In Kanti Rawal’s footsteps

Remy S. Pasquet (rpasquet@icipe.org), Dominique Dumet (d.dumet@cgiar.org), and Sunday E. Aladele (sundayaladele@yahoo.com)

Cowpea is a rich source of dietary protein for millions in West Africa. Photo by IITA.
Cowpea is a rich source of dietary protein for millions in West Africa. Photo by IITA.

Cowpea, Vigna unguiculata (L.) Walp. is the major legume crop in the African lowlands. It is the main protein supply of half of the population in sub-Saharan Africa.

Nigeria is the most populous country in West Africa, and also produces the largest amount of cowpea. Its urban population is growing in leaps and bounds, and thus, it is also importing a lot of cowpea from all its neighbors

Cowpea is considered by several authors as having been domesticated in Nigeria or within a larger area including Nigeria (i.e., Vaillancourt and Weeden 1992). In addition, the first wild cowpea accession was collected in northern Nigeria by J.M. Dalziel, a British botanist, and this led Piper (1913) to propose the African origin of cowpea1.

To some extent, this partly explains why Nigeria was the first country surveyed for cowpea accessions by IITA’s then Genetic Resources Unit. Between 1970 and 1973, Kanti Rawal traveled 38,000 km around Nigeria and Niger. He collected wild cowpea accessions as well as numerous accessions of domesticated cowpea.

According to his map, he collected wild cowpea in 68 places (Rawal 1975). Unfortunately, over the years, the passport data of these accessions were lost and, today, the place of collection is known for only four out of the 40 accessions still maintained at IITA.

Regarding Nigeria, Rawal (1975) wrote in the abstract of his paper: “As in the case with many cultivated species, Vigna unguiculata (L.) Walp. has a wild form growing in secondary forests and derived savannahs and a companion weed form adapted to disturbed habitats such as roadside ditches and fields. Evidence of introgressive hybridization between weedy and cultivated forms has been presented. The zone of extensive natural hybridization corresponds to the cultivation area of northern Nigeria and Niger and may well extend to Upper Volta (now Burkina Faso) and Senegal.”

Rawal gave very good descriptions of what he called the wild and weed forms. His wild forms were samples of subsp. baoulensis (A.Chev.) Pasquet; the weed forms were samples of subsp. unguiculata var. spontanea (Schweinf.) Pasquet. We (Pasquet and Padulosi in press) believe that the first subspecies belongs to the secondary gene pool of cowpea and the second to the primary gene pool. However, since Rawal’s material is mostly lost2, the assessment of the diversity of wild cowpea from Nigeria is impossible in the absence of new collection missions in the country.

In addition, factors such as climate change, increased incidence of pests and diseases, cultural change, or the adoption of improved lines are also likely to affect the diversity of cowpea and wild Vigna in the near future. To avoid the irreversible loss of Vigna and to secure highly viable Vigna diversity, the Global Crop Diversity Trust in association with IITA and Nigeria’s National Centre for Genetic Resources and Biotechnology (NACGRAB) organized a collecting mission for wild Vigna germplasm in 2010. The mission covered 27,000 km in Nigeria between 14 October and 7 December and collected 260 accessions (242 of var. spontanea and 18 of subsp. baoulensis . In addition, 13 populations were sampled for further population genetic analysis.

In comparison with the Rawal missions that took place in 1971–73, we surveyed more localities within a shorter time. Unlike Rawal, we focused on wild cowpea only and benefited from a much better road network, especially in northern Nigeria where we were driving more than 600 km/day. In the end, there is a general agreement between the results of Rawal’s survey and our own in terms of geographical distribution of both subspecies.

In the northern ranges, we often encountered wild cowpea in fields or at roadsides. Wild cowpea plants are easy to spot in the field, as they twine 2–3 m above soil level on sorghum or pearl millet stems. Domesticated cowpea are prostrate or short and erect but usually not twining.

