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

Abstract

Two related segregating populations of Theobroma cacao L. were analysed for their resistance to Phytophthora palmivora. The first F1 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 F1 tree with Catongo. The genetic maps obtained for each population were compared. The F1 map includes 162 loci and the backcross has 140 loci. The two maps, F1 and BC1, exhibit high co-linear loci organization 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 F1 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 F1 population. The absence of some yield QTL detection in the BC1 in comparison with the F1 population is due to the lack of transmission of the favouring alleles for these QTL from the single F1 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 Phytophthorawere 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.