Isozyme Diversity in the Cowpea Species Complex

Abstract

The cowpea [Vigna unguiculata (L.) Walp] is an important crop in many countries of tropical Africa, Asia and South America. Compared with other leguminous crops, little is known about the population structure of the cowpea and the partitioning of genetic diversity between cultivated and wild cowpea. One hundred and twelve accessions of cultivated cowpea (V. unguiculata ssp. unguiculata) and 43 of wild cowpea (V. unguiculata ssp. dekindtiana) were evaluated for genetic diversity at 26 isozyme loci. Cultivated accessions were characterized by very low genetic diversity (Ht = 0.029), with only six polymorphic loci. Four of these polymorphic loci displayed one very common allele and one rare allele. Accessions possessing these rare alleles were dispersed throughout Africa and a geographical center of isozyme diversity was not evident. Cultivated groups biflora (the catjang) and sesquipedalis (the yard‐long bean) could not be differentiated from the cowpea. Wild cowpea, on the other hand, were highly diverse (Ht = 0.168), with 19 out of 26 loci polymorphic. Our data indicate introgression from wild to cultivated must be very rare. Six wild accessions displayed high identity with the cultivated cowpea. Wild cowpea may, therefore, be the progenitor of the cultivated cowpea. Because these wild accessions were dispersed throughout equatorial Africa, there is no isozyme evidence for a narrowly defined center of domestication.