The Value of Wild Theobroma Germplasm in Genomics

Traditional Theobroma cacao (cacao) breeding has only been marginally successful in improving bean yields and disease resistance.  A little over 10 years ago, a Marker-Assisted-Selection (MAS) program was developed for cacao and MAS has significantly increased the rate of genetic gain. Whole genome sequencing is strategic for high value crops and having the complete genome sequence greatly enhances MAS. Cacao is going through a transition from specialty crop to more mainstream crop due in part to the cacao genome-sequencing project, a collaboration of Mars, Inc., USDA-ARS and IBM, begun in June, 2008 (www.cacaogenomedb.org).  A combination of Sanger, 454, and Illumina sequencing methodologies are being used along with a complete physical map. In spite of the major advances in cacao genomics, there are still few mapping populations or natural populations to either study or use for sources of disease resistance, unique flavors or yield traits.  Theobroma grandiflorum (cupuassu) is a closely related species with similar physiology and self-incompatibility system. The seeds, when fermented, taste different from cocoa and are not used to produce chocolate.  Cupuassu is susceptible to Moniliophthora perniciosa, the causal agent of witches’ broom disease in cacao; however, isolates that infect cacao do not infect cupuassu.  A population of interspecific hybrids between T. cacao and T. grandiflorum is being developed. These will segregate for many horticultural traits including witches’ broom resistance. The evaluation of these interspecific hybrids and the potential contribution to understanding distinct patterns of gene expression and how this will contribute to cacao breeding will be discussed.