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Cowpea (Vigna unguiculata (L.) Walp) is susceptible to several viruses in West Africa. Cowpea viral diseases are mainly controlled through the use of resistant cultivars. Co-infection with more tha...

The variable cowpea productivity across different environments demands evaluating the performance of genotypes in a breeding program prior to their release. The aim of this study was to assess yield stability of eight cowpea advanced breeding lines selected from participatory varietal selection in multilocational trials, and to identify mega-environments for cowpea production in Ghana. The genotypes were evaluated across five environments in 2016 and 2017 in randomized complete block design with three replications. The GEA-R version 4.0 software was used for genotype main effect plus genotype by environment interaction (GGE) biplot analyses. Analysis of variance (PROC GLM of SAS using a RANDOM statement with the TEST option) detected significant variations for location, year, genotype, environment, and their interactions. The results showed that the yield performances of the cowpea genotypes were highly influenced by genotype × environment interaction effects. The principal component 1 (PC1) and PC2 were significant components which accounted for 46.75% and 22.84% of GGE sum of squares, respectively. We showed for the first time, two mega-environments for cowpea production and testing in the major cowpea production agro-ecologies in Ghana. The genotypes SARI-6-2-6 and IT07K-303-1 were adapted to Damongo, Nyankpala, and Tumu, whereas SARI-2-50-80 was adapted to Yendi and Manga. The best ranking location was Damongo followed by Tumu, and Nyankpala. The high-yielding genotypes, IT86D-610, IT10K-837-1, IT07K-303-1, and SARI-2-50-80 had significant higher grain yields than the check (Bawutawuta) and were recommended for release as cultivars (or as breeding lines) to boost cowpea production in Ghana.

SummaryComparative analysis of preferences and key criteria for selecting cowpea (Vigna unguiculata L. Walp.) varieties was conducted using the data collected from farmers’ participatory varietal selection (FPVS) activities conducted over 3 years with a total of 2401 farmers (1230 male and 1171 female) in 13 villages in Tougouri department in northern region and Tiéfora department in southern region of Burkina Faso in West Africa. Over the 3 years, farmer criteria for variety selection remained basically stable, but some variations among the regions and years were noticed. Grain yield was the most common and the most important criterion for farmers’ choice in both regions. Farmers in Tougouri (north) put more emphasis on early maturity (90%) and drought resistance (19.7%) as selection criteria than farmers in Tiéfora (south). Farmers in Tiéfora placed statistically significant importance on seed colour and plant type, while farmers in Tougouri did not, and for these selection criteria, there were only slight differences between genders in both areas. Results of stepwise multiple regression indicated that maturity and seed colour in the north, and seed size and seed colour in the south were the most important selection factors for farmers to select cowpea varieties. Improved varieties should have sufficiently good yield to be accepted, but other favoured traits may differ by target region as a reflection of local and regional market demands as well as deep-rooted cultural preferences. Understanding local and regional differences in selection criteria for cowpea varieties is necessary to improve the acceptance of newly released improved varieties. Preferences identified in the participatory activities could inform further development of cowpea breeding strategies for north and south regions of Burkina Faso.

Three hundred and ninety accessions comprising 260 cultivated and 130 wild cowpea accessions were evaluated phenotypically using 27 cowpea descriptors. Morphological evaluation of some qualitative traits revealed 11.92% and 29.23% presence of pigmentation on the stem, 1.53% and 20.76% presence of stripes on the pod, and 0% and 20% presence of hairiness on the plant of cultivated and wild cowpeas respectively. As for the molecular analysis, sixteen SSR primers were employed for genotyping 48 accessions from both wild and cultivated cowpeas. The data generated a dendrogram with three clusters, two of which consisted of wild cowpea while the third cluster comprised all the cultivated cowpeas, including the yard-long-bean (Vigna unguiculata subsp. sesquipedalis) and Vigna unguiculata subsp. cylindrica accessions. Two wild accessions of subsp. dekindtiana, and one each of subsp. kgalagadensis and protracta clustered with cultivated cowpea indicating their relationships with cultivated cowpea, but not with other wild cowpeas. The numbers of polymorphic SSR bands in cultivated and wild cowpeas were 38 and 54, respectively, while the PIC values were 4.47 and 6.14, respectively, showing a greater genetic diversity in wild than in cultivated cowpeas. The subsp. dekindtiana had the highest number (80%) of shared SSR bands with cultivated cowpea followed by subsp. protracta with 54% of shared bands. Five species of wild cowpea have hairs and so could be used in breeding for resistance to insects.

