Investigation of the chromosomal location of the bacterial blight resistance gene present in an Australian cotton (Gossypium hirsutum L.) cultivar

Abstract

An early success of the CSIRO cotton breeding program was the incorporation of resistance to the bacterial blight pathogen Xanthomonas campestris pv. malvacearum (Xcm ) into commercial cultivars. Pedigree records suggest that the source of this resistance was a set of related, so-called immune lines carrying the B₂B₃B₇ and B_Sm genes. However, resistance to Xcm race 18 segregates as a single dominant locus in at least one Australian cultivar (CS50), so its true identity is unclear. Our study uses mapped restriction fragment length polymorphism (RFLP) markers to investigate the chromosomal location of Xcm resistance in an Australian cultivar of Gossypium hirsutum (CS50) in an inter-specific cross with a blight-susceptible Gossypium barbadense (Pima S-7). The mapping data suggest that the resistance locus is not on chromosome 20 near either the B₂ or B₃ genes, but co-segregates with a marker on chromosome 14 known to be linked to the broad-spectrum B₁₂ resistance gene originally from African cotton cultivars. Amplified fragment length polymorphisms (AFLPs) and simple sequence repeats (SSRs) were also used to search for novel markers linked to the Xcm resistance locus to facilitate introgression of this trait into G. barbadense through a program of marker-assisted selection. The overall level of polymorphism between the 2 Gossypium species used in our mapping cross was low, but one additional AFLP marker loosely linked to Xcm race 18 resistance in CS50 was found. The paucity of polymorphic loci may reflect a high degree of gene exchange between these 2 species during the breeding of the modern-day commercial Upland and Pima cultivars.