Origin, dispersal, and current global distribution of cacao genetic diversity

Abstract

Cacao (Theobroma cacao L.) is native to tropical South America, but as the unique source of cocoa butter and powder for the 200 billion USD global confectionery market, it is cultivated globally. Despite its economic importance, cocoa was, and continues to be, predominantly produced in low-input and low-output systems. Production constraints, including depletion of soil fertility on cacao farms, increasing damage due to diseases and pests, and expanding labor costs, limit cacao sustainability. Therefore, instead of increasing yields, the predominant contributing factor that keeps up with the rising demand for cocoa products has been expansion to new production regions. The future of the world’s cocoa economy depends significantly upon using germplasm with a broad genetic base to breed new varieties with disease and pest resistance, desirable quality traits, and the ability to adapt to changing environments. Cacao differs from major field crops with regard to the untapped wild populations, which are still abundant in the Amazon region where they are coevolving with the pathogens. Moreover, in the absence of reproductive barriers, these wild populations could be readily crossed with cultivated crops. Yet only a very small fraction of the wild germplasm, mostly represented by a small number of clones in the so-called Pound collection, has been used for breeding since the 1940s. Contributions from this small set of clones have made tremendous impacts in disease resistance and adaptability. However, breeding efforts in the past 70 years have been reshuffling this small fraction of genetic diversity, with little addition of new variation. The on-farm genetic diversity in Southeast Asia and West Africa is low and cannot meet the challenge of the mounting pressure from diseases and pests. New breeding strategies are needed to combine more disease resistance genes/alleles from untapped wild germplasm and provide farmers with enhanced genetic diversity.