Genetic diversity and ancestry of cacao (Theobroma cacao L.) in Dominica revealed by single nucleotide polymorphism markers.

Cacao (Theobroma cacao L.), an introduced tree crop in Dominica, is important for foreign exchange earnings from fine or flavour cocoa. The genetic structure of farmed cacao in Dominica was examined to identify varieties for conservation, breeding, and propagation to improve their cocoa industry. Cacao trees (156) from 73 sites over seven geographical regions were genotyped at 192 single nucleotide polymorphism (SNP) markers. Identity, regional differentiation, phylogenetic, multi-variate, ancestry, and core collection analyses were performed. Farmed cacao germplasm had moderate gene diversity (He = 0.320 ± 0.005) from generally unique trees, but cocoa growing regions were genetically similar. Synonymous matching (16.3%) showed that some clonal material was supplied to farmers. Cacao trees were mainly mixed from Amelonado, Criollo, Iquitos, Contamana, and Marañon ancestries, with predominantly Amelonado-Criollo hybrids. Criollo ancestry, linked to fine or flavour cocoa, was found at more than 30% in 28 unique trees. Forty-five trees, containing the SNP diversity of cacao in Dominica, are recommended as a core germplasm collection. This study identifies promising trees for improving cocoa quality; provides genetic evidence that community, regional, or country-wide pooling would not compromise the exclusive fine or flavour cocoa industry; and discusses other implications towards improving the Dominican cocoa industry.

Development of a core SNP panel for cacao (Theobroma cacao L.) identity analysis.

Single nucleotide polymorphisms (SNPs) are preferred markers for DNA fingerprinting and diversity studies in cacao (Theobroma cacao L.). Yet, a consensus SNP panel with a minimum number of SNPs for optimal identity analysis is unavailable for cacao. An initial set of 146 SNP panels of varying sizes were assembled based on heterozygosity, linkage disequilibrium (LD), linkage group (LG) distribution, major allele frequency, minor allele frequency (MiAF), polymorphism information content (PIC), and random distribution. These panels were assessed to determine their ability to distinguish among a training set of 155 accessions. The panels with the best separation ability were supplemented with additional SNPs to create 16 designer panels, which separated all 155 accessions. The 16 designer SNP panels were then assessed on a dataset of 1220 accessions coming from 10 ancestral groups. Increasing the number of SNPs generally yielded improved resolution of genetic identities with concomitant reduction of synonymous groups. The number and choice of SNPs were critical factors with LD, MiAF, and PIC being important selection attributes but an even LG distribution was unnecessary. A robust set of 96 SNPs is recommended as a minimal core SNP panel for cacao DNA fingerprinting to the international cacao community.

Morphological and genetic diversity of cacao (Theobroma cacao L.) in Uganda.

Cocoa is among the top foreign exchange earners in Uganda's agriculture sector and has benefitted the livelihood of farmers involved in production. Although cacao cultivation was adopted in the early 1900s, little is known about the on-farm diversity of the crop. A total of 125 cacao landraces were surveyed from eight districts in the Central and Western Regions to evaluate the morphological and genetic diversity of cacao in Uganda. Passport data included site, tree, fruit and seed information. Trees were genotyped using 96 single nucleotide polymorphism markers on a Fluidigm platform. Low heterozygosity was detected in the germplasm in both the Central [observed heterozygosity (H o ) = 0.295, expected heterozygosity (H e) = 0.334] and Western Regions (H o = 0.317, H e = 0.322). Genetic variation in both regions was generally comparable but the regions could be differentiated from each other. Inbreeding was noted in the Central Region while a greater sharing of genetic material was observed in the Western Region. The morphological and genetic data indicated that the Ugandan collection was an interspersed group with low to moderate variation with some separation of the Central from Western regions. Ancestry analysis indicated that the majority of the accessions were hybrids of Marañon lineage but also had Amelonado and Iquitos genetic backgrounds. These findings are consistent with the history of the movement of cacao into Uganda. A core collection of 18 individuals to represent the genetic diversity as well as 12 additional trees with possible advantageous traits is proposed.

Association mapping of seed and disease resistance traits in Theobroma cacao L.

MAIN CONCLUSION: Microsatellite and single nucleotide polymorphism markers that could be used in marker assisted breeding of cacao were identified for number of filled seeds, black pod resistance and witches' broom disease resistance. An association mapping approach was employed to identify markers for seed number and resistance to black pod and witches' broom disease (WBD) in cacao (Theobroma cacao L.). Ninety-five microsatellites (SSRs) and 775 single nucleotide polymorphisms (SNPs) were assessed on 483 unique trees in the International Cocoa Genebank Trinidad (ICGT). Linkage disequilibrium (LD) and association mapping studies were conducted to identify markers to tag the phenotypic traits. Decay of LD occurred over an average 9.3 cM for chromosomes 1-9 and 2.5 cM for chromosome 10. Marker/trait associations were generally identified based on general linear models (GLMs) that incorporated principal components from molecular information on relatedness factor. Seven markers (mTcCIR 8, 66, 126, 212; TcSNP368, 697, 1370) on chromosomes 1 and 9 were identified for number of filled seeds (NSEED). A single marker was found for black pod resistance (mTcCIR280) on chromosome 3, whereas six markers on chromosomes 4, 5, 6, 8, and 10 were detected for WBD (mTcCIR91, 183; TcSNP375, 720, 1230 and 1374). It is expected that this association mapping study in cacao would contribute to the knowledge of the genetic determinism of cocoa traits and that the markers identified herein would prove useful in marker assisted breeding of cacao.

Genetic diversity and parentage in farmer selections of cacao from Southern Sulawesi, Indonesia revealed by microsatellite markers.

Indonesia is the third largest cocoa-producing country in the world. Knowledge of genetic diversity and parentage of farmer selections is important for effective selection and rational deployment of superior cacao clones in farmers' fields. We assessed genetic diversity and parentage of 53 farmer selections of cacao in Sulawesi, Indonesia, using 152 international clones as references. Cluster analysis, based on 15 microsatellite markers, showed that these Sulawesi farmer selections are mainly comprised of hybrids derived from Trinitario and two Upper Amazon Forastero groups. Bayesian assignment and likelihood-based parentage analysis further demonstrated that only a small number of germplasm groups, dominantly Trinitario and Parinari, contributed to these farmer selections, in spite of diverse parental clones having been used in the breeding program and seed gardens in Indonesia since the 1950s. The narrow parentage predicts a less durable host resistance to cacao diseases. Limited access of the farmers to diverse planting materials or the strong preference for large pods and large bean size by local farmers, may have affected the selection outcome. Diverse sources of resistance, harbored in different cacao germplasm groups, need to be effectively incorporated to broaden the on-farm diversity and ensure sustainable cacao production in Sulawesi.

Chloroplast microsatellite primers for cacao (Theobroma cacao) and other Malvaceae.

PREMISE OF THE STUDY: Chloroplast microsatellites were developed in Theobroma cacao to examine the genetic diversity of cacao cultivars in Trinidad and Tobago. METHODS AND RESULTS: Nine polymorphic microsatellites were designed from the chloroplast genomes of two T. cacao accessions. These microsatellites were tested in 95 hybrid accessions from Trinidad and Tobago. An average of 2.9 alleles per locus was found. CONCLUSIONS: These chloroplast microsatellites, particularly the highly polymorphic pentameric repeat, were useful in assessing genetic variation in T. cacao. In addition, these markers should also prove to be useful for population genetic studies in other species of Malvaceae.