Resistance to root-knot nematode (Meloidogyne incognita) is determined by a single major gene rkn1 in Gossypium hirsutum Acala NemX cotton. Bulked segregant analysis (BSA) combined with amplified fragment length polymorphism (AFLP) was used to identify molecular markers linked to rkn1. DNA pools from homozygous susceptible (S) and resistant (R) bulks of an F 2:3 originating from the intraspecific cross NemX · SJ-2 were screened with 128 EcoR1/ Mse1 primer combinations. Putative AFLP markers were then screened with 60 F 2:7 RIL plants and four AFLP markers were found linked to rkn1. The linkage of AFLP markers to rkn1 was also confirmed in a F 2 population. The closest AFLP marker was converted to a cleaved amplified polymorphic sequence (CAPS) marker (designated GHACC1) by aligning the sequences from both susceptible and resistant parents. GHACC1 linkage to rkn1 was confirmed in the F 2 (1R:3S), F 2:7 RIL (1R:1S) and the backcross population SJ-2 · F 1 (NemX · SJ-2) (1 heterozygous: 1 homozygous). The four AFLP markers, GHACC1 plus two SSR markers (CIR316 and BNL1231) linked to rkn1 from previous work were mapped to intervals of 2.6-14.2 cM from the rkn1 locus, and the genomic region around rkn1 was spanned to about 28.2 cM in the F 2:7 population. The PCRbased GHACC1 and CIR316 markers were tested on 21 nematode resistant and susceptible cotton breeding lines and cultivars. GHACC1 was suitable for nematode resistance screening within G. hirsutum, but not G. barbadense, whereas CIR316 was useful in both species, indicating their potential for utilization in marker-assisted selection.

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