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Geographic and Genetic Population Differentiation of the Amazonian Chocolate Tree (Theobromacacao L)

Geographic and Genetic Population Differentiation of the Amazonian Chocolate Tree (Theobromacacao L) 

Juan C. Motamayor1,2*, Philippe Lachenaud3, Jay Wallace da Silva e Mota4, Rey Loor5, David N. Kuhn1, J. Steven Brown1, Raymond J. Schnell1 

Abstract

Numerous collecting expeditions of Theobroma cacao L. germplasm have been undertaken in Latin-America. However, most  of this germplasm has not contributed to cacao improvement because its relationship to cultivated selections was poorly  understood. Germplasm labeling errors have impeded breeding and confounded the interpretation of diversity analyses. To improve the understanding of the origin, classification, and population differentiation within the species, 1241 accessions covering a large geographic sampling were genotyped with 106 microsatellite markers. After discarding mislabeled samples, 10 genetic clusters, as opposed to the two genetic groups traditionally recognized within T. cacao, were found by applying Bayesian statistics. This leads us to propose a new classification of the cacao germplasm that will enhance its management. The results also provide new insights into the diversification of Amazon species in general, with the pattern of differentiation of the populations studied supporting the palaeoarches hypothesis of species diversification. The origin of the traditional cacao cultivars is also enlightened in this study. 

Introduction 

Cacao is cultivated in the humid tropics and is a major source of currency for small farmers as well as the main cash crop of several West African countries. Its fruits (pods) contain the seeds (beans) that are later processed by the multi-billion-dollar chocolate industry. Average yields are about 300 kg per hectare but 3,000 kg/ha are often reported from field trials [1]. Genetic improvement of cacao through breeding has focused on increasing yield and disease resistance. To increase yield, breeders have capitalized on heterosis that occurs in crosses between trees from different genetic groups [2]. Traditionally, two main genetic groups, ‘‘Criollo’’ and ‘‘Forastero’’, have been defined within cacao based on morphological traits and geographical origins [3]. A third group, ‘‘Trinitario’’, has been recognized and consists of ‘‘Criollo’’6‘‘Forastero’’ hybrids [3]. In parallel, botanists described two subspecies: cacao and sphaeorocarpum, corresponding to ‘‘Criollo’’ and ‘‘Forastero’’ [4,5], which, according to some authors, evolved in Central and South America, respectively [4,5]. For other authors, ‘‘Criollo’’ and ‘‘Trinitario’’ should be considered as traditional cultivars rather than genetic groups [6] . 

Two other traditional cultivars have been described: Nacional and Amelonado [7]. Nonetheless, a sound classification of Theobroma cacao L. populations, based on genetic data, is lacking for the breeding and management of its genetic resources. 

The Amazon basin contains some of the most biologically diverse tree communities ever encountered; tree species richness may attain three hundred species in one-hectare plots [8]. In cacao, flowers are hermaphrodites. However, it is an outcrossing species due to the action of self-incompatibility mechanisms in wild individuals, while the cultivated ones are generally self-compatible. Other Amazonian species of importance such as Theobroma grandiflorum show similar mating systems. Understanding the geographic pattern of differentiation of T. cacao would aid in implementing conservation strategies for many other species with similar mating systems and distribution within this important region. (p. 1) .... 

Conclusion 

The results presented here lead us to propose a new classification of cacao germplasm into 10 major clusters, or groups: Maran˜ on, Curaray, Criollo, Iquitos, Nanay, Contamana, Amelonado, Puru´ s, Nacional and Guiana. This new classification reflects more accurately the genetic diversity now available for breeders, rather than the traditional classification as Criollo, Forastero or Trinitario. We encourage the establishment of new mating schemes in the search of heterotic combinations based on the high degree of population differentiation reported. Further- more, we propose that germplasm curators and geneticists should use this new classification in their endeavor to conserve, manage and exploit the cacao genetic resources. (p.5) ....

COMPLETE STUDY AND RESULTS HERE

Category: History & Culture
Posted: Tuesday, May 10, 2011 01:21:00 AM
Views: 4224
Synopsis: Study of cacao diversity including geographic sampling which identifies 10 genetic clusters (rather than two). This study also provides insights into the diversification of Amazon species, and into the origin of the traditional cacao cultivars.