Quantitative trait locus mapping for Verticillium wilt resistance in a backcross inbred line population of cotton (Gossypium hirsutum x Gossypium barbadense) based on RGA-AFLP analysis

Verticillium wilt (VW), caused by Verticillium dahliae Kleb., is one of the most important diseases in cotton. The objective of this study was to map quantitative trait loci (QTLs) conferring VW resistance using resistance gene analog (RGA)-targeted amplified fragment length polymorphism (RGA-AFLP) markers in an interspecific backcross inbred line mapping population, consisting of 146 lines from a susceptible Sure-Grow 747 (Gossypium hirsutum L.) x resistant Pima S-7 (G. barbadense L.) cross. VW resistance was evaluated in replicated tests based on disease incidence in the field, and disease incidence and severity in the greenhouse. Of 160 polymorphic RGA-AFLP markers, 42 were significantly correlated with one or more VW traits and 41 were placed on a linkage map which covered 1,226 cM of the cotton genome and contained 251 other molecular markers. Three QTLs for VW resistance were detected, each of which explained 12.0-18.6 % of the phenotypic variation. Two of these QTLs for disease incidence and severity detected in the greenhouse inoculation tests using root wounding are located on chromosome c4. Both are closely linked to four RGA-AFLP markers and therefore considered as the same QTL for VW resistance. The other QTL detected in the field test was located on c19 and flanked by several RGA-AFLP markers. The desirable QTL allele on c4 for VW resistance detected in the greenhouse was from the VW susceptible Upland parent and absent from the resistant Pima parent which was more VW susceptible due to the disarmament of the first line of defense mechanism due to root wounding during inoculation. The other desirable VW resistance QTL allele, on c19, was from the resistant parent Pima S-7, consistent with the fact that Pima cotton was more resistant to VW when naturally infected in the field. The results should facilitate the development of more sequence specific markers and the transfer of VW resistance from Pima to Upland cotton through marker-assisted selection.