Population Genetics and Characterization of Cowpea Accessions (Vigna Unguiculata (L) Walp) from Three Agroecological Zones of Ghana Based on Isozyme

Abstract

Allozyme variation in protein encoded at 22 loci was studied in 9 cowpea accessions from three agroecological zones of Ghana (namely Deciduous forest, Guinea savannah and Sudan savannah) by electrophoresis. In addition, variation in thirteen morphological agronomic traits of the accessions were studied. The results indicated that: (i) 21 out of 22 loci were polymorphic (95.5%); (ii) cowpea accessions displayed average estimates of mean alleles per locus, A = 5.00; mean expected heterozygosity, HL = 0.561±0.037, mean observed heterozygosity, H0 = 0.264±0.007; total genetic diversity, Ht = 0.648±0.032; mean Wright's fixation index, F1S = 0.573±0,. 096,- frequency of crosspollination, 0 . 244±0.067 (iii) Coefficient of genetic distance (Dm) based on allozyme data among Deciduous forest zone accessions ranged from 0.130 to 0.167 with a mean of 0.143±0.010, among Guinea savannah zone accessions the range was 0.101 to 0.190 with a mean of 0.140±0.021 and in Sudan savannah zone accessions the range was 0.163 to 0.285 with a mean of 0.235±0.030 Coefficient of meristic distance (Dm) based on meristic data among Deciduous forest zone accessions ranged from 0.063 to 0.285 with a mean of 0.103±0.017, among Guinea savannah zone accessions the range was 0.047 0.190 with a mean of 0.123±0.034 and among Sudan savannah zone accessions the range was 0.010 0.057 with a mean of 0.038±0.012. (iv) Deviations from Hardy-Weinberg equilibrium were found in several loci in some of the cowpea accessions (v) 86.5% of total genetic variation was found to be present within accessions while 13.4% resided among accessions. (vi) Allozymic and environmental variations were partly correlated; morphological and environmental factors were partially correlated. (vii) Allozyme variations in several gene loci and morphological variations were significantly correlated with and predictable by environmental variables, primarily related to geographic, temperature and moisture indices. Pattern of genetic variation within and between accessions suggested that: (a) climatic 'selection' plays a primary role in allozymic and morphological differentiation into ecologically adaptive patterns; (b) the environmental variation model seems to be a good predictor of genetic variation in cowpea accessions. Results suggested that use of isozyme technique can (a) lead to discovery of genetic markers for quantitative traits in cowpea breeding,- (b) help in collection, multiplication and regeneration of cowpea seed germplasm. Results further suggested that cowpea accessions from Sudan savannah zone of Ghana should serve as good source of genetic material for improvement of the sum of genetic variability in materials used in cowpea breeding.