Theobroma cacao L.: a genetic linkage map and quantitative trait loci analysis.

A genetic linkage map of Theobroma cacao (cocoa) has been constructed from 131 backcross trees derived from a cross between a single tree of the variety Catongo and an F1 tree from the cross of Catongo by Pound 12. The map comprises 138 markers: 104 RAPD loci, 32 RFLP loci and two morphologic loci. Ten linkage groups were found which cover 1068 centimorgans (cM). Only six (4%) molecular-marker loci show a significant deviation from the expected 1∶1 segregation ratio.The average distance between two adjacent markers is 8.3 cM. The final genome-size estimates based on two-point linkage data ranged from 1078 to 1112 cM for the cocoa genome. This backcross progeny segregates for two apparently single gene loci controlling (1) anthocyanidin synthesis (Anth) in seeds, leaves and flowers and (2) self-compatibility (Autoc). The Anth locus was found to be 25 cM from Autoc and two molecular markers co-segregate with Anth. The genetic linkage map was used to localize QTLs for early flowering, trunk diameter, jorquette height and ovule number in the BC1 generation using both single-point ANOVA and interval mapping. A minimum number of 2-4 QTLs (P<0.01) involved in the genetic expression of the traits studied was detected. Coincident map locations of a QTL for jorquette height and trunk diameter suggests the possibility of pleiotropic effects in cocoa for these traits. The combined estimated effects of the different mapped QTLs explained between 11.2% and 25.8% of the phenotypic variance observed in the BC1 population.