Resistance gene mapping for witches' broom disease in Theobroma cacao L. in an F2 population using SSR markers and candidate genes

A genetic linkage map was created from 146 cacao trees (Theobroma cacao), using an F-2 population produced by selling an F, progeny of the cross Scab and ICS1. Simple sequence repeat (SSR) markers (170) were used principally for this map, with 12 candidate genes [eight resistance gene homologues (RGH) and four stress related WRKY genes], for a total of 182 markers. Joinmap software was used to create the map, and 10 linkage groups were clearly obtained, corresponding to the 10 known chromosomes of cacao. Our map encompassed 671.9 cM, approximately 100 cM less than most previously reported cacao maps, and 213.5 cM less than the one reported high-density map. Approximately 27% of the markers showed significant segregation distortion, mapping together in six genomic areas, four of which also showed distortion in other cacao maps. Two quantitative trait loci (QTL) for resistance to witches' broom disease were found, one producing a major effect and one a minor effect, both showing important dominance effects. One QTL for trunk diameter was found at a point 10.2 cM away from the stronger resistance gene. One RGH flanked the minor QTL for witches' broom resistance, implying possible association. QTLs mapped in F-2 populations produce estimates of additive and dominance effects, not obtainable in F-1 crosses. As dominance was clearly shown in the QTL found in this study, this population merits further study for evaluation of dominance effects for other traits. This F-2 cacao population constitutes a useful link for genomic studies between cacao and cotton, its only widely grown agronomic relative.