Introgression of Root-Knot Nematode Resistance into Tetraploid Cottons

Root-knot nematode (RKN) resistance introgression into tetraploid cotton (Gossypium spp.) and its ancestral genome origin were examined. Resistance sources ('Acala NemX', 'Clevewilt 6', Auburn 623 RNR) were compared with diverse germplasm using simple sequence repeat (SSR) markers from chromosomes 7, 11, and 14 and DNA sequence information. Differences (P < 0.05) were observed for mean root galling index (GI; scale 0-10) between the 56 resistant and susceptible entries. In resistance sources, GI ranged from 0.3 to 2.9, but no consistent differences were observed at allele-marker or DNA sequence level. Except for CIR316 allele (206207 bp) on chromosome 11, no alleles from other SSRs were observed on resistant entries (GI < 3). Allotetraploid G. hirsutum L. (AD(1)) and G. barbadense L. (AD(2)) showing the same SSR marker alleles as G. arboreum L. (A(2)) might suggest resistance introduction from the diploid cotton A(2) genome (genetic distance 0.19 to 0.27). However, percent sequence identity from MUCS088 and CIR316 revealed G. barbadense and resistant G. hirsutum (206-207 bp) also were close to diploids G. herbaceum L. (A(1)), G. thurberi Tod. (D-1), and G. trilobum (DC.) Skovst. (D-8). Other G. hirsutum DNA sequences were closer to G. raimondii Ulbr. (D-5). These analyses indicated resistance introgression into G. hirsutum allotetraploid cottons occurred by artificial hybridization with ancestral genome origin from G. arboreum as well as G. thurberi and not during cotton genome evolution.