Genetic Roots Of Cacao Trees Traced

Cocoa comes from the Theobroma cacao tree, which forms the basis of a multibillion-dollar U.S. chocolate industry. The seeds are processed into cocoa beans that are the source of cocoa, cocoa butter and chocolate. But diseases cost growers an estimated $700 million each year, and scientists have been looking for ways to produce cacao trees that can resist them.

David Kuhn, a molecular biologist at the ARS Subtropical Horticulture Research Station in Miami, Fla., and the research team published findings this fall that are a step toward that goal, shedding light on Theobroma’s genetic diversity.

The researchers extracted DNA from the leaves of 952 cacao trees maintained in germplasm collections in Miami, Puerto Rico, Ecuador, Costa Rica, Trinidad and Brazil. The trees were collected by plant explorers over the past 70 years and came from 12 South American countries.

By looking at patterns among 106 genetic markers, the researchers were able to pinpoint where cacao has the greatest genetic diversity and where it likely originated: the upper Amazon basin of Peru.

The researchers also found enough genetic diversity to realign what might be considered Theobroma’s family tree, breaking it up into 10 major genetic groups, instead of the commonly accepted three groups.

Kuhn hopes the findings will encourage breeders to increase the diversity of their cacao tree stocks by crossbreeding among the 10 groups. That would reduce outbreaks of diseases that penetrate tree fruit, destroy seed-bearing pods and can cause farmers to lose up to 80 percent of their crop. Breeders should think about using the entire palette of genetic diversity to improve cacao breeding programs and avoid certain diseases such as black pod and witches' broom, according to Kuhn.