PHYSICAL MAPPING B12 RESISTANCE IN THE COTTON

To explore the underlining molecular mechanism(s) causing bacterial blight symptoms and host plant resistance an F2 population derived from a resistant cotton line S295 (Gossypium hirsutum L.) carrying the B12 resistance gene (R-gene), and the susceptible line, Pima S7 (G. barbadense L.), was inoculated with Xanthomonas axonopodis pv. malvacearum (Xam) Race 18. The segregation of F2 phenotype was explained by a single dominant gene model. The physical location of B12 was delineated to a region of 354 kb by mapping linked DNA markers to the assembled genome sequence G. raimondii. Once the physical region was defined in G. raimondii, the entire sequence was subjected to BLAST analysis against the Plant Resistance Gene Database (PRGdb) (http://prgdb.crg.eu/wiki/Main_Page), which includes the sequences of all known plant resistance R-genes, putative and referenced R-genes, to distinguish the possible candidate resistance genes. There were73 putative plant resistance genes that aligned to the delineated sequence at 34 different positions and lengths, among which 15 aligned putative plant resistance genes have a homology length over 400 bp at 7 different positions. Both the G. raimondii sequence and PRGdb database have been valuable resources toward physical-mapping the B12 locus.