Flower bud thrips is one of the most destructive insect pests of cowpea in sub-Saharan Africa. Information on genetic variability among cowpea germplasm and interrelationships among traits under thrips infestation would facilitate the development of resistant varieties. The objectives of the study were to assess genetic variability for thrips resistance, estimate heritability of yield and other traits and investigate inter-trait relationships under thrips infestation. One hundred and fifty-six cowpea lines, including one resistant and one susceptible check, were screened for resistance under natural infestation at two locations in Nigeria, in 2016. Test lines were scored for thrips damage weekly for three consecutive weeks, after removal of spreader plants, to obtain damage scores (DS) 1, 2 and 3 while data were collected on agronomic traits. The data were subjected to analysis of variance from which genetic components of the phenotypic variance were computed. Interrelationships among traits were determined using phenotypic and genotypic correlation, and sequential path analyses. Significant variability was observed among test lines. Lines TVu 6824 and TVNu 1307 were identified as possessing thrips resistance. DS3 had significant genetic and phenotypic correlations with DS1, DS2 and yield-related traits. Number of pods per peduncle, number of peduncles per plant and DS3 were identified as first-order traits. Heritability estimates ranged from 0.55 to 0.73. Genetic variability among the lines suggests the possibility of genetic control of thrips while number of pods per peduncle, number of peduncles per plant and DS3 would serve as useful selection criteria for thrips resistance.

The narrow base of genetic diversity characteristic of cowpea can be attributed to it being self-pollinating, evolving from narrow wild germplasm and exhibiting very limited gene flow between wild and cultivated types. Backcrossing to introduce simply inherited desirable traits and utilization of improved breeding lines and varieties as parents in crossing programs further narrowed the genetic base of cowpea varieties. In most cowpea breeding programs, genes for resistance and market traits were pyramided into lines characterized by high levels of acceptance to farmers and consumers. Besides predisposing widely distributed improved varieties to genetic vulnerability, a narrow base of genetic variation may be contributing to the plateauing in cowpea grain yield, which compromises genetic gains. Cross compatible wild relatives have not been used in variety development because breeders shy away from them due to their tiny seed size, unattractive seed coat color and texture, pod shattering, and susceptibility to viruses. A number of wild cowpea relatives, both within and outside section Catiang of Vigna species, have been evaluated for their reaction to cowpea insect pests and diseases. Vigna vexillata lines were resistant to the legume pod borer (Maruca vitrata), the cowpea weevil (Callosobruchus maculatus), and Striga gesnerioides but are cross incompatible with cultivated cowpea. Some lines among the cross compatible wild relative V. unguiculata ssp. dekindtiana were found to be resistant to aphid in the seedling stage, while others showed good levels of drought and heat tolerance. Molecular markers are being generated to identify quantitative trait loci (QTL) with effects on some desirable attributes in cowpea. Modern breeding tools, including transgenics, can be applied for the improvement of cowpea, bypassing the natural barriers of traditional breeding. Transgenic cowpea with Bt gene cry1Ab showing resistance to M. vitrata has been released in Nigeria. Genome editing, a powerful emerging tool, can also be used for developing improved cowpea varieties with durable resistance to pests and diseases.

Both cowpea and yard-long bean belong to Vigna unguiculata ssp. unguiculata but have diverged through human induced evolution in sub-Saharan Africa and Asia, respectively. To map the quantitative trait loci (QTLs) for yield associated traits and derive new lines that may combine the attributes of both types, we developed a F2:3 mapping population derived from a cross between cowpea line TVu2185 and yard-long bean line TVu6642. Using DArT markers, a total of 30 QTLs accounting for 1.8-13.0% phenotypic variation was detected for pod and seed traits. Some novel major QTLs for peduncle number per plant (qPeN2.2), pod length (qPoL3), seed breadth (qSB4), length (qSL7.2) and thickness (qST9) identified on chromosomes 2, 3, 4, 7 and 9, respectively, are particularly interesting and need to be validated. Moreover, we confirmed previously reported QTLs for pod length (qPoL8) and 100-seed weight (qSW8) on chromosome 8 and for seed number per pod (qSN9.2) on chromosome 9 suggesting usefulness for marker-assisted-selection purpose. Notably, some QTLs for these traits were clustered especially on chromosomes 5, 7, 8, 9 and 10 indicating the presence of the same QTL or linked loci in these regions. Moreover, the involvement of epistasis was observed for trait expressions, but compared with the main effect QTLs, the phenotypic effects of epistatic-QTLs detected were much less. The present QTL analysis may provide a useful tool for breeders to formulate efficientbreeding strategy for introgression of the desirable alleles for yield related traits in cowpea using molecular markers.

