Diversity: the spice of life

Sarah J. Hearne, s.hearne@cgiar.org

Cowpea seed collection, IITA genebank. Photo by IITA.
Cowpea seed collection, IITA genebank. Photo by IITA.

Cowpea is an important grain legume cultivated globally in the tree-scattered open grasslands of the tropics and subtropics. In Africa, these characteristic savanna regions are the “birthplace” of cowpea. The center of diversity of wild cowpea (where you find the most variation) is in southern and southeastern Africa; the center of diversity of cultivated cowpea is in West Africa (Padulosi 1993).

As a crop, cowpea is generally grown for its dry grain used for human and animal consumption, and green pods consumed as vegetables, and also for the fiber for textiles from the long peduncles or stalks (West Africa). It is a versatile plant and is used as a green manure, a dual-purpose crop in mixed cropping systems, and alone as a forage crop for livestock. The leaves are also eaten as a vegetable in parts of East Africa and in Senegal; in Sudan and Ethiopia, the roots are eaten as well.

IITA holds more than 15,000 accessions of cultivated cowpea in its genebank collection. These accessions form an invaluable resource for conservation and improvement. To be able to fully use such a collection, it is important to characterize the materials to enable the selection of the best materials for various purposes, such as crop improvement for high yield, better agronomic traits, drought tolerance, or disease resistance.

To help characterize IITA’s global cowpea collection, Institute scientists undertook a study funded by the Generation Challenge Program. This included defining a core collection from the thousands of accessions held in the IITA genebank, characterizing the molecular diversity of this collection, and defining a smaller reference collection to enable the wider use of these important genetic resources. Seeds of the core collection accessions were virus tested and have been made available for distribution.

A core collection is a subset of accessions that are representative of the diversity of the entire collection. These core collections are needed as they provide a smaller, more manageable number of materials from which meaningful conclusions reflecting the wider collection can be made. A core collection of 2,062 accessions was derived from the 15,000 accessions in the IITA genebank, based on information held on each accession within the genebank database. The core collection contains accessions from many countries but with more from West, East, and Central Africa—the cradle of cowpea diversity.

Cowpea collection sites
Cowpea collection sites

The core collection was then subjected to further study. Molecular markers, signposts present in the DNA of all living things, were used to look for variation among the accessions in the laboratory. Using the resulting data, scientists were able to describe the molecular diversity of the accessions and identify which accessions were more like one another and those that were not. As a result, clusters of accessions that were similar to one another could be identified. Altogether, nine such clusters were identified in the cowpea core collection.

The core collection is an important resource, but it is simply too large for many users of the genebank to apply in studies, such as screening for desired traits (perhaps disease resistance) in a systematic manner. It was therefore necessary to define from the core a smaller collection of accessions, called a reference collection. The reference set of 374 accessions was defined using the clusters identifed in the molecular characterization. The reference collection is representative of the molecular diversity and descriptive diversity of the core and the entire collection.

As soon as the definition of the cowpea reference collection was publicized the genebank received many requests for the materials. The reference collection has been used widely by IITA scientists and our many partners and genebank clients in studies looking at drought, pest and disease tolerance, and in further studies of molecular diversity. The robustness of the collection was confirmed during some of these studies when comparisons of the reference collection with those from other institutes indicated that there was no novel molecular diversity present in the other collections investigated.

Reference
Padulosi, S. 1993. Genetic diversity, taxonomy, and ecogeographic survey of the wild relatives of cowpea (V. unguiculata). PhD thesis. University of Louvain La Neuve, Belgium.