F2 segregation into 15 (RS): 1 (FS) indicated that duplicate
recessive genes control the presence of the FS.
Ishiyaku, Singh, and Craufurd (2005) showed that photoperiod in
the field of 13.4 hr per day was long enough to delay flowering of
photoperiod-sensitive cowpea genotypes and photoperiod sensitivity
was found to be partially dominant. Additive and additive 9 domi-
nance interactions were the most important gene actions condition-
ing days to flower. Narrow sense heritability of 86% was observed
while at least seven major genes with an average delay of 6 days
each control time to flowering in the cross.
Cowpea aphid-borne mosaic virus (CABMV) is a major virus dis-
ease that causes substantial cowpea yield loss. Orawu, Melis, Laing,
and Derera (2013) have reported that CABMV resistance is condi-
tioned by more than one recessive gene in eight populations, single
recessive genes controlled resistance in other seven populations.
The continuous distribution of progeny for severity data observed in
the F2 populations suggests significance of quantitative inheritance
for CABMV resistance. The general combining ability effects (59.8%)
were more important than the specific combining ability effects
(40.2%) in determining virus resistance in the tested cowpea vari-
eties. In another study using genotypes KVx640 and KVx396-4-5-
2D, Barro et al. (2016) found that resistance to CABMV is governed
by two dominant genes, each variety contributing a resistant gene.
Three types of host reaction to bacterial pustule were observed
by Patel (1981) during the screening of cowpea lines: brown hyper-
sensitive resistant (BHR), non-hypersensitive resistant (R) and sus-
ceptible (S). Inheritance study of the BHR, R and S host reactions
produced by three races of the bacterial pustule pathogen (Xan-
thomonas campestris pv. vignae unguiculatae) revealed that BHR reac-
tion was dominant over R and S reactions, and R was recessive to S
reaction (Patel, 1982). BHR reaction seemed to be controlled by two
genes: one governing BHR reaction to race 1 and the other to races
1 and 2. Both of these genes were ineffective against race 3. The
study showed that R reaction seemed to be controlled by one, two
or three recessive genes that are effective against all the races.
Using six populations (Parent 1, Parent 2, F1, F2, BC1 and BC2) gen-
erated from each of four crosses involving four resistant and two sus-
ceptible cowpea varieties evaluated for resistance to Cercospora leaf
spot (CLS), Booker and Umaharan (2008) found that mode of inheri-
tance of resistance to Pseudocercospora cruenta can be oligogenic or
polygenic depending upon the cross. This is the first report of poly-
genic inheritance of CLS resistance. In another study, the evaluation of
CLS disease in F2 plants and F2:3 families derived from a cross between
“CSR12906” (susceptible) and “IT90K-59-120” (resistant) revealed that
the disease scores were continuously distributed, suggesting that the
resistance in IT90K-59-120 is a quantitative trait (Duangsong, Kaew-
wongwal, Somta, Chankaew, & Srinives, 2016).
The genetics of flower thrips resistance was studied in crosses of
four cowpea lines. Omo-Ikerodah, Fatokun, and Fawole (2009) found
that resistance to thrips is quantitatively inherited with broad sense
BOUKAR ET AL. | 3
heritability ranging from 56% to 73% and maternal effect. Additive,
dominance and epistatic gene effects contributed significantly to
thrips resistance. These authors reported that resistance to flower
thrips is oligogenic with different genes involved in the control of
resistance in TVu1509 and Sanzi.
4 | GENOMICS
With recent advances in molecular biology, some applications of DNA
marker technologies were initiated in different cowpea research pro-
grammes. These include molecular characterization of germplasm,
development of genetic and quantitative trait loci (QTL) maps.
