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 IITAâ€™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: â€œAs 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.â€
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 Rawalâ€™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 1971â€“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 Rawalâ€™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 2â€“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 â€an uncultivated plant taxon that benefits from human impacts or â€™disturbanceâ€™,” 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.
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 Rawalâ€™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.