Analysis of Genetic Diversity in Theobroma cacao with Emphasis on Witches' Broom Disease Resistance

Abstract

To facilitate the identification of Theobroma cacao L. that possess desirable traits to meet changing production and market conditions, there is a need to understand the genetic relationships among T. cacao germplasm. In addition, new cultivars are needed to provide more broadly based resistance to devastating diseases such as witches' broom disease [Crinipellis perniciosa (Stahel) Singer]. A subset of 270 T. cacao accessions based on (i) witches' broom disease resistance data, (ii) genetic characterization experiments, and (iii) a random sampling of recently acquired accessions was selected from the extensive germplasm collection at the Centro de Pesquisa do Cacau (CEPEC; Itabuna, Bahia, Brazil) in collaboration with Fazenda Almirante, a division of M&M Mars Incorporated in Itajuipe, Bahia, Brazil. Estimates of genetic distance based on random amplified polymorphic DNA (RAPD) markers were used to evaluate the genetic diversity among the selected accessions. Incorporation of recently acquired accessions (Accessions 181–270) did not increase the breadth of the distribution of genetic diversity already present within the “original” group sampled (Accessions 1–180) suggesting new accessions collected from already sampled geographic regions do not increase the existing genetic diversity in the germplasm collection. In addition, differences in RAPD marker frequencies were associated with accessions that had a high threshold of tolerance to witches' broom disease. Most accessions exhibiting tolerance to witches' broom disease were from the Upper Amazon region, with the exception of SGU 26, a hybrid from Guatemala. This suggests that the Upper Amazon is not the only region to have genes for resistance to witches' broom disease and stresses the need for further collection and examination of germplasm from other regions.

Ecophysiology of the cacao tree

“…It is considered one of the most important perennial crops in the planet, with an estimated world output of 3.5 millions tons in (ICCO, 2007. It is predominantly grown in the tropical areas of Central and South America, Asia and Africa (Marita et al, 2001). Cacao is commercially exploited for seed output mainly destined for chocolate manufacturing.…”

Assessment of genetic diversity on a sample of cocoa accessions resistant to witches' broom disease based on RAPD and pedigree data

“…In general, accessions from the Upper Amazon germplasm show higher GD than those from other regions (MARITA et al, 2001;RONNING et al, 1995;N'GORAN et al, 1994;LERCETEAU et al, 1997;WHITKUS et al, 1998). The clone set studied here consisted of Upper and Lower Amazon and Trinitario germplasm, comprising 20 TSA-644 descendants and 10 non-descendants.…”

“…These accessions, especially Scavina-6 and Scavina-12, transmit to their descendants not only resistance but also high hybrid vigor and good yielding performance, being widely used in producing countries affected by the disease. Many other resistant accessions were identified in Brazil, at the Centro de Pesquisas do Cacau (CEPEC) in the state of Bahia, and Estação de Recursos Genéticos José Haroldo (ERJOH) in the state of Pará, after field evaluation of a germplasm collection of more than 1000 accessions in each location (MARITA et al, 2001;FALEIRO et al, 2004c;PIRES, 2003). Scavina-6, Scavina-12, and other accessions identified in Brazil as resistant are being crossed in different breeding schemes to combine alleles from different resistant sources in order to develop new resistant clonal and hybrid varieties.…”

“…Molecular markers like Random Amplified Polymorphic DNA (RAPD) and microsatellites are currently the most widely used markers for assessing genetic diversity in cocoa (MARITA et al, 2001;FALEIRO et al, 2002b;FALEIRO et al, 2004a;2004b). MARITA et al (2001) using RAPD studied the genetic diversity of a representative sample composed by 280 accessions, providing an overall portrait of the relatedness of the major cocoa germplasm groups available in the world.…”

“…MARITA et al (2001) using RAPD studied the genetic diversity of a representative sample composed by 280 accessions, providing an overall portrait of the relatedness of the major cocoa germplasm groups available in the world. RAPD has also been used in genetic diversity studies in clones selected for WB resistance (FALEIRO et al, 2004a), in the identification of mislabeling and accession duplication (FALEIRO et al, 2002c), and in the development of genetic linkage maps (FALEIRO et al 2004b;QUEIRÓZ et al, 2003).…”