Genetic diversity and parentage in farmer varieties of cacao (Theobroma cacao L.) from Honduras and Nicaragua as revealed by single nucleotide polymorphism (SNP) markers

Cacao (Theobroma cacao L.) is the main source for chocolate with an annual production of four million tons worldwide. This Neotropical tree crop was domesticated in Mesoamerica as far back as 3,000 years ago. Knowledge of genetic diversity and population structure in farmer varieties of cacao in the center of domestication is essential for sustainable production of fine-flavored cacao beans and contributes to in situ/on-farm conservation of farmer varieties. Based on 70 single nucleotide polymorphism markers, we analyzed 84 fine-flavored farmer varieties collected from traditional cacao farms in Honduras and Nicaragua. The study also included 31 clones from the international cacao collections to serve as references. The SNP based multilocus matching identified six synonymous groups, including 14 Criollo and two Amelonado varieties. A moderately high level of genetic diversity was observed in these farmer varieties, indicating the possibility to further explore intra-population variation and breed for fine-flavored cocoa. Multivariate analysis showed clustering of the 84 farmer accessions in five genetic groups: ancient Criollo, Amelonado, Trinitario (including Nicaragua Trinitario and Honduras Trinitario) and Upper Amazon Forastero (only one accession). The Honduras Trinitario differed from the Nicaragua Trinitario group. The clustering results largely supported the perceived classification of cacao by local farmers and researchers, which was mainly based on morphological traits. However, the well known traditional variety Indio in this region was identified as synonymous with Amelonado. Parentage analysis showed that the variety Indio (or Amelonado) contributed more to the Trinitario type farmer varieties, whereas ancient Criollo had less influence. The present study demonstrates the efficacy of using a small set of SNP makers for cacao germplasm characterization, and further depicts the diverse origins and parentage in farmer varieties from Mesoamerica. This information thus will be highly useful for conservation and utilization of cacao germplasm from this region.