Restriction fragment length polymorphism (RFLP) markers were
used to characterize 44 accessions of different species belonging to
four subgenera of the genus Vigna (Fatokun, Danesh, Young, & Ste-
wart, 1993). Also, random amplified polymorphic DNA (RAPD) (Kaga,
Tomooka, Egawa, Hosaka, & Kamijima, 1996; Simon, Benko-Iseppon,
Resende, Winter, & Kahl, 2007), inter-simple sequence repeat (ISSR)
(Ajibade, Weeden, & Chite, 2000), amplified fragment length polymor-
phisms (AFLPs) (Fang, Chao, Roberts, & Ehlers, 2007) and simple
sequence repeat (SSR) (Gupta & Gopalakrishna, 2010; Ogunkanmi,
Ogundipe, Ng, & Fatokun, 2008) have been used to study genetic
diversity within both cultivated and wild relatives of cowpea. Huynh
et al. (2013) used more than 1,200 single nucleotide polymorphism
(SNP) markers to genotype a world collection of landraces and African
ancestral wild cowpea. Their study revealed the presence of two major
gene pools in cultivated cowpea in Africa: gene pool 1 with landraces
mostly distributed in Western Africa, while the majority of gene pool 2
are found in Eastern Africa. Each gene pool is most closely related to
wild cowpea in the same geographical region. More recently, genotyp-
ing by sequencing (GBS) was applied to discover SNPs in cowpea,
which were used to estimate genetic diversity, population structure
and phylogenetic relationships (Xiong et al., 2016).
Cowpea has a relatively small genome size estimated at 620
Mbp. Genome sequencing and analysis of the hypomethylated por-
tion of the cowpea genome selectively cloned by methylation filtra-
tion (MF) technology were carried out by Timko et al. (2008). Over
250,000 genespace sequence reads (GSRs) with an average length of
610 bp were generated, yielding ~160 Mb of sequence information.
About 74% of cowpea expressed sequence tags (ESTs) and 70% of
all legume ESTs were represented in the GSR data set. Given that
12% of all GSRs contain an identifiable SSR, the data set is a power-
ful resource for the design of microsatellite markers. The identifica-
tion of informative markers for marker-assisted trait selection and
map-based gene isolation necessary for cowpea improvement could
be possible. The longer-term goal of the Cowpea Genomics Initiative
(CGI) project is to conduct transcriptomic, proteomic and metabolo-
mic analyses to understand the basic biology of host and non-host
resistance to Striga and Alectra parasitism, and the control of key
agronomic characteristics such as drought tolerance, photoperiodic
control of flowering and seed nutritional quality.
Munoz-Amatriain et al. (2017) recently reported a whole-genome
shotgun (WGS) assembly, a bacterial artificial chromosome (BAC) physi-
cal map, and assembled sequences from 4,355 BACs using cowpea line
IT97K-499-35. In addition, WGS sequences of 36 other diverse cowpea
accessions supported the development of a genotyping assay, Illumina
Cowpea iSelect Consortium Array for 51,128 SNPs. This assay was used
to support linkage mapping, synteny analysis and evaluation of materials
currently in use in three West African breeding programmes (Institut
National de l’Environnement et des Recherches Agricoles (INERA—
Burkina Faso), Savanna Agricultural Research Institute (SARI- Ghana),
Institut Senegalais de Recherches Agronomiques (ISRA-Senegal) and
IITA. The platform was applied to five biparental RIL populations to pro-
duce a consensus genetic map containing 37,372 SNPs mapped to
3,280 bins. The marker density of this map and its resolution over an
earlier consensus map (Lucas et al., 2011) are 34-fold and fourfold
increases, respectively. The map spans 837.11 cM at an average density
of one bin per 0.26 cM and 11.4 SNPs per bin. All 11 cowpea LGs are
densely covered with 1.85 cM on LG1 being the largest gap. The
authors have investigated genetic diversity along each linkage group
and explained macrosynteny between cowpea and common bean. The
annotated reference genome assembly was recently made accessible
through Phytozome (www.phytozome.net).
Through early efforts of generating putative markers for cowpea,
about 41,949 EST sequences were produced from stressed and non-
stressed drought susceptible and tolerant cowpea materials repre-
senting 16,954 unigenes. This resource was merged with the avail-
able genespace sequences (GSS) data to create a larger unigene set
that has been mined to generate 4,958 molecular markers utilized to
generate a microarray chip for expression analysis in cowpea.
The development of cowpea genomic resources was initiated
under the CGIAR Generation Challenge Program (GCP) led Tropical
Legumes One project. The University of California, Riverside, (UCR)
played an important role in elaborating these resources. LGC Geno-
mics provided outsourcing facilities for the conversion of SNPs to
KASP based on the design/sequence information provided by UCR.
