|Schnell Ii, Raymond|
Submitted to: Journal of Crop Improvement
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2006
Publication Date: 4/20/2007
Citation: Opoku, S.Y., Bhattacharjee, R., Kolesnikova-Allen, M., Motamayor, J.C., Schnell Ii, R.J., Ingelbrecht, I., Enu-Kwesi, L., Adu-Ampomah, Y. 2007. Genetic diversity in cocoa (Theobroma cacao, L.) germplasm collection from Ghana. Journal of Crop Improvement. Vol. 20(1/2) (#39/40)p.73-87. Interpretive Summary: The amount of genetic diversity in commercial plantings and in experimental research collections of cacao (Theobroma cacao) in West Africa is unknown. During the last century, a number of cacao germplasm introductions were made from Central and South America to West Africa and this material has been utilized in West African cacao breeding programs. In Ghana the Cocoa Research Institute of Ghana (CRIG) has been involved with breeding and dissemination of hybrid seed from superior families to farmers since 1930. To survey the genetic diversity of cacao populations in Ghana molecular markers were used. Seventeen microsatellite markers were analyzed on 377 accessions. Samples from all six growing regions of Ghana were analyzed along with samples from breeders' collections and introduced parental clones. Genetic diversity was found to be higher in the breeders' collection and among the parental clones than among the farmers' collections. However, significant genetic diversity exists among the farmers collections. A number of misidentified types were found. The type thought to be West African Amelonado was determined to be of the hybrid origin. The results indicate that significant genetic variation exist within the CRIG germplasm which will allow continued genetic gain from selection in the breeding program.
Technical Abstract: Theobroma cacao L. with its center of diversity in Central and South America was first introduced to West Africa in the mid-19th century and today the region produces 70% of the world's cocoa. Several distinct cocoa types have been introduced, cultivated and intercrossed across the region. Also, bi-parental crosses involving selections from various introductions have been planted on a large scale. Consequently, a wide range of genetic diversity that could be exploited for crop improvement is expected. The present study has been therefore, undertaken to assess the degree and distribution of genetic diversity present in cocoa germplasm collections from the Cocoa Research Institute (CRIG), seed gardens and materials from farmers' plantations in Ghana, using molecular markers. Two hundred and thirty-five trees representing all the cocoa-growing regions of Ghana were sampled in situ from farmers' field and grouped as farmers' collection. Another set of 104 trees was collected from breeders' seed gardens, called breeders' collection. Thirty-eight parental clones from the CRIG's collection, used in producing the bi-parental crosses, comprised the third category, called as parental clones. The collections were screened with the set of 17 mapped microsatellite markers. Average gene diversity was high in all populations, with mean observed heterozygosity of 0.738. Although the highest was recorded in accessions from breeders' and parental collections, genetic diversity in the farmers' collection was comparable with them. Despite the low level of differentiation [Fst = 0.076] found across all the three groups, sufficient genetic differences existed between them, separating breeders' collection from farmers' collection. The study also revealed the pattern of adoption of available planting materials by farmers on their fields.