Cowpea (Vigna unguiculata(L. Walp) is an important grain legume for human and livestock nutrition, especially in sub-Saharan Africa. Aphid,Aphis craccivoraKoch (Homoptera: Aphididae), is one of the most widespread and destructive insect pests of cowpea and host-plant resistance is an effective approach to minimize the pest damage at seedling stage. This study was aimed at identifying resistant sources toA. craccivorawithin the cowpea mini core collection, a set of accessions from the largest world cowpea germplasm collection maintained at the International Institute of Tropical Agriculture (IITA). A total of 375 lines including 373 from IITA mini core collection, one resistant (TVu-801) and one susceptible (TVx-3236) checks were evaluated through artificial infestation in screening cages during the seedling stage. In cages, genotypes were planted in single rows containing four plants. They were arranged in an augmented design in which the two checks were sown in individual cages. Scoring for aphid population and damage levels were carried out on individual plants at 7, 14, and 21 days after planting. Advanced bioassays and biochemical analyses were conducted to investigate the mechanism of resistance toA. craccivora. Overall, three genotypes TVu-6464, TVu-1583, and TVu-15445 showed good levels of resistance comparable to the resistant check TVu-801. The HPLC analyses proved that both low sucrose levels in the plant, as well as high levels of kaempferol and quercetin, aglycones of phenolic compounds, were related with high resistance to aphids. The above genotypes with promising levels of resistance toA. craccivorawill be used in cowpea breeding programs to develop improved resistant lines against this pest.

Multi-parent advanced generation inter-cross (MAGIC) populations are an emerging type of resource for dissecting the genetic structure of traits and improving breeding populations. We developed a MAGIC population for cowpea (Vigna unguiculata L. Walp.) from eight founder parents. These founders were genetically diverse and carried many abiotic and biotic stress resistance, seed quality and agronomic traits relevant to cowpea improvement in the United States and sub-Saharan Africa, where cowpea is vitally important in the human diet and local economies. The eight parents were inter-crossed using structured matings to ensure that the population would have balanced representation from each parent, followed by single-seed descent, resulting in 305 F8 recombinant inbred lines each carrying a mosaic of genome blocks contributed by all founders. This was confirmed by single nucleotide polymorphism genotyping with the Illumina Cowpea Consortium Array. These lines were on average 99.74% homozygous but also diverse in agronomic traits across environments. Quantitative trait loci (QTLs) were identified for several parental traits. Loci with major effects on photoperiod sensitivity and seed size were also verified by biparental genetic mapping. The recombination events were concentrated in telomeric regions. Due to its broad genetic base, this cowpea MAGIC population promises breakthroughs in genetic gain, QTL and gene discovery, enhancement of breeding populations and, for some lines, direct releases as new varieties.

Crop genetic diversity is a sine qua non for continuous progress in the development of improved varieties, hence the need for germplasm collection, conservation and characterization. Over the years, cowpea has contributed immensely to the nutrition and economic life of the people in Togo. However, the bulk of varieties grown by farmers are landraces due to the absence of any serious genetic improvement activity on cowpea in the country. In this study, the genetic diversity and population structure of 255 cowpea accessions collected from five administrative regions and the agricultural research institute of Togo were assessed using 4600 informative diversity array technology (DArT) markers. Among the regions, the polymorphic information content (PIC) ranged from 0.19 to 0.27 with a mean value of 0.25. The expected heterozygosity (He) varied from 0.22 to 0.34 with a mean value of 0.31, while the observed heterozygosity (Ho) varied from 0.03 to 0.07 with an average of 0.05. The average inbreeding coefficient (FIS) varied from 0.78 to 0.89 with a mean value of 0.83, suggesting that most of the accessions are inbred. Cluster analysis and population structure identified four groups with each comprising accessions from the six different sources. Weak to moderate differentiation was observed among the populations with a genetic differentiation index varying from 0.014 to 0.117. Variation was highest (78%) among accessions within populations and lowest between populations (7%). These results revealed a moderate level of diversity among the Togo cowpea germplasm. The findings of this study constitute a foundation for genetic improvement of cowpea in Togo.

Cowpea is an important crop for food security, as it is a primary source of protein and cash income for farmers in the dry savanna areas of Sub-Saharan Africa. Improved varieties should contain reasonable amounts of protein in the grains, and to ensure this, timely and cost-effective analytical methods need to be developed. In this study, a procedure for estimating the grain nitrogen (protein) content in single seeds using near infrared spectroscopy was developed. Near infrared spectroscopic analysis of single seeds is a useful tool for breeders and food companies. A total of 200 cowpea germplasm accessions with wide variation in grain nitrogen content (2.44 – 5.12%N) were used for calibration. Two kinds of sample processing were performed for seed preparation (intact seed and husked seed). The models developed were validated using data from 88 and 89 genotypes. The calibration models based on both intact and husked seeds showed a high level of consistency (R2 = 0.88 and 0.88) in cross-validation. The calibration models predicted samples in validation sets with an accuracy of r2 = 0.85, RMSEP = 0.17%N, RPD = 2.55 and r2 = 0.88, RMSEP = 0.15%N, RPD = 2.90, for intact and husked seeds, respectively. With these levels of accuracy, the developed predictive models showed that NIR measurements for single seeds could be useful to determine the uniformity of grain quality, or for rapid screening of the nitrogen content in the seeds of cowpea genotypes.