Early activities using the established genotyping platform were the
implementation of marker-assisted backcross (MABC) and marker-
assisted recurrent selection (MARS) strategies in NARS and IITA
cowpea breeding programmes through the use of 100 to 400 cus-
tomized SNP markers (Boukar, Fatokun, Huynh, Roberts, & Close,
2016). Informative markers associated with quantitative trait loci
related to biotic and abiotic stresses resistances were generated
while QTLs for drought tolerance and stay green (Muchero, Ehlers,
Close, & Roberts, 2009; Muchero et al., 2013) and for heat tolerance
(reproductive stage) (Lucas et al., 2013) were identified. QTLs related
to aphid resistance (Huynh et al., 2015), bacterial blight resistance
(Agbicodo et al., 2010), Fusarium resistance (Pottorff et al., 2012),
foliar thrips resistance (Muchero et al., 2010; Lucas, Ehlers, Roberts,
& Close, 2012), Macrophomina resistance (Muchero, Ehlers, Close, &
Roberts, 2011), nematode resistance (Huynh et al., 2016) and virus
resistance (Gioi, Boora, & Chaudhary, 2012) have been reported.
QTLs for leaf shape, maturity time, grain weight (seed size), seed
4 | BOUKAR ET AL.
coat colour and patterns have also been identified. Detailed informa-
tion on QTLs was provided in recent review (Boukar et al., 2016).
Several informatics application tools were developed to manage the
different genomic resources for cowpea improvement. HarvEST:Web
(http://harve st-web.org/) and HarvEST:Cowpea (Windows software
HarvEST:Cowpea (download from http://harvest.ucr.edu) allow easy
access to available cowpea genome resources. BreedIt SNP Selector
was created to facilitate the identification of SNP sets for customized
genotyping in cowpea genetic studies. This online application (http://
breedit.org/) produces different polymorphic marker subsets according
to the marker genome coverage specified. ParentChecker is another
user-friendly tool to automate inference of parental genotypes for
assisting in QTL mapping in cowpea and other crops. It is used to
develop recombinant inbred line (RIL) populations particularly for cases
where precise parental genotypic information of parents is not available.
KBio converter is a conversion tool that allows the user to input a stan-
dard LGC Genomics SNP Viewer input file and output an equivalent file
in which each SNP has been converted to a reference strand.
5 | BREEDING
5.1 | Historical trends
Cowpea research has been underway in some African countries for
many years. In Nigeria, the Federal Department of Agricultural
Research, the Institute for Agricultural Research and Training (IAR&T)
at Ibadan, the University of Ife and the Institute of Agricultural
Research (IAR) started cowpea research in early 1960s. The Centre
National de Recherches Agronomiques (CNRA) in Senegal initiated
cowpea breeding as early as 1961 (Sene & N’Diaye, 1973). Makerere
University in Uganda started cowpea improvement programme in
1965 (Rubaihayo et al., 1973), while in Tanzania, work on cowpea
began in 1959 (Rai & Utkhede, 1973). Cowpea research was boosted
in 1977 with the involvement of two organizations: Canada’s Interna-
tional Development Research Centre (IDRC) and IITA-SAFGRAD
(Semi-Arid Food Grains Research and Development). Cowpea research
program in Mali received IDRC’s support in 1980. Many other African
countries (Togo, Botswana, Kenya, Malawi, Rwanda, DRC, Central Afri-
can Republic, Niger, Benin, Ghana, Ethiopia and Zimbabwe) started
their cowpea research activities around 1980 (Singh & Ntare, 1985).
Early breeding activities in sub-Saharan Africa involved germplasm
collection, evaluation and maintenance, followed by screening for dis-
ease resistance. Efforts were directed to breeding for insect resistance,
early maturity, improved plant types and desired seed quality. Pure line
and mass selection were initially conducted to identify the lines with
high yield potential, meanwhile followed by activities involving
hybridization, population advance and selection. Seed quality, high grain
yield potential, early maturity, insensitivity to day length, erect growth
habit, lodging resistance and best fitting to intercropping were among
the initial key desired traits. Some level of work on disease resistance
was initiated in Tanzania with technical support from IITA. Mass selec-
tion, conventional bulk method and pedigree of bulk-progeny test were
among the main breeding methods. Lines developed by IITA were
evaluated and seeds of promising lines multiplied and released for gen-
eral cultivation by national partners (Singh & Ntare, 1985). So far, more
than 80 varieties from IITA breeding nursery have been released in over
60 countries.
