Genetic structure of Guianan wild cocoa (Theobroma cacao L.) described using isozyme electrophoresis

Wild cocoa trees (Theobroma cacao L.) from southeast French Guiana, surveyed and collected between 1985 and 1995, represent an original group of germplasm that contributes significantly to the species’ genetic structure. Nineteen populations (demes) from the areas around five rivers (Oyapok, Euleupousing, Yaloupi, Camopi and Tanpok) in the main two river basins of French Guiana are represented in collections maintained in French Guiana, in a number of cocoa producing countries and on some quarantine stations. This study describes the genetic structure of the Guianan group and the genetic parameters of the group as a whole (138 clones in total) and of the eight most abundantly represented populations. Isozyme electrophoresis based on six isozyme systems (nine loci) was used. The results revealed a high average number of alleles per locus (1.9), a high percentage of polymorphic loci (77.7) and low average heterozygosity (0.085). The various populations displayed highly variable allelic frequencies. Wright’s fixation index, at around 20%, indicated substantial genetic differentiation among populations. The genetic structure of the group showed that populations from the Camopi river area fell into two sub-groups, confirming results obtained earlier with other descriptors, and that the populations from the other areas were significantly different, that of the Kérindioutou differing most from the others. The use of controls from other morpho-geographical groups (Upper and Lower Amazon Forastero, Trinitario) confirmed the uniqueness of the Guianan cocoa trees. The results of the investigations, including the low heterozygosity, suggest the potential for using clones derived from the Guianan genetic group for genetic improvement of the crop, and support arguments for continuing surveys throughout the Guianan shield