Perennial habit and floral scent are major traits that distinguish domesticated cowpeas from their wild relatives. However, the genetic basis of these two important traits remains largely unknown in cowpea. Plant longevity, a perenniality-related trait, and floral scent, an outcrossing trait, were investigated using a RIL population derived from a cross between a domesticated and a wild cowpea. QTL analysis revealed three significant loci, one on chromosome 8 associated with plant longevity and two, on chromosomes 1 and 11, for floral scent. Genes within the QTL regions were identified. Genes encoding an F-box protein (Vigun08g215300) and two kinases (Vigun08g217000, Vigun08g217800), and involved in physiological processes including regulation of flowering time and plant longevity, were identified within the perenniality QTL region. A cluster of O-methyltransferase genes (Vigun11g096800, Vigun11g096900, Vigun11g097000, Vigun11g097600, and Vigun11g097800) was identified within the floral scent QTL region. These O-methyltransferase cowpea genes are orthologs of the Arabidopsis N-acetylserotonin O-methyltransferase (ASMT) gene, which is involved in the biosynthesis of melatonin. Melatonin is an indole derivative, which is an essential molecule for plant interactions with pollinators. These findings lay the foundation for further exploration of the genetic mechanisms of perenniality and floral scent in cowpea. Knowledge from this study can help in the development of new extended-growth cycle lines with increased yield or lines with increased outcrossing for population breeding.

Cowpea, Vigna unguiculata (L.) Walp., is an important grain legume grown and consumed not only in the dry savannah areas of Sub-Saharan Africa but also in many other tropical and subtropical regions. It provides income, food and nutrition security to millions of people. Several studies have led to a better understanding of the taxonomy of cowpea and its wild relatives. The species diversity, distribution and evolution of cowpea have been intensively explored. The crop is mainly cultivated in intercropping system where its low plant population does not allow the full expression of the yield potential of the cultivars being grown. Considerable challenges affect the production of this crop despite its comparatively better adaptation to harsh environments. The available genetic resources maintained in the different gene banks are being used for the improvement of cowpea. Germplasm diversity and cultivars characterization were conducted in different studies. Sources of resistance/tolerance to key biotic and abiotic stresses are being identified and introgressed genes involved in new breeding lines are being developed. Improvement strategies were developed to address the major constraints to production while also taking consumer preferences into consideration. Breeding approaches of self-pollinated crops were used in the breeding programs. Application of biotechnology has been suggested to address intractable problems. Considerable effort has been made to genetically transform cowpea. Recent development of genomic resources should support the implementation of molecular breeding to complement conventional breeding and to enhance genetic gain. Key elements needed for successful application of molecular breeding tools include the availability of a high-throughput genotyping platform, high-quality consensus genetic maps, improved phenotyping capability and identification of markers closely linked to target traits. Progress is being recorded in many of these areas which should allow the development of modern breeding programs that will result in effective and efficient development of improved resilient cowpea cultivars.

The importance of cowpea, Vigna unguiculata, in human and animal nutrition and sustainability of soil fertility are recognized globally especially in sub-Saharan Africa (SSA) where the crop is mainly produced in the Savanna and the Sahelian agro ecologies. However, cowpea productivity is adversely affected by both biotic (insect pests, diseases, parasitic weeds, nematodes) and abiotic (drought, heat, low soil fertility) constraints. Appreciable progress has been made in the improvement of cowpea for resistance to some biotic stresses particularly diseases such as bacterial blight, ashy stem blight, marcophomina, parasitic weeds like Striga and Alectra and some insects like aphid, leaf and flower thrips among others. There is need for intensifying research activities with focus on improving cowpea resistance to abiotic stresses. As a crop grown commonly in arid regions, cowpea is subjected to seedling stage, midseason and terminal droughts. In the recent past, the amount of rainfall, during the cropping season in the dry savannah regions of SSA, is getting less. Consequently the cropping season is getting shorter occasioned by late commencement or early cessation of the rain. Farmers in the cowpea producing areas of SSA generally have no access to irrigation hence their crops are grown under rain-fed conditions. With the impending higher frequency of drought in the dry savannah region due to climate change, efforts should be made in developing climate resilient cowpea varieties that farmers will grow. Efforts have been made in enhancing tolerance to drought in some improved cowpea varieties using conventional breeding but progress has been slow in this regard. Drought tolerance is a complex trait and many genes are involved in its inheritance. Pyramiding of these genes in improved varieties would therefore, be desirable. Such varieties with pyramided genes are likely to be stable in performance over the years and across several locations in the savannahs. Recent developments in molecular biology could play significant role in the development of such resilient varieties. In a number of crops, molecular markers associated with resistance loci have been identified and are being used in marker assisted breeding. Marker assisted backcrossing (MABC) is the choice when single traits that are simply inherited are to be moved to varieties with superior performance but lacking in the trait being transferred. Also, marker assisted recurrent selection (MARS) has shown promise in accumulating multiple genes in improved varieties of some crops. Some work has been initiated in cowpea on the use of MARS to pyramid resistance to Striga, yield and drought. Results obtained so far show the potential of this method in pyramiding desirable genes in cowpea. As more resources get committed to cowpea research a solid foundation would be established for the generation of molecular tools that should facilitate their routine application to the improvement of the crop.