The priorities for cowpea improvement at IITA were reviewed and
modified regularly. In the 1970s, the focus was on diseases which
resulted in the identification of lines with high potential: TVu 201(S),
TVu 1190, TVu 1977 and TVu 4557 (Singh & Ntare, 1985). In the
1980s, focus shifted to cowpea improvement for insect and multiple
disease resistances and white rough seed coat characteristics. In the
1990s to early 2000s, the IITA cowpea breeding programme embarked
on the development of (1) extra-early-maturing (60–70 days) photo-
insensitive grain type, mainly used in sole crop and short rainy seasons,
(2) medium-maturing (75–90 days) photo-insensitive grain type, fitting
to sole crop and intercrop, (3) late-maturing (85–120 days) photo-insen-
sitive dual-purpose (grain + leaf) types, for use as sole crop and inter-
crop, (4) photosensitive early-maturing (70–80 days) grain types, for
intercropping, (5) photosensitive medium-maturing (75–90 days) dual-
purpose (grain + fodder) types, for intercropping, (6) photosensitive
late-maturing (85–120 days) fodder type, for intercropping and (7) high-
yielding, bush-type vegetable varieties (Singh, Chambliss, & Sharma,
1997). Over the years, systematic incorporation of desirable genes for
resistance in improved breeding lines as well as some selected varieties
as recurrent parents was also carried out.
A number of improved lines were developed and released by
national programs in Africa. In Nigeria, landraces/breeding lines
released for cultivation by farmers included Westbred, Prima, Dinner
(vegetable cowpea variety), Ife brown, IAR 339-1, IAR 341, IAR 345
and IAR 355. In Senegal, there were Ndambour, Mougne, Bambey
21 and 23, while in Tanzania and Burkina Faso, there were TK-1,
TK-5, TKx133-16D-2 and Cross 1-6E-2 and Suvita-2, KN-1 (Vita 7)
and TVx3236, respectively.
5.2 | Current status
From 2007, IITA’s cowpea breeding programme placed emphasis on
building on progress recorded from previous work. Activities were
initiated on the identification of additional sources of resistance to
abiotic (drought, heat) and biotic (aphid, flower thrips, striga and
alectra) stresses by exploiting the germplasm collections. Being a
self-pollinated crop, breeding methods such as pure line selection,
mass selection, pedigree, backcross and single seed descent have
been employed successfully in cowpea genetic improvement. In view
of recent developments in modern breeding, some molecular breed-
ing tools have been generated for cowpea, which are now being
deployed through marker-assisted selection (marker-assisted recur-
rent selection and marker-assisted backcrossing).
5.3 | Genetic resources
A total of 15,003 cultivated cowpea from 89 countries are main-
tained in IITA’s genebank. Based on geographical, agronomical and
botanical descriptors, a core collection of 2062 accessions was
BOUKAR ET AL. | 5
established. The diversity in the core collection was similar to that of
the entire collection and correlated traits that may be linked were
also preserved in the core collection (Mahalakshmi, Ng, Lawson, &
Ortiz, 2007). A reference set, also called mini core, composed of 370
accessions representing the entirety of the genetic diversity of the
core was constituted. The minicore is a critical resource for scientists
to study new adaptive traits, conduct comparative genomics studies,
and discover new favourable alleles and new lines for prebreeding
activities.