Based on the ecological definition of a weed, which is ”an uncultivated plant taxon that benefits from human impacts or ’disturbance’,” var. spontanea is a weed mainly encountered in disturbed places, such as in fields and gardens, at roadsides, and sometimes within towns (sewage ditches, grassy places); it was not observed within Yankari National Park. However, to some extent, var. spontanea is also a weed in the economic sense of the word since it is usually pulled out from the fields by farmers. It usually appears as isolated plants or isolated patches of fewer than 20 plants or as a few plants forgotten by the farmer while weeding.

In some places, we found fields with as many wild cowpea as domesticated cowpea (SP 815, Katsina State, for example). We suspect that, in these places, farmers were primarily interested in fodder. Var. spontanea is obviously a good fodder plant and farmers primarily cultivating cowpea for fodder would not choose to lose time weeding wild cowpea.

Domesticated cowpea close to var. spontanea (white arrow), SP 949, Borno State, Nigeria. Source: R. Pasquet, i<em/>cipe.” title=”domesticated-cowpea-close-to-var-spontanea” width=”300″ height=”252″ class=”size-medium wp-image-2510″ /></a><figcaption class=Domesticated cowpea close to var. spontanea (white arrow), SP 949, Borno State, Nigeria. Source: R. Pasquet, icipe.

Since experiments have proved pollen flow between wild and domesticated cowpea at over 30 m distances (Fatokun and Ng 2007), we checked if domesticated cowpea was grown within 30 m of the collecting sites for wild cowpea. This occurred frequently (70%) in the northern part of the range which is also the main cowpea production area. Our survey confirms Rawal’s (1975) conclusion. There are numerous situations in Nigeria in which domesticated cowpea and wild cowpea exchange genes. Therefore, its diversity may not be much higher than that of the domesticated gene pool. An evaluation of diversity among the collected material would help confirm or disprove this assumption.

The potential hybridization between wild and cultivated forms has implications for the transgenic Bt cowpea which are presently under confined field trials in Nigeria (www.aatf-africa.org/userfiles/Cowpea-Project-brief.pdf). If the Bt gene could move through the pollen from transformed to wild plants, further careful studies need to evaluate the advantage given by the Bt gene to a wild cowpea plant and whether Bt cowpea poses any risk to biodiversity.

References
Fatokun CA and Ng Q. 2007. Outcrossing in cowpea. J Food Agric Environ 5:334–338.

Pasquet and Padulosi. In press. Genus Vigna and cowpea (V. unguiculata (L.) Walp.) taxonomy: current status and prospects. Presented at the 5th World Cowpea Research Conference, Saly, Senegal, September 2010.

Piper CV. 1913. The wild prototype of the cowpea. US Dept. Agric. Bureau Plant Ind. Circular 124: 29–32.

Rawal KM. 1975. Natural hybridization among wild, weedy and cultivated Vigna unguiculata (L.) Walp. Euphytica 24(3):699–707.

Vaillancourt RE and Weeden NF. 1992. Chloroplast DNA polymorphism suggests Nigerian center of domestication for the cowpea, Vigna unguiculata (Leguminosae). Am. J. Bot. 79-10:1194–1199.

Designer (cowpea) plants

Christian Fatokun, c.fatokun@cgiar.org

Plants can be designed to order. Science has long found a way to combine good and useful characteristics in a plant by studying the genes for such traits, and putting them together in a process called “genetic engineering.”

Caterpillar boring into a cowpea pod. Photo by S. Muranaka
Caterpillar boring into a cowpea pod. Photo by S. Muranaka, IITA

Cowpea is grown mainly for its protein-rich grains and quality fodder for livestock. At present, biological control and conventional breeding methods are proving inadequate in developing cowpea varieties resistant to destructive pests, such as the legume pod borer Maruca vitrata.

M. vitrata is the most widespread cowpea pest. The adult moth lays eggs on the plant. The larvae that emerge from the eggs damage plants in the field, particularly during the reproductive stage, through feeding on young succulent shoots, flowers, pods, and seeds. This pest can cause significant grain yield reduction, between 20% and 80% if not controlled with insecticides.

Farmers usually spray insecticides to protect the cowpea crop from Maruca and other pests. Purchasing chemicals, however, adds to the production cost, thus reducing the farmers’ profit. Also, farmers are not well equipped to protect themselves when using such toxic chemicals. In some farming communities, adulterated chemicals that do not control the pests are sold to farmers. The development of cowpea varieties with resistance to Maruca and other insect pests would benefit the most resource-poor African farmers who grow the crop.

