Current Challenges of Tropical Tree Crop Improvement:Integrating Genomics into an Applied Cacao Breeding Program
Loading...
Date
2007
Journal Title
Journal ISSN
Volume Title
Publisher
Proc. IS on Biotechnol Temp. Fruit Crops & Trop. Species
Abstract
Theobroina cacao L. is an understory tree from the Amazon basin that can be cultivated in a sustainable agro-forestry system, providing income to small farmerswhile maintaining biodiversity. Four main genetic groups of cacao are traditionallydescribed: Criollo, Trinitario, and lower and upper Amazon Forastero. During the17111 and 18 centuries, plants derived from a small number of parents were distri-buted to many tropical regions of the world, resulting in commercial plantings with anarrow genetic base. Production of cacao in tropical America has been severelyaffected by two fungal pathogens causing diseases known as witches broom (WB) andfrosty 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 areconfined to tropical 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 withsuitable commercial characteristics. In 1999, the USDA-ARS in collaboration withMars Inc. initiated a project to apply modern molecular genetic techniques to cacaobreeding. The objectives were to develop an international Marker Assisted Selection (MAS) breeding program focusing on disease resistance, to identify new sources ofved with diseaseresistance in unexploited germplasm, and to identify the genes invol resistance. Over 320 microsatellite and 50 candidate gene markers are being used tomap families segregating for resistance to WB, FP, and BP diseases. Quantitative TraitLoci (QTL) have been identified for resistance to WB and FP and these are being employed in MAS. The utility of Association Mapping for productivity traits has beendemonstrated providing an alternative method to traditional mapping. Microsatelliteand candidate gene markers have also been used to estimate the genetic diversity, inover 1,300 individuals representing 70 different domesticated and semi-domesticated/wild cacao populations. Genetic diversity has been found to be much higher in thesemi-domesticated/Wild populations from the upper Amazon which may contain newsources of disease resistance. For the gene discovery effort, a Bacterial ArtificialChromosome (BAC) library has been produced from LCTEEN37, resistant to WB,and work is underway to identify and sequence gene(s) responsible for a major \\\\ BQTL. Large evaluation trials, developed using MAS, are located in Costa Rica,Ecuador, Brazil, and Papua New Guinea with additional QTL evaluation studies inGhana, Nigeria, Costa Rica, and Ecuador. All these projects are collaborations with national agricultural institutes iii the respective countries. The international MASproject is expected to produce new disease resistant cultivars by 2012. Genetic stocks developed in this project will be distributed to areas currently free of WB and FP inanticipation of the arrival of these diseases. International collaboration and sharing ofgenetic resources will ensure that crop losses due to these pathogens are manageableand will contribute to stability in the supply of cocoa beans.
Description
Keywords
Theobioma cacao, molecular markers, MAS, QTL, linkage mapping
Citation
Proc. IS on Biotechnol Temp. Fruit Crops & Trop. Species Eds. R.E. Litz and R. Scorza Acta Hon. 738, ISHS 2007