Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 10, 2012
Publication Date: April 13, 2012
Citation: Wang, C., Ulloa, M., Mullens, T.R., Yu, J.Z., Roberts, P.A. 2012. QTL analysis for transgressive resistance to root-knot nematode in interspecific cotton (Gossypium spp.) progeny derived from susceptible parents. PLoS One 7(4):e34874. doi:10.1371/journal.pone.oo34874 Interpretive Summary: The southern root-knot nematode (Meloidogyne incognita) is a major parasite of cotton that causes significant yield losses. Although cotton varieties with resistance to the root-knot nematode are available, much remains to be learned regarding the genetics of this resistance. Upland and Pima cottons with known susceptibility to the root-knot nematode were crossed to investigate the level of resistance exhibited in their progeny. Progeny from these crosses exhibited high levels of resistance to the root-knot nematode. These highly resistant progeny are called transgressive segregants. Genetic analyses, in combination with observations on nematode injury and egg production on cotton roots, revealed marker-genes (alleles) on four cotton chromosomes involved in the nematode resistance of these transgressive segregants. Data from molecular markers (small pieces of DNA that can be detected chemically) indicated the strength of the resistance to nematodes depended on specific combinations of alleles. The fact that transgressive segregants with high levels of nematode resistance can be derived from susceptible parents suggests opportunity to use genetic mapping and selection to improve nematode resistance of elite cotton cultivars.
Technical Abstract: Progeny from crosses between susceptible cotton genotypes were found to be highly resistant to the root-knot nematode (RKN, Meloidogyne incognita), a major parasite of cotton that causes significant yield losses. A recombinant inbred line (RIL) population of 138 lines developed from a cross between Upland cotton TM-1 (Gossypium hirsutum L.) and cotton Pima 3-79 (G. barbadense L.), both susceptible to RKN, was used to identify quantitative trait loci (QTLs) determining responses to RKN in greenhouse infection assays with simple sequence repeat (SSR) markers. Compared to both parents, 53.6% of RILs showed less root-galling index (GI) and 52.1 % had lower nematode egg production (eggs per gram root, EGR). Highly resistant lines (transgressive segregants) were identified in this RIL population for GI and (or) EGR in two greenhouse experiments. QTLs were identified using the single-marker analysis nonparametric mapping Kruskal-Wallis test. Four strong QTLs located on chromosomes 3, 4, 11, and 17 were identified to account for 8.0 to 12.3% of the phenotypic variance (R2) in root-galling. Two strong QTLs accounting for 9.7% and 10.6% of EGR variance were identified on chromosomes 14 and 23 (P<0.005), respectively. In addition, 19 suggestive QTLs (P<0.05) accounted for 4.5 - 7.7% of phenotypic variance (R2) in GI, and 15 QTLs accounted for 4.2 - 7.3% of phenotypic variance in EGR. In lines with alleles positive for resistance contributed by both parents, in combinations of 2 to 4 QTLs, dramatic reductions of >50% in both GI and EGR were observed. The transgressive segregants with epistatic effects derived from susceptible parents indicate that high levels of nematode resistance in cotton may be attained by pyramiding positive alleles using a QTL mapping approach.