Cowpea (Vigna unguiculata L. Walp) is a warm-season legume with a genetically diverse gene-pool composed of wild and cultivated forms. Cowpea domestication involved considerable phenotypic changes from the wild progenitor, including reduction of pod shattering, increased organ size, and changes in flowering time. Little is known about the genetic basis underlying these changes. In this study, 215 recombinant inbred lines derived from a cross between a cultivated and a wild cowpea accession were used to evaluate nine domestication-related traits (pod shattering, peduncle length, flower color, days to flowering, 100-seed weight, pod length, leaf length, leaf width and seed number per pod). A high-density genetic map containing 17,739 single nucleotide polymorphisms was constructed and used to identify 16 quantitative trait loci (QTL) for these nine traits. Based on annotations of the cowpea reference genome, genes within these regions are reported. Four regions with clusters of QTL were identified, including one on chromosome 8 related to increased organ size. This study provides new knowledge of the genomic regions controlling domestication-related traits in cowpea as well as candidate genes underlying those QTL. This information can help to exploit wild relatives in cowpea breeding programs.

Cowpea (Vigna unguiculataL. Walp) is a warm-season legume with a genetically diverse gene-pool composed of wild and cultivated forms. Cowpea domestication involved considerable phenotypic changes from the wild progenitor, including reduction of pod shattering, increased organ size, and changes in flowering time. Little is known about the genetic basis underlying these changes. In this study, 215 recombinant inbred lines derived from a cross between a cultivated and a wild cowpea accession were used to evaluate nine domestication-related traits (pod shattering, peduncle length, flower color, flowering time, 100-seed weight, pod length, leaf length, leaf width and seed number per pod). A high-density genetic map containing 17,739 single nucleotide polymorphisms was constructed and used to identify 16 quantitative trait loci (QTL) for these nine domestication-related traits. Candidate genes underlying each of those 16 QTL were identified. Four regions with clusters of QTL were identified, including one on chromosome 8 related to increased organ size. This study provides new knowledge of the genomic regions controlling domestication-related traits in cowpea as well as candidate genes underlying those QTL. This information can help to exploit wild relatives in cowpea breeding programs.Key messageThis study identified regions of the cowpea genome that played an important role in cowpea domestication, including a hotspot region for increased organ size

Cowpea (Vigna unguiculata L. Walp.) is an important cash, food and nutritional security grain legume crop in the semi-arid regions of sub-Saharan Africa. However, its productivity is hampered by several biotic stress factors including numerous insect pests that infest and damage the crop at all its development stages in the field as well as during storage. Host plant resistance is an environmental-friendly, cost-effective and sustainable pest management option for minimizing the pests’ incidence and severity. This review article aims at describing the major insect pests in cowpea and highlight key past and recent research findings in cowpea resistance to insect pests. It also provides in-depth knowledge in the host-plant resistance mechanisms in cowpea i.e. biophysical, biochemical and physiological factors that regulate the defense systems in the plant. Furthermore, the paper discusses the need for advanced investigation on the genetic basis of the plant defense systems and its application to the crop breeding program for developing new improved materials. The review would support the cowpea breeding program with the overall expectations of developing insect-resistant lines, reducing the input costs of insecticides while also enhancing cowpea productivity in sub-Saharan Africa.

