Quantitative trait loci analysis in Theobroma cacao using molecular markers.: Inheritance of polygenic resistance to Phytophthora palmivora in two related cacao populations -: Polygenic resistance to Phytophthora palmivora in cacao

Two related segregating populations of Theobroma cacao L. were analysed for their resistance to Phytophthora palmivora. The first F-1 population was obtained by crossing two susceptible cacao clones of Catongo (a highly homozygous genotype) and Pound 12 (a highly heterozygous genotype) and the second population was obtained by backcrossing a single F-1 tree with Catongo. The genetic maps obtained for each population were compared. The F-1 map includes 162 loci and the backcross has 140 loci. The two maps, F-1 and BC1, exhibit high co-linear loci organisation covering respectively, 772 and 944 cM. Phytophthora resistance was assessed by measuring the size increase of a lesion at five (DL5) and ten days (DL10) after pod inoculation. Six different QTL were detected in the F-1 and BC1 populations. One QTL was found in both populations, and appeared to be a major component of disease resistance, and explaining nearly 48% of the phenotypic variance in the F-1 population. The absence of some yield QTL detection in the BC1 in comparison with the F-1 population is due to the lack of transmission of the favouring alleles for these QTL from the single F-1 tree used for the backcross. The phenotypic variance explained by the action of the quantitative trait alleles indicated that genetic factors of both major and minor effects were involved in the control of the character studied. QTL conferring increased resistance to Phytophthora were identified in both susceptible parents, suggesting the presence of transgressive traits and the possibility of selection in cacao. Pleiotropic and epistatic effects for the QTL were also detected. Finally, the use of marker assisted selection (MAS) in cacao breeding programs is discussed.