Molecular mapping of cocoa genome for resistance to Phytophthora megakarya

Phytophthora megakarya is one of the fungus responsible of the black pod disease which promotes serious damages on cocoa in Central Africa, particularly in Cameroon. Indeed more than 50% of the fruit production may be lost due to Phytophthora megakarya. In order to understand the genetic basis of the resistance of cocoa to Phytophthora, a diallel 6x6 was set in Barombi-Kang experimental station (Cameroon) in 1974. In the present study, three of the progenies present in the diallel trial have been retained for QTL analysis according to the number of individuals available and to the differences of sensitivity of each parent to the fungus. Five years of observations of black pod disease in the field are available and supply data for QTL detection. In addition, resistance tests on fruits and leaves have also been performed. Progenies are derived from the following crosses: ICS84 (medium resistant Trinitario) x UPA134 (resistant Upper Amazon Forastero), UPA134 x SNK10 (sensitive Trinitario) and SNK413 (resistant Trinitario) x IMC67 (medium resistant Upper Amazon Forastero). For these three progenies, reciprocal crosses have been studied. The mapping of the three progenies have been addressed with microsatellites and AFLP markers and will be completed by RFLPs. Up to now, 224 markers (9 microsatellites) have been mapped on the first progeny, 182 markers (6 microsatellites) on the second one and 122 markers (6 microsatellites) on the third one. Genetic maps have been constructed with the Joinmap software and the QTL detection has been performed with Mapqtl. We present here the first QTL results obtained for resistance to P. megakarya and a comparison of QTL location accross the three progenies. (Texte intégral)