The Genus Theobroma of neo tropical origin are small under storey trees of lowland forest, distributed from the Amazon Basin through Southern Mexico, Andes, Colombia, Ecuador, Central America and Costa Rica. There are 22 species under six sections.
a. Andropetalum: (T. mammosum)
b. Glossopetalum : ( T. angustifolium, T. canumanense, T. chocoense, T. cirmolinae, T. grandiflorum, T.hylaeum, T. nemorale, T. obovatum, T. simiarum, T. sinuasum, T.stipulatum, T. subincanum)
c. Orenthes: (T. bernouillii, T. glaucum, T. speciosum, t. sylvestre, T. veluienum)
d. Rhytidocarpus: (T. bicolor)
e. Telmatocarpus: (T. gilari, T. microcarpum)
f. Theobromo: (T. cacao)
The genus Chromosome number is 2n = 2x = 20. All Theobromo sections appear to be monophyletic except for the Glossopetalum. There are two hypothesis regarding the origin and distribution of T. cacao: First, Cheesman (1944) proposed that the area between the Caquata, Napo, Putumayo Rivers is the centre of diversity for the species, and possibly spread elsewhere by humans. Alternatively, Cuatrecasas (1964) postulated that natural cacao spread from Guyana Amazon region and developed into two different varieties T. cacao subspecies cacao and T. cacao subspecies sphaerocarpum.
Its fat rich seeds are important source of cocoa solids and cocoa butter, raw materials for the chocolate, confectionary and cosmetic industries. Cacao breeding faces the limitation of narrow genetic base, restricted genetic information, price and political instability. The devastating diseases are Witches broom disease caused by C. periciosa, Monilia Pod rot caused by Moniliopthora roreri, Cocoa shoot badnavirus (CSSV) and vascular streak dieback caused by Oncobasidium theobrome; black pod rot caused by Pantropical ubiquitos Phytopthera complex ( P. palmivora, P. capsicii, P. citropthora, P. megakarya).Bacteria and pests are not big problems. Cacao pod borer (Conophomorpha cramerella) and mirids spread by mealy bugs are common in some parts. Various mired species (sahlbergella singularis, Distantiella theobromae and helopeltis spp.) deposit eggs on young stems, branches and pods that are later eaten by the larvae.
Cacao breeding was initiated since 1920s, with following achievements already at the hand; selection for heterosis, germplasm collection, resistance to witches’ broom and production of homozygous parents through inbreeding or production of double hybrids.
Cacao has a small haploid genome between 388 Mbp to 415 Mbp, three times the size of the Arabidopsis genome. The genome size measured by reassociation kinetics, has been estimated to be 201 Mbp, with 36% G + C, a low degree of methylation and a small amount of highly repetitive DNA.Genes cloned; based on cDNA and/or genomic library screening are:- 21 kDa trypsin inhibitor gene; 67kDa vicilin like storage protein gene; the chitinase 1 gene. A 2S albumin storage peptide has been identified and characterized and the corresponding full length has been cloned. cDNAs of two seed aspartic proteinases ( TcAP1 and TcAP2) associated with the development of flavor precursors are cloned and characterized. Sequences of 15.8 kDa olesins and 16.9 kDa carboxy peptidase type III, both seed proteins, are deposited in the genebank. The amplified and sequenced nuclear genes include:- argenine decarboxylase (spe2), the 18 S rDNA, the trypsin inhibitor gene, Plastid genes, H+ transporting ATP synthetase (atpB), Ribulose 1, 5 biphosphate carboxylase large subunit (rbcL) and the nicotinamide adenine dinucleotide dehydrogenase(NADH) subunit F ( ndhF)
Genetic studies of cacao has been hampered by the long juvenile cycle, by the heterozygosity and by the high cost needed to maintain large population for long time. In cacao, F1 hybrid varietal trials, derived from the crosses between two heterozygous genotypes are frequently used to develop linkage maps, exploiting the ‘pseudo test cross’ configuration for various loci. A map is generated for each heterozygous parent based on segregation of co-dominant markers (RFLP, SSRs, Isozymes, etc.). Both the maps are joined to make a reference map.A high density linkage map of T. cacao has been established from a population of 195 individuals derived from a cross of two heterozygous plants: a Trinitario and a Forastero. The map comprises 473 loci linked in 10 groups which correspond to the ten chromosomes of T. cacao. These loci correspond to 5 isozymes, 178 RFLP probes, 30 RAPD and 191 AFLP markers and 69 microsatellites. The length of the map is 887 cM with a 1,9 cM average distance between two markers.
Cacao has been cultivated for more than 400 years and this crop still faces numerous problems with diseases, pests and the lack of high yielding clone materials. In vitro culture methods could play an important role to complement existing breeding efforts and to establish cacao plantations with superior germ plasm. Recent progress has now been made on the development of methods for production of somatic embryos (SE) from non-sexual explants (petals and nucleus tissues). Large numbers of SE have been produced and resulting cacao plants have been raised to maturity. The regeneration rate has been 4.3% for petals, (9,000 explants) and 2.0% for nucellus (29,000 explants).
Since the first studies on somatic embryogenesis in 1975, only floral explants, particularly staminodes, have had some success. So far, it is possible to define the proper leaf stage and culture conditions allowing a good callogenesis whatever the genotype and to obtain and maintain in liquid media cell suspensions with embryogenic characteristics. The recovery of the haploid plant from cultured anthers and microspores has not been reported. However, Dublin (1972-73) reported the recovery of haploid, n= x=10, cacao plants from naturally occurring polyembryonic seeds and later he described the production of dihaploid plants.