Cowpea is grown extensively in the savanna region of sub-Saharan Africa (SSA). At least one major insect pest attacks cowpea at every stage in the life cycle, including seeds in storage. These pests are significantly responsible for the low grain yield in farmers’ fields.

Through conventional breeding, some varieties have been developed that show resistance to some of the pests, such as aphids and flower thrips, and low levels of resistance to the storage weevil. However, not much progress has been made in host plant resistance, especially M. vitrata.

Efforts continue to identify parasites and predators that could be used as biocontrol agents. When deployed, such agents would greatly reduce the population of the Maruca larvae in the field, giving the cowpea plant some respite for the production of flowers and pods containing whole and well-formed seeds.

Using conventional breeding, several hundreds of accessions of cultivated cowpea and its wild relatives have also been screened for resistance to this pest. Accessions belonging to Vigna vexillata were found to be resistant to M. vitrata. These accessions were found to be closest to cowpea in a phylogenetic study of diversity in the Vigna species. The study was based on data obtained after DNA genotyping. Efforts were made to cross cowpea with V. vexillata but without success.

This strong cross-incompatibility makes gene exchange between the two species impossible. This is where biotechnology comes to the rescue. Two major steps are needed to develop genetically modified cowpea with resistance to M. vitrata. First is developing a transformation system and the second is identifying the transgene that would be effective against the pest when introduced into cowpea. Since Maruca is a Lepidopteran, some of the genes from Bacillus thuringiensis (Bt) should be effective against the insect’s larvae. IITA screened several Bt protoxins on Maruca by incorporating different concentrations in the diet fed to the larvae. The protoxin of Bt gene ”Cry1ab” was found to be most effective even at very low concentrations in the artificial diet. This Bt gene (Cry1ab) was therefore selected as the candidate gene for designing Maruca-resistant cowpea.

Scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia developed the transformation system using an IITA-developed breeding line ”IT86D-1010” derived from a cross between ”TVx4659-03E” and ”IT82E-60”. CSIRO scientists have now obtained cowpea lines containing the Bt gene. Monsanto donated the Bt gene used to transform cowpea by licensing it to the African Agricultural Technology Foundation (AATF) for use in Africa. The Rockefeller Foundation and USAID funded this cowpea transformation project.

Farmers transporting cowpea harvest. Photo by S. Muranaka
Farmers transporting cowpea harvest. Photo by S. Muranaka, IITA

The Bt cowpea had been tested in the CSIRO laboratory in Australia and found to be effective against the larvae of another Lepidoptera, Helicoverpa armigera. The Bt gene in cowpea is expected to be effective against M. vitrata, the cowpea pest, but needs to be tested in an environment where Maruca thrives. Apart from Burkina Faso, none of the African countries where cowpea is an important crop has a biosafety law in place. A few lines of the Bt cowpea were, therefore, taken to Puerto Rico for field testing. The field trial was carried out in late 2008. If the Bt gene in the cowpea lines is found to be effective against Maruca, the next step would be to transfer the Bt gene into popularly grown cowpea varieties selected from interested countries. The line presently containing the Bt gene is not high yielding, and farmers are not likely to accept it readily.

Under the international biosafety protocol (Cartagena protocol on biosafety) it is necessary to carry out risk assessment on the Bt cowpea before it is introduced to another country. The data obtained from risk assessment form part of the dossier that accompanies applications requesting for importation to any country. Risk assessment would entail studies on gene flow, the effect of the transgene on nontarget organisms, food safety, and resistance management strategies.

A meeting of experts in these various fields is planned in March 2009 at the Donald Danforth Plant Science Center, St. Louis, Missouri, USA. The experts would design studies to address the different questions that may arise from biosafety regulators in the countries where the Bt cowpea is meant to be grown. Many of the proposed studies are necessary, because cowpea is an indigenous food crop in SSA where cross-compatible wild relatives are found growing in agroecologies similar to farmers’ fields. Biosafety reviews in the African countries would, therefore, be rigorous.