Theobroma cacao L. is the most important species of the Theobroma genus. This species belongs to the Malvaceae family. It is a diploid plant (2n chromosome) with a genome size ranging from 411 to 494 Mb (Argout et al., 2011).
In Côte d'Ivoire, the cocoa genetic improvement program began in 1958. It was based on the selection of clone hybrids obtained from crosses between High Amazonian clones and "local" Low Amazonian clones or Trinitarios. The first hybrids resulting from these crosses were released in 1975. They were characterized in particular by their earliness (production starts at two years) and high yield (2 tonnes of merchantable cocoa/ha) (Besse, 1977).
Since 1990, recurrent and reciprocal selection, based on the improvement of a population of Haut-amazoniens (HA) on the one hand, and a composite population of Bas-amazoniens and Trinitarios (BA + T) on the other, has been adopted, with production, production-vigor ratio, resistance to Phytophthora and mirids and earliness of production as selection criteria (Lachenaud et al., 2001). Over several cycles, this more efficient strategy should enable the accumulation of favorable genes for the selected criteria, leading to the creation of hybrid progenies that maintain an acceptable level of production, and also offer better adaptation to the environment.
Implemented in 2000, the second cycle of recurrent selection comprises two intra-population mixing plots, each using a 20 x 20 incomplete factorial crossing design, in which each female sire was crossed in a disconnected manner with 4 male sires. For the second cycle, broodstock selection was based on an "individual/family" selection. Individuals and families were selected on the basis of their agronomic performance (Lachenaud et al., 2001).
Several studies on the productivity and field tolerance to brown rot of SRR populations have shown genetic gains and good agronomic performance obtained between each selection cycle (Tahi et al., 2017). The work carried out, focusing mainly on populations from the second cycle of recurrent and reciprocal selection, has enabled on index-based selection of elite cocoa trees for production and rate of rotted pods (Trébissou, 2021).
This study was carried out to assess the genetic diversity of these cocoa trees selected on indexes for production and rotted pod rate. The aim is to determine the identity, genetic structure and phylogenetic relationships of the cocoa trees selected of first and second cycles of recurrent and reciprocal program, with a view to a better appreciation of the genotypes presenting a genetic make-up consistent with the different groups to which they belong.
Study site
Samples were taken from four plots of reciprocal recurrent selection. These plots are located at the CNRA (Centre National de Recherche Agronomique) station in Divo (5°50'27.8''N; 5°21' 30.1''W), Côte d'Ivoire. The station is located 17 km from Divo, in the Lôh Djiboua region. Annual rainfall varies from 1,200 mm to 1,400 mm/year. Average temperature is 27°C and relative humidity is 85% (Ehounou et al., 2019).
Plant material
The plant material used consists of 152 cocoa genotypes from the second cycle of the recurrent and reciprocal selection program currently underway in Côte d'Ivoire (Fig. 1). The plant material is made up of 53 genotypes from the High Amazonian (HA)2, 26 genotypes from the mixed Bas Amazonian and Trinitario population identified as (BA + T)2, 43 genotypes from the inter-group population of the 2nd cycle of the RRS [HA2 X (BA + T) 2] and 46 genotypes from the 1st cycle of recurrent and reciprocal selection HA1 and (BA + T)1.