|Schnell Ii, Raymond|
Submitted to: American Association for the Advancement of Science Meeting
Publication Type: Proceedings
Publication Acceptance Date: 2/13/2004
Publication Date: 2/13/2004
Citation: Schnell II, R.J. 2004. Germplasm and genome resources in theobroma cacao: current challenges of tropical tree crop breeding. American Association for the Advancement of Science Meeting. Interpretive Summary: Abstract only.
Technical Abstract: Theobroma cacao L. is an understory tree from the Amazon basin that can be cultivated in a sustainable agro-forestry system that provides income to small farmers while maintaining biodiversity. Four main genetic groups of cacao are traditionally described: Criollo, Lower Amazon Forastero, Trinitario, and Upper Amazon Forastero. During the seventeenth and eighteenth centuries, plants derived from a small number of parents were distributed to many tropical regions of the world, resulting in commercial plantings with a narrow genetic base. Production of cacao in tropical America has been severely affected by two fungal pathogens causing diseases known as witches' broom (WB) and frosty pod (FP). These, along with another pan-tropical fungal disease, black pod (BP), were responsible for over 700 million USD in losses in 2001. Currently, WB and FP are confined to Central and South America; however, commercial populations in West Africa and South Asia are highly susceptible to both diseases. Traditional cacao breeding programs have only been marginally successful in producing resistant material with suitable commercial characteristics. In 1999, the USDA-ARS in collaboration with Masterfoods Inc. initiated a project to apply modern molecular genetic techniques to cacao breeding. The objectives were to develop an international Marker-Assisted-Selection (MAS) breeding program focusing on disease resistance, to identify new sources of resistance in unexploited germplasm, and to identify the genes involved with disease resistance. Over 300 microsatellite and 30 candidate gene markers are being used to map families segregating for resistance to WB, FP, and BP diseases. Two Quantitative Trait Loci (QTL) have been identified for resistance to WB and are being employed in MAS. Microsatellite and candidate gene markers have also been used to estimate the genetic diversity in over 1,500 individuals representing 70 different domesticated and semi-domesticated/wild cacao populations. Genetic diversity has been found to be much higher in the semi-domesticated/wild populations that are upper Amazon in origin and are believed to contain new sources of disease resistance. For the gene discovery effort, a BAC library has been produced from LCT-EEN 37 and SCA 6, both resistant to WB. A 3,000 unigene set has been developed for gene expression analysis and a large international EST sequencing project is underway. In conclusion, the rapid accumulation of information on disease resistance genes will continue and contribute to the efficiency of the overall genetic improvement program. The international MAS project is expected to produce new disease resistant cultivars by 2010. Information and genetic stocks developed in this project will be distributed to areas currently free of WB and FP in anticipation of the arrival of these diseases. International collaboration and sharing of genetic resources will ensure that crop losses due to these pathogens are manageable and will contribute to stability in the supply of cocoa beans.