| dc.description.abstract | The assessment of genetic diversity In cowpea (Vigna unguiculata (L.) Walp), is an
important component In the genetic improvement of the crop for agriculture. Simple
sequence repeat markers (SSRs) are often the markers of choice for diversity
assessments. Here SSR markers are used to assess diversity in cowpea. This study was
aimed at: (i) optimizing the PCR conditions for the amplification of cowpea DNA using
(SSRs), (ii) characterizing SSR diversity in cowpea (iii) assessing the distribution of
genetic variation within and among cowpea accessions and within and among
geographical regions and (iv) determining the genetic relationship of cowpea accessions
by country of origin. A core set of 102 cowpea accessions from lITA genebank in
Nigeria, which conserves 15003 cowpea accessions were used for this study. The
germplasm was sampled from 37 different countries including 6 from East Africa, 6 from
Southern Africa, 13 from West Africa and 12 from the Rest of the World. In each case, a
maximum of 10 individuals per accession were ~~pled making a total of 955 individual
plants. One hundred and twenty one microsatellite DNA markers were screened to assess
their amplification and polymorphism; of these, 15 primer pairs were polymorphic and
amplified a single locus from all. the individuals studied and were thus used in this study.
A total of 96 alleles were observed with an average of 6.4 alleles per marker and a range
of two to 13 alleles. The polymorphic information content (PIC) of the 15 SSRs ranged
from 0.06 for VM54 to 0.87 for VM70, with an average of 0.53. These findings revealed
VM70 as the most informative marker of all the SSRs tested. West Africa showed the
greatest diversity among the geographical regions with mean number of alleles per locus
being 5.5 compared to Southern Africa 4.0, East Africa 3.9 and Rest of the World 5.0.
However, according to Nei's unbiased estimate of gene diversity (ll), Southern Africa
had marginally the greatest diversity (0.5095) compared to West Africa (0.5085), East
Africa (0.4718) and Rest of the World (0.5044). In addition, Nigeria and Niger were the
countries of greatest cowpea diversity (H=0.4740 and 0.4669 respectively). The presence
of two centers of diversity (Southern Africa and West Africa) in cultivated cowpea can be
explained by the presence of the majority of wild species in Southern Africa some of
which cross regularly with cultivated cowpea increasing diversity, and a concentration of
farmer varieties in West Africa which have accumulated diverse alleles through being
exposed to diverse farmer environmental selection pressures. Greater genetic diversity
was observed within accession Tvu-2382 from Botswana with mean number of alleles
(MNA) of2.6 and H=0.4746. The limited genetic variation within the accessions was due
to low levels of heterozygote's and limited number of polymorphic loci within the
accessions. The observed heterozygosities were highest in West Africa (0.0346),
followed by Rest of the World (0.0350), Southern Africa (0.0254) and East Africa
(0.0197). Furthermore, there was a large difference ..between the observed heterozygosity
and Nei's unbiased estimate of gene diversity (H), in all the populations, suggesting non
random mating and a deviation from Hardy-Weinberg equilibrium. Cluster analysis did
not reveal any significant population structure according to geographical origin. This was
supported by Wright's F-statistics. Population differentiation was greatest at the
accession level (theta S =0.077) compared to the region level (theta P= 0.004). Cowpea
accessions from West Africa were most closely related to those from the Rest of the
World (D=0.1173) and those from Southern Africa were most distantly related to those
from East Africa (D=0.1894). This implies that much of the germplasm sampled from the
Rest of the World originated from West Africa. Tvu-566 from Gabon and Tvu-337 from
Botswana were the most closely related accession (D=O.0353) while Tvu-3641 from
Niger and Tvu-1886 from Botswana were the most distantly related accessions
(D=O.5774). | en |