In addition to the cowpea minicore, the first eight-parent cowpea
multiparent advanced generation intercross (MAGIC) population was
developed recently as an important genomic community resource for
trait discovery and breeding. The eight founder parents were selected
based on abiotic and biotic stress tolerance or resistance and agro-
morphological trait variability (Huynh et al., 2017). The eight parents
were intercrossed using structured matings as described by Cavanagh,
Morell, Mackay, and Powell (2008) with some modifications. Pairwise
crosses (AxB; CxD; ExF; GxH) were performed followed by 600 double
crosses producing F1 seeds (300 ABxCD; 300 EFxGH). A total of 300
four-way random pair crosses were made (ABCD x EFGH). Three hun-
dred and sixty-five (365) F1s were generated from the crosses and
advanced by single seed descent (SSD) till F8 generation (Huynh et al.,
2017). Prior to the advancement of the population through SSD, each
of 365 eight-way individuals was genotyped using SNP genotyping
with the cowpea Illumina 60K iSelect BeadArray to confirm each indi-
vidual was derived from an eight-way cross.
6 | COWPEA BREEDING UNDER TLII
Cowpea is grown mostly in the dry northern guinea savannah, Sou-
dan Savannah and Sahel agro-ecologies characterized by low annual
rainfall. In recent times, the amount of rainfall received in these
areas is declining and the distribution of the rains is irregular espe-
cially during early or late stages of the cropping seasons. These
predispose cowpea to drought while also undermining yield in
farmers’ fields. The development and release of cowpea varieties
with enhanced levels of drought tolerance to farmers in drought
prone areas of SSA was the main plank of objective three of
Tropical Legumes II project. However, most of these varieties
remain on shelves of various breeding programmes. Following the
inauguration of the Tropical Legumes project which aims to
improve livelihoods of peoples by enhancing cowpea productivity
and production in drought prone areas of sub-Saharan Africa, it
was possible to disseminate some of the shelved cowpea varieties.
Farmer participatory variety selection (FPVS) was employed to help
facilitate uptake of the improved breeding lines by farmers in vari-
ous communities.
In Mozambique, farmers showed interest in adopting the follow-
ing breeding lines, IT00K-1263, IT97K-1069-6 and IT82E-16
because of their high yield performance and drought tolerance while
the following cowpea breeding lines IT00K-1263 and IT99K-1122
were released in Tanzania. During FPVS, we observed that the best
lines in terms of grain yield in some locations were not necessarily
the most preferred by farmers. Attributes such as striga resistance
was a very important selection criterion coupled with grain and fod-
der yields by farmers in West Africa hence they selected IT97K-
499-35 in Mali, Niger and Mali whereas farmers in Mozambique pre-
ferred IT00K-1263 because of its high grain yield ability (Fatokun,
Boukar, Kamara, et al., 2012). More than 24 IITA breeding lines were
released during the last 10 years in 13 different countries in sub-
Saharan Africa (Table 2).
6.1 | Sustenance of the breeding pipeline
In order to identify sources of new genes to use in the development
of better performing breeding lines, some germplasm and improved
breeding lines available at IITA were screened for different desirable
traits under the Tropical Legumes Project.
6.2 | Screening for drought tolerance
About 1,200 germplasm lines from the Genetic Resources Center at
IITA were evaluated in the field for their responses to drought con-
ditions. The evaluation was carried out under irrigation during the
dry season. Drought stressed plants were exposed to terminal
drought following withdrawal of irrigation at 5 weeks after sowing.
It was observed that drought depressed grain yield in all the germ-
plasm lines tested. However, there were significant differences in
the extent of grain yield reduction due to drought among the lines.
Most of the lines flowered and matured earlier under drought. A
number of lines remained green at 7 weeks after irrigation water
TABLE 2 List of released cowpea varieties from 2008 to 2017
Year of
release Variety Country
2008 IT97K-499-35 Nigeria
2009 IT89KD-288, IT89KD-391 Nigeria
IT97K-499-35, IT97K-499-38, IT98K-205-8 Niger
2010 IT97K-499-35, IT93K-876-30 Mali
IT99K-573-1-1 Niger
2011 IT82E-16, IT00K-1263, IT97K-1069-6 Mozambique
IT99K-494-6 Malawi
IT99K-573-1-1, IT99K-573-2-1 Nigeria
2012 IT99K-7-21-2-2-1, IT99K-573-1-1 Tanzania
2013 IT99K-573-2-1, IT98K-205-8 Burkina Faso
IT95K-193-12 Benin
2015 IT00K-1263, IT99K-1122 Tanzania
IT07K-292-10, IT07K-318-33 Nigeria
IT05K-321-2, IT97K-390-2, IT82E-16,
IT-82E-18, IT99K-494-4
Swaziland
IT99K-573-2-1, IT99K-573-1-1 Sierra Leone
2016 IT90K-277-2, IT07K- 211-1-8 South Sudan
IT99K-573-2-1, IT99K-573-1-1 Ghana
6 | BOUKAR ET AL.