Flower bud thrips, Megalurothrips sjostedti is the most severe field pest of cowpea that causes massive flower abortion which eventually results to substantial yield reduction in Africa. There is paucity of information on the mode of gene actions controlling inheritance of resistance to flower bud thrips in cowpea in the literature. The objectives of study were to assess the genetic variability for thrips resistance among the cowpea germplasm, determined the mode of inheritance of genes that conferred resistance and both broad and narrow-sense heritability estimates for the inheritance of thrips resistance in cowpea. Twelve cowpea lines were used in crosses in the screen house at IITA, Ibadan. The mating was accomplished using North Carolina design II to generate 48F1 hybrids, which were eventually evaluated with the parents. Data on number of peduncles, number of pods and number of thrips per flower were recorded and subjected to analysis of variance using random model by SAS 9.2. Significant variability was observed for most agronomic and thrip-adaptive traits among the cowpea germplasm, parental-lines and F1 genotypes evaluated. General combining ability (GCA) and specific combining ability (SCA) mean squares were significant (P

Development and analysis of Multiparent Advanced Generation Inter-Cross (MAGIC) populations have been conducted with several crop plants to harness the potential for dissecting the genetic structure of traits and improving breeding populations. We developed a first MAGIC population for cowpea (Vigna unguiculataL. Walp.) from eight founder parents which are genetically diverse and carry many abiotic and biotic stress resistance, seed quality and agronomic traits relevant to cowpea improvement in sub-Saharan Africa (SSA) where cowpea is vitally important in the human diet and in local economies. The eight parents were inter-crossed using structured matings to ensure the population would have balanced representation from each of the founder parents, followed by single-seed descent, resulting in 365 F8 recombinant inbred lines (RILs) each carrying a mosaic of genome blocks contributed from all founders. This was confirmed by SNP genotyping with the cowpea Illumina 60K iSelect BeadArray. Following filtering to eliminate duplicates, sister lines and accidental selfing events, a core set of 305 F8 RILs was chosen as the primary population. The F8 lines were on average 99.74% homozygous while also diverse in agronomic traits including flowering time, growth habit, maturity, yield potential and seed characteristics across environments. Trait-associated SNPs were identified for most of the parental traits. Loci with major effects on photoperiod sensitivity and seed size were also verified by genetic mapping in biparental RIL populations. The distribution of recombination frequency varied considerably between chromosomes, with recombination hotspots distributed mostly in the telomeric regions. Due to its broad genetic base, this cowpea MAGIC population promises breakthroughs in genetic gain and high-resolution genetic mapping for gene discovery, enhancement of breeding populations and, for some lines, direct releases as new varieties.

Cowpea is one of the most important grain legumes in sub-Saharan Africa (SSA). It provides strong support to the livelihood of small-scale farmers through its contributions to their nutritional security, income generation and soil fertility enhancement. Worldwide about 6.5 million metric tons of cowpea are produced annually on about 14.5 million hectares. The low productivity of cowpea is attributable to numerous abiotic and biotic constraints. The abiotic stress factors comprise drought, low soil fertility, and heat while biotic constraints include insects, diseases, parasitic weeds, and nematodes. Cowpea farmers also have limited access to quality seeds of improved varieties for planting. Some progress has been made through conventional breeding at international and national research institutions in the last three decades. Cowpea improvement could also benefit from modern breeding methods based on molecular genetic tools. A number of advances in cowpea genetic linkage maps, and quantitative trait loci associated with some desirable traits such as resistance to Striga, Macrophomina, Fusarium wilt, bacterial blight, root-knot nematodes, aphids, and foliar thrips have been reported. An improved consensus genetic linkage map has been developed and used to identify QTLs of additional traits. In order to take advantage of these developments single nucleotide polymorphism (SNP) genotyping is being streamlined to establish an efficient workflow supported by genotyping support service (GSS)-client interactions. About 1100 SNPs mapped on the cowpea genome were converted by LGC Genomics to KASP assays. Several cowpea breeding programs have been exploiting these resources to implement molecular breeding, especially for MARS and MABC, to accelerate cowpea variety improvement. The combination of conventional breeding and molecular breeding strategies, with workflow managed through the CGIAR breeding management system (BMS), promises an increase in the number of improved varieties available to farmers, thereby boosting cowpea production and productivity in SSA.

Cowpea is an important grain legume crop in sub-Saharan Africa (SSA) where, on a worldwide basis, the bulk is produced and consumed. The dry savanna area of SSA is where cowpea is mostly grown under rain-fed conditions. The crop is therefore prone to drought which may occur early, mid and/or late in the cropping season. Compared with many other crops, cowpea is drought tolerant, even though drought is still a major constraint limiting its productivity in SSA. Increasing the level of drought tolerance in existing cowpea varieties grown by farmers would enable them to obtain more and stable yield from their cowpea fields. As a first step towards enhancing drought tolerance in existing cowpea varieties, 1288 lines were selected randomly from cowpea germplasm collections maintained at the International Institute of Tropical Agriculture, and evaluated for their drought tolerance at Ibadan. Drought was imposed by withdrawal of irrigation from 5 weeks after sowing. On average, drought reduced the number of days to flower by 12 d, and the mean grain yield per plant was also reduced by 67.28%. A few of the cowpea lines stayed green for up to 6 weeks after irrigation was stopped, even though some of these produced no pods when the study was terminated. Further evaluation in the screenhouse of 142 selected drought-tolerant lines helped to identify six lines that could be potential parents for developing breeding lines with enhanced drought tolerance.

