Gene action analysis by inheritance and quantitative trait loci mapping of resistance to root-knot nematodes in cotton

P>Host-plant resistance is highly effective in controlling crop loss from nematode infection. In addition, molecular markers can be powerful tools for marker-assisted selection (MAS), where they reduce laborious greenhouse phenotype evaluation to identify root-knot nematode (RKN) Meloidogyne incognita Kofoid and White (Chitwood) resistant genotypes. To better understand host-plant interactions, we investigated gene action and genomic locations of RKN resistance genes in cotton (Gossypium hirsutum L). Twelve parents, 17 intraspecific (G. hirsutum x G. hirsutum) and four interspecific (G. hirsutum xG. barbadense L.) F-1, and 11 F-2 populations were investigated by examining different RKN resistance backgrounds. The F-1 and F-2 generation means and distributions, and differences between F-1 and mid-parent values for Galling Index (GI) indicated that allelic interaction, epistasis and heterosis operated in these crosses. Genetic and quantitative trait loci (QTL) analyses in crosses NemX x SJ-2 and SJ-2 x Clevewilt revealed resistance because of at least one major recessive QTL with strong additive effect, while in PS 7 x NemXresistance was because of at least one major QTL with strong dominant effect. Simple sequence repeat (SSR) markers CIR316 and MUCS088 on chromosomes 11 and 21 showed the association of two different chromosomes with the RKN resistance. This also supported the two-gene model of observed resistant : susceptible ratios in F-2 and F-3 (PS 7 x NemX) populations. Selection success rate for MAS was > 85% for identifying resistant genotypes with GI < 3 from these crosses with SSRS CIR316, BNL1231 and MUCS088, and CAPS (GHACC1). QTL analyses validated the importance of a major genome-telomeric region on chromosome 11, harbouring RKN resistance genes from different backgrounds. We conclude that breeding for optimal resistance must be based on selection of progeny with combinations of genes homozygous for resistance.