was suspended. Some of these stay-green type had delayed flower-
ing under drought (Fatokun, Boukar, & Muranaka, 2012). A total of
190 lines were found to show enhanced levels of drought tolerance
as proportions of grain yield reduction due to drought were rela-
tively lower among them. These were further evaluated, and the
best 10 were selected for use in making crosses aimed at developing
populations segregating for drought and from where selections have
been carried out for breeding lines with superior drought tolerance.
6.3 | Screening for phosphorus use efficiency
The marginal soils in which cowpea is mostly grown in the Sahel
and Soudan savannah agro-ecologies are generally sandy with very
low levels of organic matter. The soils are also deficient in minerals
needed by plants for good growth. Cowpea is a legume, which can
fix atmospheric nitrogen in the crop’s root nodules. It can therefore
obtain some of its needed nitrogen from the root nodules while
also leaving some in the soil for crops that follow in rotation. For
good nodulation to occur phosphorus is an essential mineral that is
generally lacking in the marginal soils where most cowpea is grown.
Fifty improved breeding lines from IITA cowpea breeding nursery
were evaluated in pots placed on benches in the glass house for
their responses to phosphorus applications (0, 30, 60 and
90 kg/ha). Nodulation was highest in a dual-purpose breeding line
IT98K-166-44. Response to added phosphorus was higher in
IT89KD-288 than in IT98K-166-44. Line IT89KD-288 was superior
to all others in terms of efficiency in the utilization of P, while line
IT99K-7-21-2-2 was found to be least efficient under low P appli-
cation. The study showed that the application of 90 kg P per hec-
tare could adversely affect cowpea’s performance; hence,
application of P between 30 and 60 kg per hectare would be most
beneficial to many lines. It is possible, from this work, to develop
cowpea breeding lines that can utilize efficiently low amounts of
phosphorus application.
6.4 | Screening for resistance to aphid
Aphid (Aphis craccivora) is a major insect pest of cowpea especially
during the crop’s seedling stage in the field. Aphid attack on cowpea
is most devastating when drought occurs in the seedling stage.
Entire crop can be wiped out in such situations. The single dominant
gene that confers resistance to this pest and which had been incor-
porated in several improved breeding lines and varieties has now
become ineffective as such plants now succumb to the pest. There
is need for identification of new sources of resistance. A number of
germplasm lines and some cross compatible wild relatives were
screened for resistance to aphid in the seedling stage. A wild cowpea
relative, line TVNu 1158, was found to be resistant to aphid, and
this has been crossed to some improved breeding lines with the aim
of transferring the resistance gene to good background through
backcrossing. A set of about 210 recombinant inbred lines (RILs) has
been generated from the cross which has been used to produce a
genetic linkage map of cowpea comprising about 17,739 SNP mark-
ers. The RILs have also been phenotyped for aphid resistance, and
the data will be used for QTL analysis.
6.5 | Screening for resistance to bacterial blight
Bacterial blight, caused by Xanthomonas axonopolis pv. vignicola, is
one of the major diseases of cowpea. It causes considerable yield
losses in susceptible lines. Fifty improved cowpea breeding lines
(Table 3) were inoculated by spraying a suspension containing the
bacterium obtained from a culture maintained in IITA’s pathology
laboratory. The results showed that fourteen of the 50 improved
breeding lines tested were resistant to bacterial blight. These breed-
ing lines are good sources of genes for resistance to bacterial blight.