Cowpea, an indigenous crop to sub-Saharan Africa, is found mainly in the dry savanna regions along with cereals such as millets and sorghum. Cowpea is grown primarily for human consumption of the grains, which are rich in protein, carbohydrates and contain some minerals. The development and deployment of cowpea varieties with higher nutritional value will be of immense benefit to consumers. As a first step in the enhancement of mineral content in cowpea grains, several germplasm lines (1541) of different origins and obtained from the genetic resources unit at IITA were sown in the experimental field in Minjibir, Kano State, Nigeria. The grains were analysed for protein and nine mineral contents. However, in this study, we shall report data on crude protein, Fe, Zn, Ca, Mg, K and P. The data generated from the chemical analysis were subjected to the ‘mixture of normal distributions’ clustering method, which distributed the cowpea lines into nine groups. Groups G7 and G9 contained 174 genotypes and these were characterized by high mineral concentrations. The mean nutritional content values for group G9 were 24.7% for protein, and 58.9, 41.5, 1107, 2132, 15,282 and 5664 mg/kg for Fe, Zn, Ca, Mg, K and P, respectively. On the other hand, the 363 lines making up groups G2, G3 and G6 showed low amounts of nutrients in their grains. Some of the lines in G7 and G9 would be good as parents to use in breeding programmes that aimed at developing nutrient-dense cowpea varieties.

Cowpea bacterial blight (CoBB), caused by Xanthomonas axonopodis pv. vignicola (Xav), is a worldwide major disease of cowpea [Vigna unguiculata (L.) Walp.]. Among different strategies to control the disease including cultural practices, intercropping, application of chemicals, and sowing pathogen-free seeds, planting of cowpea genotypes with resistance to the pathogen would be the most attractive option to the resource poor cowpea farmers in sub-Saharan Africa. Breeding resistance cultivars would be facilitated by marker-assisted selection (MAS). In order to identify loci with effects on resistance to this pathogen and map QTLs controlling resistance to CoBB, eleven cowpea genotypes were screened for resistance to bacterial blight using 2 virulent Xav18 and Xav19 strains isolated from Kano (Nigeria). Two cowpea genotypes Danila and Tvu7778 were identified to contrast in their responses to foliar disease expression following leaf infection with pathogen. A set of recombinant inbred lines (RILs) comprising 113 individuals derived from Danila (resistant parent) and Tvu7778 (susceptible parent) were infected with CoBB using leaf inoculation method. The experiments were conducted under greenhouse conditions (2007 and 2008) and disease severity was visually assessed using a scale where 0 = no disease and 4 = maximum susceptibility with leaf drop. A single nucleotide polymorphism (SNP) genetic map with 282 SNP markers constructed from the same RIL population was used to perform QTL analysis. Using Kruskall-Wallis and Multiple-QTL model of MapQTL 5, three QTLs, CoBB-1, CoBB-2 and CoBB-3 were identified on linkage group LG3, LG5 and LG9 respectively showing that potential resistance candidate genes cosegregated with CoBB resistance phenotypes. Two of the QTLs CoBB-1, CoBB-2 were consistently confirmed in the two experiments accounting for up to 22.1 and to 17.4% respectively for the first and second experiments. Whereas CoBB-3 was only discovered for the first experiment (2007) with less phenotypic variation explained of about 10%. Our results represent a resource for molecular marker development that can be used for marker assisted selection of bacterial blight resistance in cowpea. peerReviewed

Vigna vexillata is a wild cross-incompatible relative of cowpea. It is highly resistant to several diseases and pests plaguing cowpea. A linkage map was developed for V. vexillata comprising 120 markers, including 70 random amplified polymorphic DNAs, 47 amplified fragment length polymorphisms, one simple sequence repeat and two morphological traits namely, the cowpea mottle carmovirus resistance locus (CPMoV) and leaf shape (La), utilizing an F 2 generation of the intra-specific cross 'Tvnu 1443' × 'Tvnu 73'. The genetic map comprised 14 linkage groups spanning 1564.1 cM of the genome. Thirty-nine quantitative trait loci (QTLs) associated with nine traits were detected on the linkage map, explaining between 15.62 and 66.58% of their phenotypic variation. Seven chromosomal intervals contained QTLs with effects on multiple traits.

