Submitted to: Forest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2007
Publication Date: 4/17/2008
Citation: Lachenaud, P., Zhang, D. 2008. Genetic diversity and population structure in wild stands of cacao trees (Theobroma cacao L.) in French Guiana. Forest Science. 65(3):310. Interpretive Summary: Cocoa is an important tropical crop since it is the source of cocoa butter and powder for the confectionery industry. New genetic resources of cocoa are important for breeding new cocoa varieties that improve sustainable cocoa production.The wild cacao trees (Theobroma cacao L.) in French Guiana are an important source of genetic diversity of this species. Several expeditions to south-eastern French Guiana resulted in the collection of wild varieties of cocoa. Understanding the genetic diversity of this collection is essential for its conservation and it use in breeding programs. In the present study, we fingerprinted cocoa trees sampled from 19 wild populations from the upper Oyapok River and its tributaries. Our results show that the wild cocoa trees in French Guiana are unique in the South American cocoa gene pool. The results also showed that tree diversity had a particular distribution in the Amazon rainforest. These results improved our understanding of the wild cocoa populations. This will contribute to a better management of cocoa genetic diversity and will improve the breeding of better cocoa varieties thus directly benefiting chocolate consumers and cocoa farmers around the world.
Technical Abstract: Background and Aims The native cacao trees (Theobroma cacao L.) in French Guiana represent an important fraction of genetic diversity of this species. Several scientific surveys have led to the collection of germplasm accessions from south-eastern French Guiana, which provides an ideal opportunity for investigating the pattern of genetic diversity in natural populations of cocoa. The purpose of this work was to assess the genetic structure of these natural cocoa populations and provide baseline information for conservation and utilization of these germplasm in breeding. Methods Using a capillary electrophoresis genotyping system, we fingerprinted 189 wild trees in 19 natural populations from the upper Oyapok River and its tributaries in south-eastern French Guiana. Thirty-nine clones, including three semi-spontaneous clones, fourteen domesticated local clones, and 15 clones from Peruvian Amazon were used as controls. Based on the 15 loci SSR profiles, we analyzed the intra- and inter-population variation of these wild trees and examined their relationship with the domesticated clones and clones from other regions of South America. Key Results Major structuring was discovered in the metapopulation of French Guianan cacao trees, with an overall Fst of 0.22. The Camopi populations were divided into two main groups that were distant from another group containing the Euleupousing and Upper-Oyapok populations. Overall allelic richness was 4.87 alleles per locus, but fell to 2.4 on average within the populations. Gene diversity was 0.368, observed heterozygosity was low (0.160), and the inbreeding coefficient was 0.626. There had therefore been large fixation in the populations, which could be explained by the biological characteristics of the cacao tree, and the contraction-expansion phases of the French Guianan forest, due to the climatic variations of the Quaternary period. Conclusions The present results confirmed that the wild cocoa trees in French Guiana have a unique genetic composition in the South American cocoa gene pool. Its genetic diversity is spatially structured into small populations. Substantial allele fixation has taken place in the wild populations, which is an advantage for using these wild cocoa germplasm in breeding. The improved understanding of genetic structure in the natural populations of cocoa is also highly useful for the development of in situ conservation strategies for cocoa genetic diversity.