6.6 | Screening for resistance to bean common
mosaic virus in cowpea
The bean common mosaic virus (BCMV) is one of the important viruses
affecting cowpea in SSA. About 100 germplasm lines were screened
for resistance to this virus, and two showed good levels of resistance
using both visual scores and enzyme-linked immunosorbent assay
(ELISA) test. An evaluation of F1s from a diallel cross involving 10 (both
resistant, tolerant and susceptible) lines revealed that both additive
and non-additive genetic variability are important which suggests that
integrated breeding strategies can be adopted to efficiently utilize the
TABLE 3 Severity of bacterial blight on 50 cowpea accessions 4 weeks after inoculation with Xanthomonas axonopolis pv vignicola
Variety Disease severity Group
IT98-692, IT04K-405-5, IT99K-573-2-1, TVu-7778 3–3.67 HS
IT89KD-288, IT03K-378-4, IT97K-1069-6, IT97K-1042-3, IT04K-227-4, IT98K-1103-13, IT98K-128-3, IT99K-573-1-1,
IT99K-377-1, IT00K-1207, IT98K-412-13, IT03K-316-1, IT99K-494-6, IT98K-589-2, IT99K-216-24-2, IT98K-205-8,
IT99K-529-2, IT96D-610, IT97K-390-2, IT97K-568-18, IT00K-901-5, IT98D-1399, IT98K-1092-1, IT97K-499-35,
2–2.83 S
IT93K-452-1, IT03K-351-1, IT98K-131-2, IT00K-898-5, IT99K-216-44, IT99K-7-21-2-2, IT98K-503-1, IT98K-628,
IT98K-491-4, IT98K-506-1,
1.13–1.92 MR
DANILA, IT00K-1263, IT03K-324-9, IT97K-819-118, IT98K-1092-2, IT98K-1111-1, IT98K-1263, IT98K-133-1-1,
IT98K-166_4, IT98K-311-8-2, IT99K-1060, IT99K-1122, IT99K-529-1, IT00K-835-45
1 R
Mean 1.92
SE 0.13
HS, Highly susceptible; S, susceptible; MR, moderately resistant; R, resistant.
BOUKAR ET AL. | 7
additive as well as non-additive genetic variability (Inegbenose, 2016)
in the development of BCMV resistant cowpea breeding lines.
7 | CONCLUSION
Although cowpea has for some time now been regarded as an
orphan crop in view of the relatively low level of research attention
given to the crop, modest progress has been made in the assemblage
and conservation of its germplasm, generation of genomic tools for
more effective breeding and development of improved varieties
some of which are already available in SSA farmers’ fields. The chal-
lenge of striga to cowpea production especially in the dry savannah
agro-ecologies is being effectively contained with the development
of varieties that show immunity to this parasitic weed. There is how-
ever room for the development of better performing varieties that
will be characterized by high stable grain yield, good amount of pro-
tein, large seed size, white or brown seed coat colour, resistance to
some insect pests which presently creating huge losses to the crop.
The over 15,000 and 2,000 accessions of cultivated and wild com-
patible wild relatives in the gene bank at IITA, respectively, need to
be systematically screened for adaptive genes that control the abi-
otic and biotic stresses still limiting its productivity. In addition, cow-
pea breeding programmes at IITA and NARS are currently engaged
in the implementation of modern breeding practices that will
improve their efficiency. With the development of various platforms
such as phenotyping, genotyping and data management coupled with
necessary resources now being committed to cowpea research, more
impacts in term of variety development and integrated crop manage-
ment practices will be achieved.
ACKNOWLEDGEMENTS
This review summarizes most of the work performed under cowpea
breeding component of Tropical Legumes projects of Bill and
Melinda Gates Foundation and USAID Feed the Future projects (IITA
project, Legume Innovation Lab and Climate Resilient Cowpea Pro-
ject) for which the authors are grateful. We also would like to thank
Prof. Tim J. Close, Prof. Phil A. Roberts, Dr. Lam Bao Huynh and Dr.
Maria Munoz-Amatriain for their contributions to the development
of cowpea genomic resources, Dr. Ranajit Bandyopadhyay and Dr.
Robert Abaidoo for their contributions to the work on bacterial
blight and P use efficiency, respectively.
ORCID
Ousmane Boukar http://orcid.org/0000-0003-0234-4264
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How to cite this article: Boukar O, Belko N, Chamarthi S,
et al. Cowpea (Vigna unguiculata): Genetics, genomics and
breeding. Plant Breed. 2018;00:1–10.
https://doi.org/10.1111/pbr.12589
10 | BOUKAR ET AL.