Cowpea mottle carmovirus (CPMoV) causes grain yield losses of up to 75% in cowpea (Vigna unguiculata [L.] Walp.). There is no resistance to this virus among cultivated cowpea lines, but a high level of resistance exists in Vigna vexillata, a wild Vigna species. Fifty-four accessions of V. vexillata germplasm collection at IITA were tested for resistance to CPMoV. Seedlings were mechanically inoculated with the virus and susceptibility or resistance was assessed by visual scoring of disease symptoms and serological analysis using antigen-coated plate enzyme-linked immunosorbent assay (ACP-ELISA). All but three V. vexillata lines belonging to the variety angustifolia were resistant to CPMoV. Crosses were made between two resistant V. vexillata lines and the three susceptible lines. Segregation patterns observed in the F2 and the backcross populations of all the crosses showed that resistance to CPMoV in V. vexillata is controlled by a single dominant gene, and the level of resistance conferred by this gene in V. vexillata is very high.

Cowpea [Vigna unguiculata (L.) Walp.] is a major tropical legume crop grown in warm to hot areas throughout the world and especially important to the people of sub-Saharan Africa where the crop was domesticated. To date, relatively little is understood about its domestication origins and patterns of genetic variation. In this study, a worldwide collection of cowpea landraces and African ancestral wild cowpea was genotyped with more than 1200 single nucleotide polymorphism markers. Bayesian inference revealed the presence of two major gene pools in cultivated cowpea in Africa. Landraces from gene pool 1 are mostly distributed in western Africa while the majority of gene pool 2 are located in eastern Africa. Each gene pool is most closely related to wild cowpea in the same geographic region, indicating divergent domestication processes leading to the formation of two gene pools. The total genetic variation within landraces from countries outside Africa was slightly greater than within African landraces. Accessions from Asia and Europe were more related to those from western Africa while accessions from the Americas appeared more closely related to those from eastern Africa. This delineation of cowpea germplasm into groups of genetic relatedness will be valuable for guiding introgression efforts in breeding programs and for improving the efficiency of germplasm management.

Restriction fragment length polymorphism (RFLP) analysis has several advantages over traditional methods of genetic linkage mapping, one of these being the starting point for map-based cloning. The recent development of an RFLP map of cowpea (Vigna unguiculata L. Walp) has allowed the investigation of associations between genes of interest and RFLP markers. A cross between an aphid (Aphis craccivora Koch) resistant cultivated cowpea, TT84S-2246-4, and an aphid susceptible wild cowpea, NI 963, was screened for both aphid phenotype and RFLP marker segregation. One RFLP marker, bg4D9b, was found to be tightly linked to the aphid resistance gene (Rac 1) and several flanking markers in the same linkage group (linkage group 1) were also identified. The close association of Rac 1 and RFLP bg4D9b presents a real potential for cloning this insect resistance gene.

Background Cowpea [Vigna unguiculata (L.) Walp] exhibits a considerable variation in leaf shape. Although cowpea is mostly utilized as a dry grain and animal fodder crop, cowpea leaves are also used as a high-protein pot herb in many countries of Africa. Results Leaf morphology was studied in the cowpea RIL population, Sanzi (sub-globose leaf shape) x Vita 7 (hastate leaf shape). A QTL for leaf shape, Hls (hastate leaf shape), was identified on the Sanzi x Vita 7 genetic map spanning from 56.54 cM to 67.54 cM distance on linkage group 15. SNP marker 1_0910 was the most significant over the two experiments, accounting for 74.7% phenotypic variance (LOD 33.82) in a greenhouse experiment and 71.5% phenotypic variance (LOD 30.89) in a field experiment. The corresponding Hls locus was positioned on the cowpea consensus genetic map on linkage group 4, spanning from 25.57 to 35.96 cM. A marker-trait association of the Hls region identified SNP marker 1_0349 alleles co-segregating with either the hastate or sub-globose leaf phenotype. High co-linearity was observed for the syntenic Hls region in Medicago truncatula and Glycine max. One syntenic locus for Hls was identified on Medicago chromosome 7 while syntenic regions for Hls were identified on two soybean chromosomes, 3 and 19. In all three syntenic loci, an ortholog for the EZA1/SWINGER (AT4G02020.1) gene was observed and is the candidate gene for the Hls locus. The Hls locus was identified on the cowpea physical map via SNP markers 1_0910, 1_1013 and 1_0992 which were identified in three BAC contigs; contig926, contig821 and contig25. Conclusions This study has demonstrated how integrated genomic resources can be utilized for a candidate gene approach. Identification of genes which control leaf morphology may be utilized to improve the quality of cowpea leaves for vegetable and or forage markets as well as contribute to more fundamental research understanding the control of leaf shape in legumes.